Li-Ion Myth-Buster
Poking holes into some common beliefs
about Li-Ion cells and Li-Ion BMSs.
Davide Andrea
Battery Power Conference 2010, 1,
Myths
A) Cells
B) Batteries
C) BMS
D) Balancing
Battery Power Conference 2010, 2,
A: cell myths
A: Cell myths
Group A:
Myths about Li-Ion cells
- Cell capacity myths
- Cell swelling myths
Battery Power Conference 2010, 3,
A: cell myths
Charge efficiency
Myth A1:
"A cell puts out less charge
than you put into it"
FALSE
Battery Power Conference 2010, 4,
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, 5,
A: cell myths
Charge efficiency
Current
1A
1 hr
0A
Time
2 hr
1 A in for 1 hour, 1 A out for 1 hour
-1 A
SOC
100 %
1 Ah cell
0%
1 hr
Time
2 hr
Voltage
4V
1 hr
Time
2 hr
1 hr
Time
2 hr
0V
SOC: 100 %, to 0 %, back to 100 %:
charge efficiency is 100 %
Charge voltage is higher than
OCV, lower during discharge
Power
4W
0V
Charge power is more than
discharge power
-4 W
Energy
4.5 Wh
1 Wh
0 Wh
1 hr
Time
2 hr
Energy doesn't go back down to 0:
energy efficiency is < 100 %
Battery Power Conference 2010, 6,
A: cell myths
Capacity vs rate
Myth A2:
"Cell capacity depends on
rate of discharge"
FALSE
Battery Power Conference 2010, 7,
A: cell myths
Capacity vs rate
Yes, at high discharge a
fixed cutout will stop
discharge sooner
Cell voltage
4V
0.1 C
3V
10 C
2V
2.0 V cutoff
1V
DOD
0V
0%
22 % 40 %
65 %
90 % 100 %
Battery Power Conference 2010, 8,
A: cell myths
Capacity vs rate
But the rest is still there;
you can still get it out,
with a constant voltage
load.
Cell voltage
4V
0.1 C
3V
10 C
2V
02V constant voltage load
1V
DOD
0V
0%
22 % 40 %
65 %
90 % 100 %
Battery Power Conference 2010, 9,
A: cell myths
Capacity vs rate
But the rest is still there;
you can still get it out,
with a lower cutout
Cell voltage
4V
0.1 C
3V
10 C
2V
1V
0 V cutoff
0V
0%
22 % 40 %
65 %
DOD
90 % 100 %
Battery Power Conference 2010, 10,
A: cell myths
Capacity vs cycles
Myth A3:
"Cell capacity decreases
with number of cycles"
MIX
Battery Power Conference 2010, 11,
A: cell myths
Capacity vs cycles
Cell capacity
Degradation =
100 Ah
capacity loss
Cycles
Cell voltage
Term vtg,
new cell
(high R)
4.2 V cutoff
4V
Term vtg,
new cell
(low R)
Higher resistance
+ fixed cutoffs =
OCV
unused capacity
0V
SOC
85 % 100 %
Battery Power Conference 2010, 12,
A: cell myths
Cell swelling
Myth #A5: "Cells swell due to charging"
Myth #A6: "Cells swell due to heat"
Myth #A7: "With a cell cooling system,
there's no need for cell retention"
FALSE
FALSE
FALSE
Battery Power Conference 2010, 13,
A: cell myths
Cell swelling
Internal pressure: high at ~ 100 % SOC,
whether sitting on a shelf or in use.
It's not the process of charging
It's not the temperature, not the I2R power
It's the state of being fully charged.
Pressure
SOC
100 %
Battery Power Conference 2010, 14,
A: cell myths
Cell swelling
MUST use retention plates on prismatic
or pouch cells.
Battery Power Conference 2010, 15,
B: Battery myths
B: Battery myths
Group B:
Myths about batteries
- Reliability myths
Battery Power Conference 2010, 16,
B: Battery myths
Paral. strings are better
Myth B1: "It's better to use multiple
strings, connected in parallel"
vs.
MIX
Battery Power Conference 2010, 17,
C: BMS myths
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, 18,
C: BMS myths
No BMS: I use a cutoff
C1: "I don't need a BMS
my charger has a high voltage limit"
C2: "I don't need a BMS
my load has a low voltage cut-off"
FALSE
FALSE
Battery Power Conference 2010, 19,
C: BMS myths
No BMS: I use a cutoff
There's no way of knowing the state of
individual cells from the pack voltage.
13.2 Vtot
13.2 Vtot
13.2 Vtot
3.3 V
3.8 V
2.9 V
3.3 V
1.8 V
4.5 V
3.3 V
3.8 V
2.9 V
3.3 V
3.8 V
2.9 V
Battery Power Conference 2010, 20,
C: BMS myths
No BMS: I use a cutoff
Top balancing won't help
Fully charged,
Top balanced
14.4 Vtot
Discharged.
All same capacity
Discharged.
One has low capacity
11.2 Vtot = LV cutoff
11.2 Vtot = LV cutoff
3.6 V
2.8 V
3.2 V
3.6 V
2.8 V
1.6 V
3.6 V
2.8 V
3.2 V
3.6 V
2.8 V
3.2 V
Battery Power Conference 2010, 21,
C: BMS myths
No BMS: I use a cutoff
Yes: a CCCV will protect a topbalanced 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, 22,
C: BMS myths
No BMS: I use a cutoff
Bottom balancing won't help
Fully discharged,
Bottom balanced
Charged.
All same capacity
Charged.
1 has low capacity
12.0 Vtot
14.4 Vtot = Charger CV
14.4 Vtot = Charger CV
3.0 V
3.6 V
3.3 V
3.0 V
3.6 V
4.5 V
3.0 V
3.6 V
3.3 V
3.0 V
3.6 V
3.3 V
Battery Power Conference 2010, 23,
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, 24,
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, 25,
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, 26,
C: BMS myths
More cells: no BMS
Myth #C4:
"Extra cells
are cheaper
than a BMS"
FALSE
Battery Power Conference 2010, 27,
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, 28,
D: balancing myths
D: Balancing myths
Group D:
Myths about cell balancing
- Balance purpose myths
- Balance point myths
Battery Power Conference 2010, 29,
D: balancing myths
Balancing purpose
D1: "Balancing protects a battery"
D2: "Balancing compensates for
variations in cell capacity"
D3: "Balancing allows use of the
entire charge in a battery"
FALSE
FALSE
FALSE
Battery Power Conference 2010, 30,
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, 31,
D: balancing myths
Bottom balancing
Myth D4:
"Bottom balancing
protects the cells"
MIX
Battery Power Conference 2010, 32,
D: balancing myths
Bottom balancing
Cell voltage
4V
Stop charging
at 70 % SOC
to stay away from
overcharging
2.5 V
Manually balance
all cells at 0 % SOC
SOC
0V
0%
70 %
No BMS &
limited charge:
bottom balance
may work
Batt 35
voltage 25
Turn off load
Cell 4 V
voltage
2.5 V
Turn off charger
SOC
70 %
0V
0%
Charge
Discharge
Charge
Time
Battery Power Conference 2010, 33,
D: balancing myths
Bottom balancing
Cell
voltage
4V
Otherwise,
only top balance
makes sense
BMS balances
all cells at Vhi
(100 % SOC)
2.5 V
Stop discharging
when voltage of
lowest capacitance
cell reaches Vlow
0V
DOD
0%
100 %
Turn off charger
Cell 4 V
voltage
2.5 V
Turn off load
SOC
100 %
0V
0%
Charge
Discharge
Charge
Time
Battery Power Conference 2010, 34,
Thank you
"Battery Management Systems for
Large Lithium Ion Battery Packs"
Davide Andrea
book.LiIonBMS.com
elithion.com