ISRN-UTH-INGUTB-EX-E2015/01-SE

Examensarbete 15 hpApril 2015

Pre-Study for a Battery Storage

for a Kinetic Energy Storage System

Henrik Svensson

Teknisk- naturvetenskaplig fakultet UTH-enheten Besöksadress: Ångströmlaboratoriet Lägerhyddsvägen 1 Hus 4, Plan 0 Postadress: Box 536 751 21 Uppsala Telefon: 018 – 471 30 03 Telefax: 018 – 471 30 00 Hemsida: http://www.teknat.uu.se/student

Abstract

Pre-Study for a Battery Storage for a Kinetic EnergyStorage System

Henrik Svensson

This bachelor thesis investigates what kind of battery system that is suitable for an electric driveline equipped with a mechanical fly wheel, focusing on a battery with high specific energy capacity. Basic battery theory such as the principle of an electrochemical cell, limitations and C-rate is explained as well as the different major battery systems that are available. Primary and secondary cells are discussed, including the major secondary chemistries such as lead acid, nickel cadmium (NiCd), nickel metal hydride (NiMH) and lithium ion (Li-ion). The different types of Li-ion chemistries are investigated, explained and compared against each other as well as other battery technologies. The need for more complex protection circuitry for Li-ion batteries is included in the comparison. Request for quotations are made to battery system manufacturers and evaluated. The result of the research is that the Li-ion NMC energy cell is the best alternative, even if the cost per cell is the most expensive compared to other major technologies. Due to the budget, the LiFeMnPO4 chemistry is used in the realisation of the final system, which is scaled down with consideration to the power requirement.

ISRN-UTH-INGUTB-EX-E2015/01-SEExaminator: Nóra MassziÄmnesgranskare: Anders HagnestålHandledare: Johan Abrahamsson

Sammanfattning

Detta examensarbete undersoker vilket typ av batterisystem som ar lampligt for en elekt-risk drivlina utrustad med ett mekaniskt svanghjul, dar fokus ligger pa ett system medhog specifik energikapacitet istallet for hog e↵ektkapacitet. Grundlaggande batteriteo-ri sasom funktionen i en elektrokemisk cell, begransningar, ”C-rate”, skillnaden mel-lan primara och sekundara battericeller forklaras samt vilka sorts batterisystem somfinns tillgangliga pa marknaden. Fakta om de vanligast forekommande batterikemier-na, sasom bly-syra, Nickel Kadmium (NiCd), Nickel-metallhydrid (NiMH) och Lithium-jonbaserade (Li-ion) redovisas och jamfors med varandra. Olika typer av lithium-jon cellerundersoks, redovisas och jamfors med varandra samt med de andra batteriteknologiernadar ocksa behovet av ett mer komplext skydds- och overvakningssystem ar medtaget ijamforelsen. O↵ertforfragan till mojliga leverantorer gors och o↵erterna utvarderas. Slut-satsen av undersokningen ar att lithium-jon cellen av typen LithiumNickelManganKo-boltoxid (NMC) ar den lampligaste alternativet, aven om kostnaden for varje cell ar denhogsta jamfort med de andra vanligen forekommande batteriteknologierna. Med hansyntill budget anvands LiFeMnPO4 i det realiserade systemet, eftersom att detta mojliggorett nerskalat system som anda uppfyller kravet pa e↵ekt.

Contents

1 Introduction 21.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.2 Purpose and project goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theory 52.1 The Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 The Electric Battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 Operation Of A Cell . . . . . . . . . . . . . . . . . . . . . . . . . 72.2.2 The Limitations Of A Cell . . . . . . . . . . . . . . . . . . . . . . 92.2.3 C-Rate and E-Rate . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2.3 Primary Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4 Secondary Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.5 Types of Secondary Batteries . . . . . . . . . . . . . . . . . . . . . . . . 14

2.5.1 Lead Acid Accumulators . . . . . . . . . . . . . . . . . . . . . . . 142.5.2 NiCd and Nickel–Metal Hydride . . . . . . . . . . . . . . . . . . . 172.5.3 Lithium Metal and Lithium-Ion . . . . . . . . . . . . . . . . . . . 18

2.6 Battery Control and Protective Circuitry . . . . . . . . . . . . . . . . . . 222.6.1 State of Charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.6.2 State of Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.6.3 Battery Charging . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3 Method 273.1 Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273.2 Finding the right cells; performance comparison . . . . . . . . . . . . . . 273.3 BMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.4 Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Results 30

5 Discussion 365.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365.2 Further research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

1

1 Introduction

The word battery refers to ”any large group or series of related things: a battery ofquestions.” In electrical terms, a battery is two or more electrochemical cells stackedtogether.

1.1 Background

In the year 1800, the Italian Alessandro Volta invented the first modern battery in formof the ”voltaic pile” which cells consisted of zinc and copper that were separated by clothdrenched in sulphur acid. The standard unit for electric voltage, [V] is named in honourof Alessandro Volta. During the 19th century, the electric battery was further developedand was very important to the development of the telegraph network. The first lead acidrechargeable battery was invented by Gaston Plante in 1859 and was called secondary dueto the fact that it was invented after the non rechargeable primary cell and these whereneeded to recharge the secondary cells. In the beginning of the 20th century electric carswere relatively more common compared to today and electric batteries were of courseimportant for this application.

Due to development of the internal combustion engine (ICE) and worldwide findings ofoil in parallel with improvements in road infrastructure, petrol powered cars became lessexpensive than electric ones. Because of the limited technology development of batteries,electric cars at that time were slow and had a very limited range compared to a car withan ICE. With the discovery of the semi conductive properties of germanium and siliconmaterials which led to the development of the transistor and later on the microprocessor,it is now cheaper and more easy than ever to build sophisticated power electronics systemsfor electric motor controllers, inverters, rectifiers and battery control systems.

With the recent findings that strengthens the theory of a correlation between a globalclimate change and the use of fossil fuel, the need for smarter and more e↵ective powersystems has never been more obvious.1 Since the electric motor has a compact designand very high e�ciency compared to the ICE, it is the the better alternative for a vehiclefleet of the future. Recent development in battery research and smarter control systemsenables a greater action range of electric vehicles which now is a serious alternative foran every-day car. Of course, the total e�ciency and environmental impact of the energystored in the batteries of the cars depends on how the energy is produced. The demandson a battery system for electric vehicles are high due to the need for both specific poweroutput and specific energy storage capacity and also the ability to absorb power whileusing regenerative breaking. To optimise power flow and energy conservation, a vehiclein which the power train is divided in an energy storage part and a power handling partcould have benefits of both properties. If the propulsion system would have a robustenergy bu↵er able to handle high power flow and an energy storage system optimised for

1http://report.mitigation2014.org/spm/ipcc_wg3_ar5_summary-for-policymakers_

approved.pdf

2

high energy storage capacity, the system would have a better durability and possibly agreater action range. In Figure 1.1 the Ragone plot shows the specific power and specificenergy capacity of di↵erent energy storage systems.

Figure 1.1: Ragone plot of various means of storing electric energy. Power rating corre-sponds to 95% e�ciency. The specific energy of the flywheels includes the vacuum chamber andauxiliary equipment.(Image and data: Abrahamsson 2014)

The purpose and final goal of the flywheel research project at Uppsala University isto develop a robust and reliable energy storage system, optimised for both high poweroutput capacity and high specific energy storage capacity. The system is depicted inFigure 1.2 consists of a high power side and a low power side. The low power side is tothe left of the flywheel and consists of an inverter and a battery system, which can beoptimised for high specific capacity and deliver a low continuous power to the flywheel.The high power side is to the right and consists of a three phase traction motor, a rectifierand inverter. Due to the design of the flywheel, the double winded machine will separatethe low power side from the high power side.

Figure 1.2: The system overview (Image: Abrahamsson, 2012)

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1.2 Purpose and project goals

The experimental set up of the research flywheel system does not yet have a batterysystem as power source but is powered from the grid via a variable three phase transformerand a rectifier.

The purpose of this thesis is to investigate which type of battery that is suitable for adriveline such as the research flywheel system. The battery system should be optimisedfor high energy storage capacity, be equipped with relevant control and managementsystems and also su�cient safety systems so that it does not pose any danger for users.The system is intended to be be installed in a commuter bus and the battery will bedimensioned thereafter with consideration to the minimum required power for a vehicleof that size. The total budget is set to 50 kkr and will be the limiting factor for thecapacity of the system.

Aspects to be investigated are:

• Battery theory

• Energy density

• Type of battery

• Battery system safety and protection

• Cost

For safety reasons, the investigation is limited to solutions that already exists on thecommercial market.

Specifications of the desired system

• A minimum terminal voltage of 150 V

• 15 kW of continuous power

• Modular system with possibility for capacity expansion

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2 Theory

2.1 The Flywheel

A flywheel is a kinetic energy storage system (KESS) that stores energy as rotationalenergy, as shown in equation (2.1)[1].

E =J!2

2(2.1)

In the beginning of the industrial era, the flywheel was used to enable translation ofthe linear movement of the piston in a steam engine to rotational movement and alsosmooth the torque. These flywheels were made heavy and large to enable as much energystorage capacity as possible as the the moment of inertia J is linearly proportional to themass and quadratic proportional to the radius of a rotating solid cylinder, as describedin Equation (2.2)[1].

J =mr2

2(2.2)

Since the kinetic energy E is is proportional to the square of the angular velocity !,the mass and size of the flywheel can be decreased if the rotational speed is increased.Recent progress in materials engineering enables more compact flywheel applications,such as energy bu↵er systems in the powertrain for electric or hybrid vehicles, whichhandles the transient power fluctuations that occurs during a standard driving cycle.

The research flywheel at Uppsala University[2], shown in Figure 2.1, is divided in alow power side (LP) and a high power side (HP), separated by the electric machine insidethe flywheel. The battery delivers a low, constant power to the flywheel through thepower electronics at a relatively low voltage on the LP side while the flywheel deliversand receives a variable power from the traction motor at a higher voltage.

If the flywheel is used for transient power handling and storage for regenerative break-ing energy instead of the batteries in an electric or hybrid vehicle, the batteries can beoptimised for specific energy storage capacity, instead of compromising between this andspecific power capacity.

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Figure 2.1: The latest version (2014) of the experimental flywheel at Uppsala University.(Image: Abrahamsson 2014)

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2.2 The Electric Battery

A battery consists of one or more electrochemical cells that function as an electrolytic cell,converting electrical energy into chemical energy (a charging battery) or as a galvaniccell, converting the stored chemical energy into electrical energy (a discharging battery).

2.2.1 Operation Of A Cell

The fundamental principle in an electrochemical cell is spontaneous redox reactions intwo electrodes separated by an electrolyte, which is a substance that is ionic conductiveand electrically insulated. An electrolyte exist in di↵erent forms but is often either asolid, a liquid or an ionomeric polymer. In Figure 2.2 the electrolyte consists of a saltbridge, allowing ionic exchange of cations and anions. The redox reaction that occur inthe Daniell Cell1 is shown in Figure 2.2 and is described as

Oxidation (Anodic reaction): Zn(s) ��! Zn2+(aq) + 2 e� (2.3a)

Reduction (Cathodic reaction): Cu2+(aq) + 2 e� ��! Cu(s) (2.3b)

Redox (Cell reaction): Zn(s) + Cu2+(aq) ��! Zn2+(aq) + Cu(s) (2.3c)

Figure 2.2: Electrochemical cell (Daniell Cell), consisting of one half cell of a zinc plate inzinc sulphate and one half cell of a copper plate in copper sulphate.(Image: Wikipedia Commons)

1Invented by the British chemist and meteorologist John Frederic Daniell in 1836 (Tansjo, Levi. JohnFrederic Daniell Nationalencyklopedin. http://www.ne.se/john-frederic-daniell 2014-05-12.)

7

Equation (2.3c) shows a cell reaction where the zinc anode plate is oxidised and electronsflow through the conductor, reacting with the aqueous copper ions. Simultaneously thereis a corresponding ionic exchange through the electrolyte. The electrical current willcause a potential di↵erence and a electromotive force, E which is equal to energy (E(J))per charge (q(C)), E = E/q[3]. The spontaneous reaction will occur because of zinc beinga better reducing agent than copper and thereby more able to emit electrons. Thereby,copper is more noble than zinc. A stronger ability to emit electrons at the anode willcause a stronger E . This ability is measured with the standard hydrogen electrode (SHE)as reference which is called the standard electrode potential, E0 and is shown in Table 2.1.

Table 2.1: The standard electrode potential rel-ative to the standard hydrogen electrode at thetemperature 298.15 K, concentration 1M andpressure 101.3 kPa.[4]⇤ The hydrogen electrode potential is -0414 V atpH = 7

Oxform Redform E�

V

Li+ +e– Li -3.04K+ +e– K -2.93Ca2+ +2 e– Ca -2.87Na+ +e– Na -2.71Mg2+ +2 e– Mg -2.37Al3+ +3 e– Al -1.66Zn2+ +2 e– Zn -0.76Fe2+ +2 e– Fe -0.44Cd2+ +2 e– Cd -0.40Co2+ +2 e– Co -0.28Pb +

2 +2 e– Pb -0.13

2H+ +2 e– H2 0.00⇤

Cu2+ +e– Cu+ +0.15Cu2+ +2 e– Cu +0.34Fe3+ +e– Fe2+ +0.77Ag+ +e– Ag +0.80AuCl –

4 +3 e– Au + 4Cl– +1.00Cl2 +2 e– 2Cl- +1.36F2 +2 e– 2F- +2.87

In a galvanic cell (E0 > 0), the electro-motive force is driven by the di↵erence ofthe Gibbs free energy and can, under thestandard condition2 be described as

�G� = �nFE� (2.4)

where �G is the di↵erence in the Gibbsfree energy and n is the number of elec-trons transferred in the reaction. F is theFaraday constant (9.648·104C/mol and E�

is the standard electrode potential. Thecondition (�Gcat < �Gan, E

�cat > E�

an)gives that the standard electric potentialof the cell is

E�cell = ��G�

cell

nF(2.5)

or as

E�cell = E�

cathode � E�anode (2.6)

and the electric potential in the Daniellcell in Figure 2.2 is

E�cell = E�

Cu � E�Zn

E�cell = +0.34V � (�)0.76V

E�cell = 1.1Volt

(2.7)

Note that values in (2.7) are given in thestandard condition. In other conditions,the cell voltage Ecell will deviate from stan-dard cell potential and is given by theNernst equation.[5, p. 2.3]

E = E� � RT

nFlnQ (2.8)

where Q is the reaction quotient, R is the universal gas constant (8.314JK�1mol�1)and T is the absolute temperature in Kelvin.

2At temperature 298.15 K, concentration 1 mol L�1 and the pressure 101.3 kPa

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By applying an external voltage that drives the current in the opposite direction asthe spontaneous reaction of the galvanic cell is reversed and the battery is charged. Theexternal voltage Erev must be greater than E�, as shown in figure 2.3. The total chargethat can be stored in the cell is dependent of the amount of the electroactive materialwithin the cell.

(V)

(A)

E0

Current i �!

CellVoltageE

C!

Electroly

tic Cell

Galvanic Cell

ir

ir

iR

ED

Figure 2.3: Schematic variation of cell voltage EC against the current i where E� is thestandard electrode potential of the cell or the voltage with no load. R is an external load and ris the internal impedance. (Image: Reproduced from ”Electrochemistry” [6])

2.2.2 The Limitations Of A Cell

The theoretical value of the cell voltage is described in Equation 2.6 and is given bythe di↵erence of the standard electrode potentials in the anode and the cathode. Theenergy storage capacity of a cell is referred to as coulumbic capacity and is the quantityof charges (electrons) that can be exchanged in the reaction. The number of electronscan be theoretically determined by calculating the quantity of active materials in thecathode and anode. The energy/charge capacity of a cell is ampere-hours (Ah). Onemole of electrons, the quantity of Avogadro’s constant, NA ⇡ 6.022 ⇥ 1023, contains atotal charge of 96.487⇥ 104C which is equal to the Faraday’s constant and an energy of26.8 Ah.

z =

✓VNL

VFL

� 1

◆R (2.9)

Internal impedance

According to the Thevenin theorem[7, p. 139], the galvanic battery cell can be describedas a ideal voltage source in series with an impedance. The Thevenin impedance is oftencalled the internal impedance of the cell, denoted z in Figure 2.4. If the voltage ismeasured between the terminals A and B in an open circuit or if the external loadRload � z , the value of the voltage is equal to E or VNL. If the value of the external loadR ! 0, the value of the current I ! E/z , is limited only by the internal impedance z ,for which the voltage drop of the circuit will occur (VFL). The internal impedance is alsoreferred to as ”ohmic polarisation” and is proportional to the drawn current[5, p. 2.1].

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The internal impedance is caused by inertia within the cell; such as the decompositionof ions, the ionic resistance within the electrolyte, di↵usion of ions on the surface of theelectrodes and also losses due to complex polarisation e↵ects within the cell. These factorsof internal impedance can be divided into to lumped elements as seen in Figure 2.5. Toovercome the impedance of the polarisation e↵ects, an extra force in the form of greaterelectric potential is needed. This is called the overpotential of the reaction.

Since the internal impedance is caused by the chemical reactions in the cell, it istemperature dependent. The temperature for the standard condition is 298, 15 K or 25�Cand any deviation will change the internal impedance of the cell and also its performanceto deliver current and power. A rise in temperature will increase the reaction speed,the internal impedance will decrease and the cell will be able to drive more current andpower but will shorten the lifespan of the cell over time. Low temperatures will slowdown the reaction rate and increase the internal impedance, decreasing the accessiblepower capacity of the battery.

I

Rload

Az

BE

Battery

Figure 2.4: The schematic circuit withthe battery modelled as the Thevenin equiv-alent.

CDL

RERCT RW

Figure 2.5: The lumped elements modelof a battery cell where CDL is the doublelayer capacitance, RCT the charge transferresistance, RW the Warburg resistance andRE the resistance of the electrolyte

Self Discharge

Although it would be e�cient to slightly rise the temperature for batteries during dis-charge it also a↵ects the rate at which the spontaneous reactions within the cell willoccur, even if the electrical circuit is open and not galvanic conductive. This is calledthe self discharge and how fast it will occur depends on factors as the electrode material,design and storage temperature. A lower temperature reduces the rate of self discharge.The self discharge phenomena can also contribute to shorter discharge time during verylight loads as shown in Figure 2.6.

Overpotential and Limit of Charge Transfer

The charge transfer overvoltage or activation polarisation and concentration polarisationof the electrodes in the cell can be described as the magnitude of deviation from theequilibrium electrode potential of the cell, E�

cell is defined as

⌘ = E � E�cell (2.10)

where E is the actual voltage[6, p. 159]. The overpotential is dependent on thecurrent density in the cell and is at low currents a↵ecting the rate at which electronsare transferred across the electrode and the ion-conductive solution. This is termedthe electron transfer overpotential. At high current density, the limitation that causesthe overpotential phenomena is dependent on the rate at which the reactants can betransferred in the electrolyte, inducing di↵usion overpotential.

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In a cell that is charging (electrolytic cell), the threshold voltage for the reaction willbe increased. For a discharging cell (galvanic cell), the output voltage will be reduced.The energy loss of the overpotential will go away as heat and the charge/discharge over-potential may di↵er in ”rechargeable” cells. The ratio of E�/E gives a value of the voltagee�ciency.

Figure 2.6: E↵ect of self discharge on battery capacity(Image: Lindens Handbook of Batteries [5])

Figure 2.7: The relationship between current density and overpotential. The dashed curvesrepresent the partial current densities of anode (above) and the cathode (bottom). The curve inbetween is the equilibrium potential. (Image: Scrosati[8])

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2.2.3 C-Rate and E-Rate

A common method for indication of the current flow of a battery is the C rate which givesa value of the rate at which the battery is discharged or charged and can be expressed as

I = kCh (2.11)

where I is the current i amperes (A), C is the rated capacity of the battery in ampere-hours (Ah), h is the time in hours of the discharge rate and k is a fraction or multipleof the rated capacity C. For example; if a battery is rated to contain 1000 mAh anddischarges 1 ampere for one hour, the C-rate is 1C. If it discharges in two hours, the Crate is 0.5C and 2C if the discharge is for 30 minutes.

A similar way to indicate power is the E-rate which refers to a value of the ratedpower discharge in the cell and can be expressed as

P = kEh (2.12)

where P is the power in watts (W) and E is the rated energy of the battery in Watt-hours (Wh). For the E-rate, 1E is the discharge power to discharge the entire battery in1 hour.

Figure 2.8: Discharge time for di↵erent C rates. The C rate is in non-linear in practice.(Image: BatteryUniversity.com)

As seen in Figure 2.8, the C-rate is non-linear and will, depending on the chemistryof the cell set-up, discharge faster than the theoretical discharge time at higher C ratesand may provide power longer than the calculated discharge time at low C rates.

Battery systems that are optimised and designed for high power performance canhandle high C rates but have limited energy storage capacity.

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2.3 Primary Batteries

Non-rechargeable batteries are referred to as primary batteries and are disposable, theyare discarded after use. This means that the redox reaction within the cell is not re-versible. Primary cells have higher energy density than the rechargeable secondary cell,as shown in Figure 2.9, but most types of primary cells have high inner impedance andwill therefore cause a big voltage drop during high discharge current, limiting the powercapacity. This means that a cell that may seem depleted after having supplied energyto a power demanding application can deliver power to devices that do not need a highcurrent to work for some time, i.e a wall clock.

The most common type of primary battery cells is the Zinc/Carbon cell or theLeclanche battery, developed by Georges Leclanche in 1866, and the Alkaline battery(Zink/alkaline/Manganese Dioxide). The Leclanche cell is still commonly used but hasbeen much improved and further developed and is the cheapest type of cell on the mar-ket. The alkaline cell was introduced to the market in 1959 but did not become morecommon than the Zinc/Carbon cell until around 1980. Lithium based primary cells havethe lowest self discharge rate hence the longest available shelf time, up to 10 years and intemperatures up to 70�C which is extreme for batteries. The Zinc/air primary cell hasthe highest energy density, up to 370 Wh/kg, and is commonly used in hearing aids.

Due to its lack of ability for recharge the use of primary battery cells as a power sourcein the flywheel driveline is discarded.

Figure 2.9: The comparison of specific energy density in primary and secondary cells.(Image: Battery University)

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2.4 Secondary Batteries

Rechargeable cells can be recharged by driving electric current in the opposite directionof the discharge current, as explained in section 2.2.1, and are referred to to as secondarybatteries. Primary cells have better energy storage capacity, as seen in the comparisonin Figure 2.10, but secondary cells have better power output capabilities compared toprimary cells and are used for high power applications. They are also known as ”accumu-lators” or ”storage batteries” and the applications can be divided in two categories, onewhere the battery is used as storage device for electric energy which is generated froma prime power source, as in emergency backup systems (UPS), aircrafts, automotives,hybrid vehicles and most common as starter, lightning and ignition batteries (SLI) in astandard car.

The other field of application is where the secondary battery is used as a main powersource and is discharged as a primary but instead of being discarded are recharged.Examples for this are mobile phones, laptops, power tools and plug-in hybrid or electricvehicles.

The most important characteristics among secondary batteries are high e�ciency, highenergy density and long cell life meaning that the physical impacts of the componentsof the cell should be least possible during the conversion of chemical energy to electricenergy and back again. The battery should also perform over a wide temperature rangewhich limits the alternatives of materials used in a rechargeable battery system.

2.5 Types of Secondary Batteries

The range of secondary battery cells includes special solutions such as hydrogen basedbatteries and batteries that is constructed as a combination with capacitors such as theCSIRO Ultrabattery™and Nilar Bipolar Battery. These battery types will not be includedin this text since it focuses on the most common types.

2.5.1 Lead Acid Accumulators

The Lead Acid battery is one of the most common batteries and the design has mainlybeen the same since it was invented by Plante in 1859.3 It has become the standardbattery for the automotive industry and is commonly used as the power source for start,lightning and ignition (SLI) in cars. Industrial fields of applications for lead acid batteriesare as traction power for mining vehicles, forklifts and as stationary power sources suchas emergency back up power storage (UPS) and signalling stations for rail roads andtelecommunication.

In the 70s, the ”maintenance free” lead acid battery was introduced and since thenseveral types of sealed lead acid have emerged and the most common type is valve-regulated lead acid (VRLA) which contains a silica type gel that suspends the electrolytein a paste, or an absorbent glass mat (AGM). The VRLA battery has a very low internalimpedance and is capable to deliver high power and may have a relatively long service life.Even when deep-cycled, VRLA batteries o↵ers a depth-of-discharge of 80 % comparedto flooded batteries which is specified at 50 % DoD to attain the same cycle life. TheVRLA design is lighter than the flooded lead acid type, has a low self-discharge and canbe charged up to five times faster than the flooded but has relatively low energy density.

3The share of lead acid batteries was 70% of the total market of secondary batteries in 2008

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Figure 2.10: Theoretical and actual specific energy of di↵erent battery chemistries(Image: Lindens handbook of Batteries (p 1.15))

The lead acid battery can be used for deep cycling such as in traction power appli-cations and can be optimised for more specific energy storage capacity or for high powercapacity such as in SLI configuration.

The deep cycle design that is modelled in Figure 2.13 has thick lead plates, whichenhances the abilities to withstand the attrition caused by cycling and is optimal fortraction applications. In Figure 2.12 the battery is optimised for high power output,using many thin plates of lead, maximising the surface contact and decreasing the internalimpedance. This design does not allow deep cycling and the battery will be worn outafter 10-15 cycles if deep cycled.

The lead acid battery has many advantages as a stationary power storage cell. It isvery cheap and sturdy compared to other battery systems and up to 97% of the leadplates can be recycled, which is good from a sustainability point of view even if lead isan environmental hazard if not handled correctly. Both VRLA designed batteries andconventional lead acid has high power capacity if designed as shown in Figure 2.12 andhave good performance in both high and low temperatures, but the VRLA design is moresensitive to higher temperature environment.

The disadvantages of this battery chemistry is that it is very sensitive to deep cycling,compared to other battery systems and due to the high density of lead, the specific energyof the batteries is quite low; 252 Wh/kg in theory and 35-40 Wh/kg for a practical battery.For most designs the cycle life is limited of up to 500 cycles, even if there are special designs

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Discharge Current[A]

Cap

acity[A

h]

Low-rate (high capacity) battery

High-rate battery

Figure 2.11: The ratio of specific power vs. specific energy capacity

Figure 2.12: Lead Acid battery designed forhigh power output, such as SLI batteries.(Image: Battery University)

Figure 2.13: Lead Acid battery designed fordeep cycling and high specific energy capacity,such as traction power batteries.(Image: Battery University)

that allow up to 2000 cycles. Charging a lead acid battery system is slow and it can takeup to 16 hours for a full charge. It also requires a current and voltage limiting algorithm,hence limiting the charging current and power. If the charging current is too high, itwill cause sulfation which may degrade the performance and cycle life of the battery.Furthermore, the charging voltage must be regulated as the maximum voltage appliedis limited by the ambient temperature, due to the risk of hydrogen gas generation. Thecharging procedure for a lead acid battery system is shown in Figure 2.16.

The keywords for lead acid battery systems are:

Advantages:Cheap, sturdy, sustainable, low temperature performance, easy to handle

Disadvantages:Heavy, low energy density, not suitable for deep cycling, slow charging time

16

2.5.2 NiCd and Nickel–Metal Hydride

Nickel Cadmium - The first Nickel Cadmium battery was invented in 1899. Itsperformance and advantages were superior to the lead acid cell but the components werefar more expensive and it would abide until 1947 that the improvements where madewhich led to the the sealed NiCd cell that still is modern today. The battery has lowinternal impedance resulting in high power capabilities but lower energy storage capacitycompared to other battery systems. It has long cycle life and capability of rapid rechargebut may su↵er from voltage depression or memory e↵ect, meaning that that maximumcharge voltage will decrease and hence the energy capacity if continuously dischargedshallowly. The greatest disadvantage is the content of cadmium. Unfortunately, cadmiumis extremely toxic and therefore the NiCd will not be an alternative for a modern batterysystem.

Nickel Metal Hydride - [5]: The NiMH battery were commercially introduced in1989 and where mainly used as a power source in portable personal computers. Sincethen, the NiMH battery system has become very popular in electric hybrid vehicles andmake up 10% of the total market for rechargeable batteries.4 The performance of thebattery has also been improved; the specific energy has doubled and the service life spanhas been extended since introduction. Compared to the NiCd battery, the NiMH provides40 percent higher specific energy and less a↵ected by voltage depression, but the mainadvantage is the absence of the toxic cadmium.

The NiMH is maintenance free and is tolerant to less careful handling, including over-discharge and overcharge. It has good power flow abilities and can utilize regenerativebreaking energy in HEV and also have good rapid charge abilities. The electronics incharging systems and control circuits for NiMH is simple and inexpensive and the batteryis considered to be a safe battery system, which is one of the main reasons why thechemistry is popular among manufacturers of HEV or PHEV. The cell potential of nickelmetal hydride based batteries is 1.20 V and the theoretical specific energy is 240 Wh/kgand 100 Wh/kg for a practical battery with the energy density of 235 Wh/L. This ismore than twice the specific energy compared to the lead acid battery but the drawbackis that it is more expensive than lead acid per kWh. The NiMH battery also has high selfdischarge and can loose up to 20 % of its charge during the first 24 hours and thereafter10 % per month. Special designs of the hydride alloy will lower the self discharge andincrease the robustness and cycle life but with the cost of the specific energy capacity,this is often done for applications for power trains in electric or hybrid vehicles.

The keywords for the NiMH battery are:

Advantages:Safe, rapid recharge capability, good power flow performance, less expensive thanlithium based battery systems

Disadvantages:High self-discharge, no suitable for shallow cycling, lower specific energy and specificpower than lithium based battery systems

4In 2008 [5, p. 22.1]

17

2.5.3 Lithium Metal and Lithium-Ion

Since lithium has the atomic number three, it is the lightest of all metals and has thelowest density of all solid elements. Lithium is very reactive and never occurs in its freeform in nature but often in ionic compounds[3, p. 335↵]. As seen in Table 2.1, lithium hasthe greatest electrode potential and can provide the highest specific energy and energydensity but due to the reactive properties in lithium, there are di�culties to make thelithium metal battery safe during charging. If the cell is short circuited, the temperaturerises quickly and approaches the melting point of lithium, causing thermal runaway andpossible a fire. This risk is a disadvantage for lithium based cells but can be reducedby using compounds with lithium ions instead of lithium metal. The use of protectivecircuits that limits the charging voltage and currents provides a much safer and feasiblebattery technology, even if the specific energy capacity is lower than the lithium metal.

The lithium ion battery was introduced on the market by Sony in 1991 and has cometo replace the NiMH in applications such as laptops and cordless power tools and theinterest in lithium ion has become a fast growing market due to the development of EVsand HEVs. The research on safe lithium metal solutions is still in progress, for examplethe Lithium -Sulphur battery that have a theoretical specific energy of 2500 Wh/kg andup to 350 Wh/kg for a practial battery5.

The Li-ion battery works according to a swing principle as the Li+ ions exchange backand forth between the electrode materials when the battery is cycled[5, p.26.1↵]. Theelectrochemical active materials in the negative electrode is often a lithiated graphite andfor the positive electrodes lithium phosphate or lithium metal oxide. The cells can bemanufactured in several form factors; either prismatic, cylindrical or in polymer pouchcells. The pouch cells gives a very compact and modular design. The most commonpackage of lithium-ion batteries is the 18650 cell which measures 18 x 65mm, was designedin the mid 90’s and have been a standard power cell for laptops but even as multi-cellpacks in EVs such as the Tesla Model S.6 Modern laptops have more and more switchedover to the more compact polymer pouch format due to the demand for lighter computerswith longer run time.

Compared to other battery systems, the advantages of Li-ion cells are numerous. Thebattery type have long cycle life and the self discharge rate is as low as 2% per month.The cell voltage (3.8V) is three times the cell voltage of NiMH (1.2V) and the specificenergy is twice as high, depending on the type of chemistry used in the Li-ion cell. It ismaintenance free, has rapid charge capabilities, high specific energy, power and e�ciencyand lack the issue with voltage depression that NiMH batteries has.

The Li-ion battery may seem like the perfect battery, but even if it has superiorabilities compared to other systems, the drawbacks with Li-ion systems is an expensiveinitial cost, and the risk of thermal runaway if abusively handled or overcharged. Thesystem does also degrade when operated or stored in high temperatures and may becomeunstable if charged in sub zero temperatures. The greatest drawback is that a Li-ionsystem need a protective circuitry, which can monitor the charging process and in multicell batteries balancing the rate of charge, voltage and current.

5Sion Power Battery http://www.sionpower.com/technology.html

6http://www.greencarcongress.com/2013/10/20131030-tesla-1.html

18

Types of Lithium Ion batteries

When designing Li-ion battery cells, there is a balance between high specific power andhigh specific energy. Di↵erent designs and choice of material give the cell either morespecific power or specific energy hence cells are arranged in power cells and energy cells.The following chemistries and variations of the cathode materials give the cell di↵erentproperties:

Li-cobalt/LCO - Lithium Cobalt Oxide, LiCoO2 was the first type of lithium ionbattery that was commercialised, and is still today the most common cell type. It hashigh specific energy but short cycle life and the specific power rating is low which limitthe field of application to low power devices such as cell phones and laptops. The lowspecific power rating also limits the pace at which the cell can be charged and dischargedand C-rates above 1C could damage the cell and shorten the cycle life. The cell type alsohas a narrow span of operation temperature and the capacity is significantly decreasedin low temperatures. Another drawback is the price; LCO has a content of 60% cobalt,which is an expensive raw material.

The keywords for Lithium Cobalt Oxide batteries are:

Advantages:Most common, high specific energy and energy density

Disadvantages:Low specific power rating, expensive, narrow temperature gap

NMC - Lithium Nickel Manganese Cobalt Oxide, LiNiMnCoO2 is a further develop-ment of lithium cobalt cells that have a content of 10–20% of cobalt, hence cheaper thanLCO. By altering the proportions of nickel and manganese the chemistry can be designedfor optimisation of high specific energy (more nickel) or for high specific power (moremanganese) but not both.

As seen in Table 2.2, the ratio of specific energy and power di↵ers alot and a standard18650 energy cell can hold up to 2,250 mAh when discharged at 0.2C and with a limitedC-rating of 2C. A power NMC cell has the ability to output as high C-rates as 30Ccontinuous power and up to 100C in short pulses but has a limited energy capacity of1,500 mAh. A NMC energy cell has similar (but slightly better) cycle life as LCO as ithas 87% capacity left after 300 cycles [5, p26.55f]. The power cell has better cycle lifewith a capacity of 90% even after 500 cycles. The NMC cells has wider temperature gapthan LCO and the NMC power cell has slightly better low temperature performance thanthe energy cell.

The NMC chemistry is modular and can be tailored for a specific application evenif it excels in specific energy capacity. It is very safe compared to LCO and it is lessexpensive.

The keywords for Lithium Nickel Manganese Cobalt Oxide batteries are:

Advantages:Modular chemistry, safe, high specific energy OR high specific power

Disadvantages:Not the best in either highest specific power nor energy compared to other chemistries

19

Li-phosphate/LiFe - Lithium Iron Phosphate, LiFePO4 is the lithium ion chemistrythat has the best thermal stability of all cell types available on the market. This gives asturdy cell with superior specific power ratings which is very safe and have a long cyclelife; a capacity of 95% after 1000 cycles at 1C discharge in 25�C and the wear down e↵ectof discharge during high temperatures is less than in other li-ion chemistries[5, p.26.63].The trade-o↵ with the sturdiness and splendid high current capabilities is a lower cellvoltage at 3.3 Volt, hence lower specific energy and due to complex manufacturing bysynthesises in inert gas make LiFe - cells very expensive measured in price per kWh,compared to other types of li-ion cells. As seen in Table 2.2, the LiFe - cell can deliverextreme current in up to 250C but have the lowest specific energy of all the comparedchemistries.

The keywords for LiFePO4 batteries are:

Advantages:Very safe, sturdy, long cycle life, superior power rating

Disadvantages:Low specific energy, expensive

LTO - In this cell the graphite in the anode is replaced by Lithium Titanate, Li4Ti5O12.The cell voltage is lower than other available chemistries (2.3V) and also the specificenergy (70 Wh/kg). The advantage of lithium titantate cells is a superior cycle life andlow temperature performance. The cell type is also very safe and have good C-ratingcapabilities with up to 10C in continuous output.

The keywords for Li4Ti5O12 batteries are:

Advantages:Very safe, superior cycle life, good performance in low temperatures

Disadvantages:Low specific energy, expensive

20

NCA - Nickel-Cobalt-Aluminium, LiNiCoAlO2 is a relative new cathode material, stillbeing in the beginning of the emerging stage much e↵ort to overcome the safety problemby partial substitution of nickel with other elements and the best known mixture isLi(Ni0.8Co0.15Al0.05)O2. The strengths of the NCA-system include high energy (up to240 Wh/kg) and high power due to its relatively high specific charge of about 180 mAh g-1 and an average voltage of about 3.9 V. Although the properties of NCA seem promising,work remains to be done in terms of safety, costs, and the useful fraction of the stateof charge range. A crucial problem of high Ni content materials, like NCA, is the rapidreaction with air resulting in the formation of Li2CO3 and LiOH on the surface, leadingto a reduction in capacity and increased irreversible capacity of the active material[9].

The keywords for LiNiCoAlO2 batteries are:

Advantages:Good cycle life, superior specific energy capacity

Disadvantages:Expensive, bad thermal stability rating.

Chemistry LiCoO2

NMCNCA

LMONMC

LiFePO4 LMO/Li4Ti5O12

Characteristic Energy Cells Power Cells Power Cells Neither

Voltage range [V] 2.5-4-35 2.5-4.2 2.5-3.6 2.8-1.5Average voltage [V] 3.7 3.7 3.3 2.3Specific Energy [Wh/kg] 175-240 100-150 60-110 70Energy Density [Wh/L] 400-640 250-350 125-250 120Continuous output capability [C] 2-3 Over 30 10-125 10Pulse output capability [C] 5 Over 100 Up to 250 20Cycle life +500 +500 +1000 +4000Charge temperature [�C] 0-45 0-45 0-45 -20 - 45Discharge temperature range [�C] -20 - 60 -30 - 60 -30 - 60 -30 - 60Power density [W/L] (pulse) 2000 10000 10000 2000Specific power [W/kg] (pulse) 1000 4000 4000 1100

Table 2.2: The general performance characteristics of Li-ion cells [p.26.47][5]

21

2.6 Battery Control and Protective Circuitry

A battery pack needs proper control and protective circuitry to perform according tospecifications and it is very important to handle the energy flow in the chemical cells toensure a longer service life. The most commonly used component for protective circuitryin battery packs is the thermistor. The device works as resistor but its resistance variesas a function of the temperature. The type of thermistor used in battery applicationshas a positive temperature coe�cient (PTC) and its resistance will increase significantlywhen the temperature rises. When current flowing in or out from the battery cell, thetemperature will rise and the thermistor will limit the current at a certain value oftemperature and will also work as a protection for short circuit. As seen in Figure 2.14,the resistance of the thermistor will increase rapidly in the temperature span of 50-100�C.

Figure 2.14: The characteristics of a PTC device.(Image: Reddy and Linden[5])

The development and innovation of new and more energy dense battery technologiessuch as lithium-ion needed the implementation of more complex protective circuits andmanagement systems. In the mid 90’s, microprocessors were begun to be implementedin systems for protection, control of charge/discharge rate and battery performance op-timisation. With help of electronics, discharging and charging of the battery can besafer and more e↵ective. The complexity of the protection circuits varies according tothe complexity and stability of di↵erent battery systems. The least complex and mostfundamental circuit in a modern battery system is the protective circuit module, PCM,that is common in most modern battery cells but is more or less mandatory for lithium-ion cells. The PCM protects the cell from over(under)voltage and over(dis)charge and isoften contained within a single cell or a cell pack. For a more complex multi cell batterysystem a Battery Management System (BMS) is needed.

The main task of the a BMS is to keep the service life of the battery as long as possibleand optimise the e�ciency of the energy exchange during charge and discharge. To dothis, the BMS should have functions such as cell protection,charge control, cell balancing,data logging, State of Health and State of Charge determination.

22

The complexity of a BMS increases with an increment of the amount of cells in thebattery pack. In multi-cell packs, the pack is divided in several modules that each hasits own slave-BMS. The slave-BMS in the modules communicate via CANbus7 and arecontrolled by a master-BMS. Figure 2.15 shows a schematic model of how a BMS can bedesigend. In this case the BMS is equipped with cell monitoring and balancing features aswell as a current sensor and temperature sensor. It also has the possibility to communicateusing CAN bus.

Figure 2.15: A schematic model of a Battery Management System.(Image: Public Domain)

2.6.1 State of Charge

The State of Charge (SoC) is the approximation of the maximum possible charge left inthe battery, often presented in percentage of the full capacity. For applications in EVand HEV the SoC is translated to the possible driving range. In the electrochemical cell,the SoC refers to the amount of active materials left.

There are several methods used to determine the State of Charge. Depending on therequired precision and the complexity of the battery system, these methods can be usedone by one but more often combined.

Direct Measurement These methods refers to the physical properties of the cells,such as voltage over the terminals and impedance of the cell. The terminal voltage canbe measured in open circuit. Lead-Acid batteries has shown to have an approximatelylinear relationship of the SoC and open circuit voltage and can be described as

7Controlled Area Network, a standard bus used in the vehicle industry

23

VOC(t) = V1 · SOC(t) + V0

where V0 is the open circuit voltage when SoC is zero and V1 is obtained by knowingV0 and SoC = 100%. This is not a good SoC estimation method for Li-Ion batteriesdue to the very flat discharge voltage in Li-Ion cells. Another voltage method is tomeasure the EMF of the battery by using thermodynamic data of the battery cell andthe Nernst equation or by measuring the terminal voltage during discharge. The EMF ofthe battery is approximately linearly proportional to the SoC. At the end of the discharge,the measurement error will be large due to a sudden drop of voltage caused by an increaseof the internal impedance.

Book-keeping systems , such as Coulomb counting is the integral of the batterycurrent over time. The estimation is done by measureing the voltage drop over a currentshunt; a well defined resistance with low ohmic value or by using a Hall e↵ect sensor. Byknowing the rated capacity in amp-hours of the battery the SoC can be approximated.The precision of the method depends on factors such as the ambient temperature, depthof discharge (DoD) and ageing process and therefore this needs to be accounted for. TheSoC must be re-calibrated on regular basis to contain a reference point for which the SoCis 100%.

Self learning systems are systems which implement the use of artificial intelligenceand adaptive algorithms such as neural networks and Kalman filtering. A Kalman filteris a recursive algorithm that can estimate and predict a value from a very noisy anduncomplete signal which increases the precision. There are several advantages with theuse of adaptive systems due to the many factors that can cause error in the estimationof the SoC.

2.6.2 State of Health

A newly-produced battery cell is assumed to be at its best condition and has a capacityin ampere hours that is close to the rated specifications and has a low internal impedance.As the battery is cycled its electrodes and electrolyte degrade and the performance willdecrease over time and consequently the impedance within the cell will increase. TheState of Health is a figure of merit that refers to the degradation of the performancecompared to the ideal battery, which has a SoH = 100%, down to the ”end of life”threshold, commonly accepted around 80%. The rate of which the SoH is decreased isa↵ected by several simultaneous mechanisms. For a well designed battery the degradationwill be balanced so that one mechanism does not cause a premature breakdown of the cell.For lithium ion cells the porosity and the relative mass of active material (stoichiometry)is the parameters that most likely will cause the decrease of capacity [10]. The Stateof Health can be estimated by using Electrochemical Impedance Spectroscopy where thebattery is modelled as seen in Figure 2.5. By probing the battery with AC-signals the realand imaginary parts of the total internal impedance can be obtained. The impedancedata, along with parameters such as historical logged temperature, depth of dischargeand time of use can be translated into an estimation of the SoH.

There is a strong correlation between the estimation of SoH and SoC. To accuratelydecide the current State of Charge, the parameter of the State of Health must first becalculated.

24

2.6.3 Battery Charging

Each battery type has its own charging algorithm which is more or less complex. Leadacid batteries are sturdy and do not really need a complex protective circuit although thecycle life can be greatly extended if the battery is charged properly. The circuit shouldbe able to limit both charge current and applied charge voltage. The proper procedurefor charging lead acid batteries are shown in Figure 2.16.

Figure 2.16: The charging procedure of lead acid batteries. In stage 1, the charging current isconstant, in stage 2 the voltage is constant. In stage 3 the charging prevents self discharge.(Image: Buchmann [11])

25

A lithium-ion battery require more complex charging algorithms, depending on whichtype of lithium-ion chemistry that is used. Most chemistries can only withstand a chargingrate of 1C, and the most common recommendation from manufacturers is a charge rate of0.8C. If the charge rate exceeds the limited level, there is a risk for instability and thermalrunaway. The process of lithium-ion battery charging is described in Figure 2.17.

Figure 2.17: The charging procedure of lithium -ion batteries.(Image:Buchmann [11])

26

3 Method

3.1 Approach

To investigate the properties of di↵erent battery systems and the important parametersand limitations of the electrochemical nature of a battery cell, adequate information wasfound in specialist literature such as ”Linden’s Handbook of Batteries” [5], ”Electro-chemistry” [6], ”Lithium Batteries: Advanced Technologies and Applications” [8] andthe website ”Battery University” (www.batteryuniversity.com”) to sort out which bat-tery chemistry that is most suited for the application as a power source in the flywheelsystem. The important properties and di↵erences of battery chemistries examined in thisstudy is written in the Theory part. A comparison of the three di↵erent chemistries wasrequired to find out which one that is most suitable for the system.

The prerequisites of the system is a nominal power of 15 kW and a minimal terminalvoltage of 150 Volt. In this setup the battery must be able to deliver a nominal currentof 100 Amperes, given by Irated = Prated/Vrated. By increasing the terminal voltage, theoutput current can be decreased. This can be done by increasing the number of cellsstacked in series but also leads to a higher cost.

3.2 Finding the right cells; performance comparison

The theoretical voltage is determined by the electrode materials, as described in Table2.1, the practical voltage is what can be achieved in a real battery. The practical voltageof NiMH batteries is around 10% lower than the theoretical value but for Lead-acid andLi-ion the real voltage only di↵ers 5-8%. This also a↵ects the specific energy, which isthe energy storage capacity of the battery in watt-hours (Wh) divided by the mass ofthe battery in kilograms (kg). The theoretical capacity in ampere-hours/gram (Ah/g)is based on the equivalent weight of the active materials participating in the electro-chemical reaction. Multiplying the theoretical capacity and voltage gives the theoreticalspecific energy in Wh/kg. As described in 3.1, NiMH has the lowest capacity with 240Wh/kg, Lead-acid has 250 Wh/kg. The Li-ion battery theoretically yields a much larger450 Wh/kg specific energy, depending on which chemistry that is chosen; the chemistryspecified in the table is LiCoO2. In practice, however, none of the chemistries lives upto its theoretical potential. Li-ion is still the highest at 200 Wh/kg, which is 45% oftheoretical value. NiMH batteries demonstrate at most 100 Wh/kg in practice, around40% of the theoretical value. Lead acid cells only perform at 14% of their theoreticalspecific energy, or 35 Wh/kg. The high energy density of Li-ion batteries at 570 Wh/Lis 2.4 times higher than NiMH and 8.1 times Lead-acid.

Although Lead-acid has the lowest specific energy, the approximative price per kWhis $100 and is the cheapest of all compared batteries.1 Despite the low price per kWh,

1http://ultralifecorporation.com/downloads/275

27

the Lead-acid battery is not suitable for vehicular applications due to the fact of thelow specific energy density, low practical available energy, long charge time and that thesystem with lead-acid batteries would be heavy and bulky.

With an approximative price of $600-$1,000 per kWh for Li-ion cells, the cost is almosttwice as high as for NiMH, which costs $250 to $400 per kWh. This makes the NiMHtechnology suitable for hybrid electric vehicles, where the battery is used as an energybu↵er instead of as a main power source. For example, The Toyota Prius HEV, uses abattery pack with 168 NiMH cell packs that produce 1.2 volts each.2.

Given the far more superior properties of Li-ion cells in terms of specific energy ca-pacity and power rating, it is clear that Li-ion batteries have an advantage in weightand volume-sensitive applications like electrical or hybrid vehicles, despite the highercost. The reason for higher cost is that Li-ion is a newer technology than NiMH andthe more complex protection circuitry. The most economical Li-ion battery in terms ofcost-to-energy ratio is the cylindrical 18650 cell, due to the massive production volume.

Theoretical values Practical battery

Battery type [V] [g/Ah] [Ah/kg] SpecificEnergy[Wh/kg]

Nominalvoltage[V]

Specificenergy[Wh/kg]

EnergyDensity[ Wh/L]

Lead-acid 2.1 8.32 120 252 2.0 35 70NiMH 1.35 5.63 178 240 1.2 100 235Li-ion 4.1 9.14 109 448 3.8 200 570

Table 3.1: Theoretical and practical values for voltage, specific energy and energy density ofthe examined battery systems. [5, p.1.13]

As mentioned in section 2.5.3, Li-ion batteries can be made with di↵erent chemistriesand therefore optimised for maximum power or energy. Since the driveline is dividedin a high power part where the flywheel handles the high power flow, and a low powerhigh energy storage part, the battery for the flywheel system should be the latter. Fo-cusing only on the aspect of specific energy excludes chemistries such as LiFePO4 andNMC Power cells, which have better specific power ratings but poorer specific energy.LMO/LTO, Lithium-Titanate has neither outstanding specific energy capacity nor powerrating but is the most durable cell type and may be a good choice for applications inharsh conditions but due to the higher cost per kWh of stored energy this is not suitablefor the flywheel system.

The Li-ion chemistries that where left to examine are NCA, LCO and NMC energycells. Lithium Cobalt Oxide is the oldest lithium ion cell type available on the commercialmarket and still the most common cell type. It has high specific energy capacity but due tothe poor power ratings performance and is mostly used in cellular phones and laptops andbecause of the high content of cobalt, LCO becomes expensive in large scale applicationssuch as a main power source for a commuter vehicle equipped with a flywheel system.

Nickel-Cobalt-Aluminium has shown the best characteristics in terms of specific en-ergy capacity but is a relatively new technology and yet not produced in any high volumewhich make the cell type expensive. The thermal stability is also an issue for NCA andLCO and even with a good BMS system, the battery types are limited to C-rating of 1C.

2http://www.forbes.com/sites/rosskennethurken/2013/02/12/tesla-debacle-highlights\

-need-for-new-ev-battery-technology/

28

Energy cells made of Lithium Nickel Cobalt Oxide or NMC have the advantages ofhigh energy capacity such as LCO but have better thermal stability and is less expensive,due to the lower content of cobalt. It is the second most common cell type on the marketand is still the cell that has the best ratio of price per kWh and weight.

3.3 BMS

Using a lithium ion based battery system will require a Battery Management Systemthat contains charge and discharge algorithms, current and voltage measurement andalso a cell balancing feature. A more advanced BMS is more expensive than using amanagement system for lead acid batteries but also mandatory and very important whenusing a lithium based battery system. To fulfil the requirements of the project goals, itshould be programmable and able to be modular for future expansions and changes.

The battery pack will also need a charger that can safely charge the cells. Using acharger that just measure the entire pack voltage and not the cell voltages will under-charging some cells and overcharging others, hence risking thermal runaway. Thereforethe charger must have a possibility to measure the voltages and temperature for eachcell. Here, there are two alternatives; Using a smart charger that has certain chargingalgorithms and is able to communicate with the BMS so that it can control the chargingprocedure or using a charger that is controlled by the BMS itself, only providing enoughvoltage and current.

The first alternative will increase the complexity of the charger and also the costbut may reduce the cost of the BMS as some features can be handled by the charger,even if the BMS still must have the possibility to measure the total current flow andthe voltages for each cell. The second alternative may imply a more expensive BMSbut the charging hardware needed can be built by using a rectifier and a relay or switchfor charging shut-of, controlled by the BMS. Fortunately, there is already a controllablerectifier included in the laboratory set-up for the flywheel and this can be used as acharger for the battery pack, reducing the cost of an expensive separate charger. So,implementing this alternative, a BMS which is able to control the charging procedureand a switch or relay is needed.

3.4 Budget

The budget of 50 kkr is quite low to find system that will fit within the economical frame.The cost for a lithium based system is more than for a lead acid batteries, due to theneed of a more advanced BMS. A pre-built system is more expensive than a assemblykit of components and is the only feasible approach to find a functional system. Anotherapproach to find a suitable battery system that meet the requirements of power output of15 kW is to focus on the specific power capacity of the cells, instead of the specific energy.This enables the use of a smaller battery pack with fewer cells but with a loss of energycapacity. For this approach the LiFePO4 cell type is suitable due to the advantages ofpower capacity. The cell type also has very good thermal stability, which is good forsafety aspects. A scaled down battery system is also cheaper witch is more convenientfor a strict budget and can be feasible in a laboratory setup.

29

4 Results

Requests for quotes that matched the required specifications where made to battery cellmanufacturers and importers. Since the requested battery system is a one copy build,all but one of the consulted supplier, ”CellTech Abatel”, was willing to give quotes for asystem.

The first proposed system is designed with the high specific capacity rating in mind.It is manufactured by the South Korean manufacturer ”Incell” and consists of 6 batterytrays, containing NMC Li-ion 18650 cells with 2000mAh per cell. Each tray has a voltageof 43.2 volt and the total system voltage is 259 volt. The total capacity is 10 kWh witha operating capacity of 8 kWh, which corresponds to a operation time of 30 minutes ifdelivering the rated power of 15 kW to the system. The cost of the total system is 140kSEK, or approximately $2100 per kWh, which is over the given budget. See Figures 4.1and 4.2 for an overview of the proposed battery system.

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Figure 4.2: A battery tray for the system

Since the price of this system is 2.8 times the budget a strictly cost reduced alternativeis necessary to reach the budget goals. A scaled down approach with focus on specificpower capacity give the possibility to find a system that fits the budget. Also, a systemthat is delivered as a kit of components for the user to assemble, rather than a completepre-built system is much cheaper and gives the possibility to adjust the price to the givenbudget. The kit includes a BMS, linkage and cells. The chosen BMS is a LithiumateMaster Pro manufactured by Elithion and seen in Figure 4.3. The cells are viewed inFigure 4.4 and are manufactured by Zhejiang GBS Energy Co. Ltd, situated in PeoplesRepublic of China. The complete system consists of 56 prismatic cells stacked togetherin 8 banks with 7 cells in each bank. The stacks are coupled in series, giving a terminalvoltage of 179 Volts. The Lithiumate Master Pro BMS is field programmable and canbe customised according to the users need and can communicate using RS232 and CANprotocols. It can handle large battery packs and since it has a distributed circuitry, ithas the ability to measure current, voltage for each cell as well as the possibility of cellbalancing.

The complete system will have a rated energy storage capacity of 7168 Wh and for therated power of 15 kW it will deliver a nominal current of 84 Amperes, which correspondsto a discharge rate of approximately 2C. The available capacity is 5700 kWh at a DOD of80%, giving a run time of 23 minutes. Since there already exists a fully functional rectifierin the fly-wheel setup, there is no need for a separate charger. The charging procedurewill be taken care of by the Lithiumate PRO BMS, controlling a NO-relay (normal open)that can disconnect the pack from the rectifier bridge, see Figure 4.5 for an explainingschematic. For increased safety, a current limiting circuit should also be implemented tothe system. The approximate cost of the system is 60,000 SEK; of which the cells makeup for 31.3 kkr and the BMS totals 28 kkr, not including tax of 25%. There may be someadditional cost when the system is assembled; casing, cables, relays and so forth. The

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total cost is still 20% above the given budget but still acceptable. Building the systemwith fewer cells would have reduced the cost, but would result in a decreased systemvoltage. To make up for the reduced voltage, a higher current would have been necessaryto meet the power requirement of 15 kW.

Due to several delays and long delivery time of the cells, it was not possible to buildthe battery system during the writing of this thesis. A CAD model of the system wasmade using SolidWorks™and can be seen in Figure 4.6. The suggested casing is designedwith consideration to the laboratory setup, the ”breastwork” gaps will let cooling air flowthrough the casing with ease, provide easy access to the cells, allowing cables to be addedand removed easily. The model is shown without the distributed cell boards, power andsignal cables, current sensor, battery cell lids and the casing lid.

• Supports all cell form factors such as prismatic, cylin-drical and pouch cell.

• Can handle mid-voltage cell chemistries such as LCO,LMO, NMC, NCA and Lithium Iron Phosphate.

• Able to handle up to 255 individual cells

• Individual cell measurement ability for voltage, tem-perature and current. Passive cell balancing.

• Communicates using RS232 and CAN protocols

• Fully configurable and field programmable using aGUI

Figure 4.3: The Lithiumate Master Pro BMS from Elithion(Image by courtesy of Elithion)

Cell type:

Lithium Iron Manganese Phosphate, LiFeMnPO4

Cell weight: 1.4 kg

Cell Voltage: 3.2 Volt

Rated Capacity: 40 Ah @ 0.2C

Discharge currents:

Rated: 40 Ampere (@ 1C)

Maximal, continuous: 3C (120 Ampere)

Maximal, impulse: 5C (400 Ampere for about10 sec.)

Charge currents:

Standard: @ 0.5C, 20 Amperes

Best: @ 0.25C, 10 Amperes

Fast: @ 1C, 40 Amperes

Self discharge: ⇡ 3% per month

Specific power: 800 W/kg

Specific energy: 92 Wh/kg

Figure 4.4: The 40 Ah LiFeMnPo4 cell(Image with courtesy of GBS Energy Co.)

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Battery Pack BMS

AC/DC3-Ph. Power line

DC-bus

NO Relay

Figure 4.5: A schematic of how the rectifier working as a charger for the battery system. TheBMS controls the conduction between the DC-bus and the battery using a NO relay.

Figure 4.6: The CAD model of the complete system. The casing lid, power cables, circuitboards and battery lids are not shown in this image.

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The Lithiumate PRO battery management system can handle up to 255 single cells,distributed in up to 16 separate banks. Each cell has it’s own circuit board that mea-sures the cell voltage, temperature and handles the cell balancing. The circuit boardsare mounted across the poles of the cell and each bank has a positive end, with a redcircuit board as seen in Figure 4.7 and a negative end with a black circuit board, seen inFigure 4.8. The cells in between will have green circuit boards can be seen in Figure 4.9.All circuit boards in the bank will be linked together and a signal cable will go to themaster unit from each negative and positive end circuit boards. The distribution forthe circuit boards and the planned layout for the cells with linkage can be seen in theoverview of the cells in Figure 4.10. To minimise the use of cables, each cell in the packshould be placed so that it is flipped 180� from the nearest cell. A hall e↵ect currentsensor will also be installed in the complete system but is excluded in the figures.

Figure 4.7: Positive end of battery bank circuit board.

Figure 4.8: Positive end of battery bank circuit board.

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Figure 4.9: Circuit board for the middle cells in the battery bank.

Figure 4.10: The layout of the coupling of the cells. Each cell will be flipped 180� from theneares cell. The placement of the circuit boards are seen as the red, green and black fields andalso the distribution of the banks.

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5 Discussion

5.1 Conclusion

The price of the first proposed system is about twice the average market price per kWh.The customisation of the system make it more expensive and also, investing in s ready-to-use system like this drives up the cost. Building an equivalent system in the laboratorywould decrease the cost but with the risk of reduced safety and system dependability.

For example: Consider that the price for a 18650 cell is $7.75 per cell, when buyingfrom China or USA.1 To build a battery pack that contains the same amount of energyas the Celltech system using 18650 cells that has a nominal voltage of 3.7 volt and thecapacity of 2,600 mAh, the number of needed cells and the cost is:

3.7V ⇥ 2, 600mA = 9.62Wh

10, 000Wh(total)

9.62Wh(cell)

= 1040 cells

1040⇥ $7.75 = $8060

(5.1)

As seen in Equation (5.1), the total cost for battery cells is $8060 (53 518 SEK). Thisresult in a cost of $806 per kWh, just for the battery cells. In addition, there is anassembly cost, a cost for the battery management system, cost of a charging unit and thecasings. Building a custom made BMS could reduce the total cost but for the amount ofenergy contained in a battery pack such as this, the consequences of management systemfailure could be hazardous if not built properly. Therefore could the cost of $2100 or140 kSEK could be regarded as a fair price for a battery system such as the one proposed.

The scaled down system is not designed with a high specific energy capacity in mindbut with a high specific power. This alternative do di↵erentiate from the sought forpower system with a battery pack that focus on high energy density but to obtain theneeded specific power of 15 kW but this solution make it possible to use fewer cells toreach the power demand, with the disadvantage of some loss in energy storage. Like allof the LiFe- chemistries, the LiFeMnPO4 has good thermal stability characteristics andis a safer choice compared to other lithium based battery cells. This is an importantaspect when designing a system for a lab setup, even if a sophisticated BMS such asthe Lithiumate Master Pro is used and correctly calibrated. Due to the delays and longdelivery time of the cells, the system was not able to be assembled in time to finish thisthesis. That is why the CAD model of the system is shown and not the finished system.

1http://www.megabatteries.com/item_details2.asp?id=17073&cat_id=1204&uid=1837

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The conclusion made from this study is that designing a battery system is not trivialand there are many parameters and aspects that needs to be included, as following:

• What is the purpose of the battery system? Is its main purpose to deliver poweror is it as an energy storage?

• Is the volume and weight an aspect that is important or the total capacity or maybeboth? This is an important aspect for the choice of cells used in a battery system.

• Is thermal stability an very important aspect?

• The price, overall budget?

• The time of the project is aslo an important aspect. Do you have time to wait fora custom build special system that is good for all the other parameters or do youneed to go with more common and generic systems?

These parameters are important to think of when choosing in the many alternativesof di↵erent solutions of battery systems. Lithium based systems may have a theoreticallysuperior advantage regarding power and energy per mass unit, but for a solution for anUPS or backup power system the weight and volume aspect may not be as important asfor a mobile application. Here may less expensive lead-acid cells be superior regarding tocost per kWh and even if the cost for BMS systems tends to get less expensive, the leadacid solution would not have the need for an advanced BMS such as for lithium basedbattery systems.

When designing batteries for a mobile application such as a commuter bus or a car, thedemands are higher due to the need of good performance in all aspects and parameters.This will a↵ect the price of the battery system and is the major cause of why electriccars are so expensive compared to cars with ICE.

5.2 Further research

In a future development of a system such as the flywheel research system, the use of fuelcells instead of batteries as the main power source could be implemented in a flywheelsystem. The fuel cell works according to the same principles as the electric battery but willbe continuously fed with hydrogen gas that reacts with oxygen, supplied from surroundingair. The hydrogen could be stored in compressed form or in form of a hydrocarbon suchas petrol. The fuel cell does not cope with the same load factor as battery does and thefly wheel could work excellent as a power bu↵er in a vehicular driveline equipped with afuel cell.2

One battery concept that has not been discussed in this thesis is flow cell batteries.The concept of a flow battery, viewed in Figure 5.1 is similar to solid-state cells but alsoto fuel cells. Instead of a solid anode and cathode the flow cell battery consist of twohalf cells where the reactants are dissolved in liquids and separated with a membrane.In a flow battery the energy is stored in the liquid electrolyte instead of the solid-stateelectrode, as in a conventional battery. The ionic exchange occurs across the membraneand the electrons flow through the electrodes, which does not take part in the reaction.Therefore there is no loss of performance from repeated cycling, caused by electrodematerial deterioration, as for most rechargeable solid-state batteries. When the batteryis discharged, meaning that all the active fluid has been used, the battery can either

2http://batteryuniversity.com/learn/article/fuel_cell_technology

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Figure 5.1: The concept of a flow cell battery.(Image: Public domain)

be charged by applying a current in the opposite direction such as the procedure whenrecharging a conventional battery. Or the electrolyte can be drained and replaced withnew, like refuelling a car with ICE or with a fuel cell.

The advantage of a flow battery is the possibility of fast ”recharge” when replacing theelectrolytes, which could be a great advantage when used in electric cars. The electrolytescan be reused and with no loss of electrodes the flow cell battery will have a long life time.The disadvantages is low energy density and the price, even if some manufacturers claimthat the energy density is up to 85 Wh/kg for a flow battery based on Zinc Bromide3.

The reason of excluding flow batteries is that it is still a technically di�cult solutioncompared to conventional, solid-state batteries. As the need for alternative solutions formobile energy storage, the flow battery could be a competitor on the battery market inthe future.

3http://www.zbbenergy.com/products/flow-battery/zn-br-battery-technology/

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