li-ion myth-buster
DESCRIPTION
Li-Ion Myth-Buster. Poking holes into some common beliefs about Li-Ion cells and Li-Ion BMSs. Davide Andrea Elithion - Electronics for Lithium Ion. Battery Power Conference 2010, 1 ,. EXTRA! EXTRA!. As heard this morning: "With few large format cells, each cell is monitored. - PowerPoint PPT PresentationTRANSCRIPT
Li-Ion Myth-Buster
Battery Power Conference 2010, 1,
Poking holes intosome common beliefs
about Li-Ion cells and Li-Ion BMSs.
Davide AndreaElithion - Electronics for Lithium Ion
EXTRA! EXTRA!
As heard this morning:"With few large format cells,
each cell is monitored. With many small format cells,
monitoring each cell is impractical."
Battery Power Conference 2010, 2,
EXTRA! EXTRA!
The same BMS with work equally well with both formats.
Battery Power Conference 2010, 3,
FALSE
S m a l l c e l l s
B M S c e l l b o a r d s
L a r g e c e l l s
B M S c e l l b o a r d s
Myths
A) CellsB) BatteriesC) BMSD) Balancing
Battery Power Conference 2010, 4,
A: Cell myths
Group A:Myths about Li-Ion cells
- Cell capacity myths
- Cell swelling myths
Battery Power Conference 2010, 5,
A: cell myths
Charge efficiency
Myth A1:"A cell puts out less charge
than you put into it"
FALSE
Battery Power Conference 2010, 6,
A: cell myths
Charge efficiency
Li-Ion charge efficiency = 100 %:
Every electron that goes in
can come back out
(Though, energy efficiency is < 100 %)
Battery Power Conference 2010, 7,
A: cell myths
Charge efficiency
1 A in for 1 hour, 1 A out for 1 hour
SOC: 100 %, to 0 %, back to 100 %:charge efficiency is 100 %
Charge voltage is higher than OCV, lower during discharge
Charge power is more than discharge power
Energy doesn't go back down to 0: energy efficiency is < 100 %
Battery Power Conference 2010, 8,
A: cell myths
1 0 0 %
0 %
S O C
T i m e
1 A
0 A
C u r r e n t
T i m e
- 1 A
4 V
0 V
V o l t a g e
T i m e
4 W
0 V
P o w e r
T i m e
3 . 5 W h
0 W h
E n e r g y
T i m e
1 h r 2 h r
1 h r
1 h r 2 h r
1 h r 2 h r
2 h r
1 h r
1 A h c e l l
- 4 W
C H A R G ED I S C H A R G E
O p e n C i r c u i t
T e r m i n a l
T e r m i n a l
O p e n C i r c u i t
I d e a l
I d e a l
A c t u a l
A c t u a l
Capacity vs rate
Myth A2:"Cell capacity depends on
rate of discharge"
FALSE
Battery Power Conference 2010, 9,
A: cell myths
Capacity vs rate
Battery Power Conference 2010, 10,
A: cell myths
Yes, at high discharge a fixed cutout will stop discharge sooner4 V
0 V
C e l l v o l t a g e
0 %
3 V
2 V
1 V
1 0 C
D O D
0 . 1 C
2 2 % 6 5 % 9 0 % 1 0 0 %4 0 %
2 . 0 V c u t o f f
Capacity vs rate
Battery Power Conference 2010, 11,
A: cell myths
But the rest is still there; you can still get it out, with a constant voltage load.
4 V
0 V
C e l l v o l t a g e
0 %
3 V
2 V
1 V
1 0 C
D O D
0 . 1 C
2 2 % 6 5 % 9 0 % 1 0 0 %4 0 %
0 2 V c o n s t a n t v o l t a g e l o a d
Capacity vs rate
Battery Power Conference 2010, 12,
A: cell myths
But the rest is still there; you can still get it out, with a lower cutout4 V
0 V
C e l l v o l t a g e
0 %
3 V
2 V
1 V
1 0 C
D O D
0 . 1 C
2 2 % 6 5 % 9 0 % 1 0 0 %4 0 %
0 V c u t o f f
Capacity vs cycles
Myth A3:"Cell capacity decreases
with number of cycles"
MIX
Battery Power Conference 2010, 13,
A: cell myths
Capacity vs cycles
Battery Power Conference 2010, 14,
A: cell myths
Degradation =
capacity loss
Higher resistance
+ fixed cutoffs =
unused capacity
C e l l c a p a c i t y1 0 0 A h
C y c l e s
4 V
0 V
C e l l v o l t a g e
S O C
1 0 0 %
4 . 2 V c u t o f f
8 5 %
O C V
T e r m v t g ,n e w c e l l( l o w R )
T e r m v t g ,n e w c e l l( h i g h R )
C: BMS myths
Group C:"I don't need a BMS" myths
- Cutoff myth
- Small SOC range myth
- Extra cells myth
Battery Power Conference 2010, 15,
C: BMS myths
No BMS: I use a cutoff
C1: "I don't need a BMSmy charger has a high voltage limit"
C2: "I don't need a BMSmy load has a low voltage cut-off"
FALSE FALSE
Battery Power Conference 2010, 16,
C: BMS myths
No BMS: I use a cutoff
There's no way of knowing the state of individual cells from the pack voltage.
3.3 V
3.3 V
3.3 V
3.3 V
13.2 Vtot
2.9 V
4.5 V
2.9 V
2.9 V
13.2 Vtot
3.8 V
1.8 V
3.8 V
3.8 V
13.2 Vtot
Battery Power Conference 2010, 17,
C: BMS myths
No BMS: I use a cutoff
Top balancing won't help
3.6 V
3.6 V
3.6 V
3.6 V
14.4 Vtot
3.2 V
1.6 V
3.2 V
3.2 V
11.2 Vtot = LV cutoff
2.8 V
2.8 V
2.8 V
11.2 Vtot = LV cutoff
Fully charged,Top balanced
Discharged.One has low capacity
Discharged.All same capacity
2.8 V
Battery Power Conference 2010, 18,
C: BMS myths
No BMS: I use a cutoff
Yes: a CCCV will protect a top-balanced pack: when the pack voltage is at the max, all the cell voltages will be equally at the max.
However, when discharging without a BMS, the voltage of the least capacity cell will drop too far and be damaged.
Battery Power Conference 2010, 19,
C: BMS myths
No BMS: I use a cutoff
Bottom balancing won't help
3.0 V
3.0 V
3.0 V
3.0 V
12.0 Vtot
3.3 V
4.5 V
3.3 V
3.3 V
14.4 Vtot = Charger CV
3.6 V
3.6 V
3.6 V
14.4 Vtot = Charger CV
Fully discharged,Bottom balanced
Charged.1 has low capacity
Charged.All same capacity
3.6 V
Battery Power Conference 2010, 20,
C: BMS myths
No BMS: I use a cutoff
Yes: a LV cutoff will protect a bottom-balanced pack: when the pack voltage is low, all the cell voltages will be equally low.
However, when charging without a BMS, the voltage of the least capacity cell will go too high.
And that's a fire danger!
Battery Power Conference 2010, 21,
C: BMS myths
No BMS: I mind the SOC
Myth #C3:"I'll won't use
the entire SOC range,so I don't need a BMS"
FALSE
Battery Power Conference 2010, 22,
C: BMS myths
Nice and easy: no BMS
The assumptions are that:
1) The pack SOC is known
2) All the cells are at that same SOC
Not so.
Without a BMS, the pack SOC is not known (and even a BMS doesn't always know the SOC)
Battery Power Conference 2010, 23,
C: BMS myths
More cells: no BMS
Myth #C4:"Extra cellsare cheaperthan a BMS"
FALSE
Battery Power Conference 2010, 24,
C: BMS myths
More cells: no BMS
The assumption is that a pack without a BMS will simply slowly degrade. So, instead of buying a $ 1000 BMS, I'll buy 10 extra cells.
Not so.
A SINGLE overcharge or over-discharge event can kill a cell. And it will keep on happening after you replace that cell.
Battery Power Conference 2010, 25,
C: BMS myths
D: Balancing myths
Group D:Myths about cell balancing
- Balance purpose myths
- Balance point myths
Battery Power Conference 2010, 26,
D: balancing myths
Balancing purpose
D1: "Balancing protects a battery"
D2: "Balancing compensates for variations in cell capacity"
FALSE FALSE
Battery Power Conference 2010, 27,
D: balancing myths
Balancing purpose
The point of balancing is to maximize battery capacity.
Balancing brings all the cells to the same SOC
at ONE point.
The SOC is balanced.Battery Power Conference 2010, 28,
D: balancing myths
Bottom balancing
Myth D4:"Bottom balancing protects the cells"
MIX
Battery Power Conference 2010, 29,
D: balancing myths
Bottom balancing
No BMS & limited charge:
bottom balancemay work
Battery Power Conference 2010, 30,
D: balancing myths
4 V
0 V
C e l l v o l t a g e
S O C
0 % 7 0 %
M a n u a l l y b a l a n c ea l l c e l l s a t 0 % S O C
S t o p c h a r g i n g a t 7 0 % S O Ct o s t a y a w a y f r o mo v e r c h a r g i n g
2 . 5 V
4 V
0 V
C e l lv o l t a g e
7 0 %
2 . 5 V
T i m eC h a r g e D i s c h a r g e C h a r g e
S O C
B a t tv o l t a g e
3 5
2 5
T u r n o f f c h a r g e r
T u r n o f f l o a d
0 %
Bottom balancing
Otherwise, only top balance
makes sense
Battery Power Conference 2010, 31,
D: balancing myths
4 V
0 V
C e l lv o l t a g e
D O D0 % 1 0 0 %
B M S b a l a n c e sa l l c e l l s a t V h i( 1 0 0 % S O C )
S t o p d i s c h a r g i n g w h e n v o l t a g e o f l o w e s t c a p a c i t a n c ec e l l r e a c h e s V l o w
2 . 5 V
4 V
0 V
C e l lv o l t a g e
1 0 0 %
2 . 5 V
T i m eC h a r g eD i s c h a r g e D i s c h a r g e
S O C
T u r n o f f c h a r g e r
T u r n o f f l o a d
0 %
Thank you
"Battery Management Systems for Large Lithium Ion Battery Packs"
Davide Andrea
book.LiIonBMS.com
elithion.com