li-ion myth-buster - battery management systems (bms)...

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."


EXTRA! EXTRA!

The same BMS with work equally well with both formats.


FALSESmall cells

BMS cell boards

Large cells

BMS cell boards

Myths

A) CellsB) BatteriesC) BMSD) Balancing


A: Cell myths

Group A:Myths about Li-Ion cells

- Cell capacity myths- Cell swelling myths


A: cell myths

Charge efficiency

Myth A1:"A cell puts out less charge

than you put into it"

FALSE


A: cell myths

Charge efficiency

Li-Ion charge efficiency = 100 %:

Every electron that goes incan come back out

(Though, energy efficiency is < 100 %)


A: cell myths

Charge efficiency1 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 %


A: cell myths

100 %

0 %

SOC

Time

1 A

0 A

Current

Time

-1 A

4 V

0 V

Voltage

Time

4 W

0 V

Power

Time

3.5 Wh

0 Wh

Energy

Time

1 hr 2 hr

1 hr

1 hr 2 hr

1 hr 2 hr

2 hr

1 hr

1 Ah cell

-4 W

CHARGEDISCHARGE

Open Circuit

Terminal

Terminal

Open Circuit

Ideal

Ideal

Actual

Actual

Capacity vs rate

Myth A2:"Cell capacity depends on

rate of discharge"

FALSE


A: cell myths

Capacity vs rate


A: cell myths

Yes, at high discharge a fixed cutout will stop discharge sooner4 V

0 V

Cell voltage

0 %

3 V

2 V

1 V

10 C

DOD

0.1 C

22 % 65 % 90 % 100 %40 %

2.0 V cutoff

Capacity vs rate


A: cell myths

But the rest is still there; you can still get it out, with a constant voltage load.

4 V

0 V

Cell voltage

0 %

3 V

2 V

1 V

10 C

DOD

0.1 C

22 % 65 % 90 % 100 %40 %

02V constant voltage load

Capacity vs rate


A: cell myths

But the rest is still there; you can still get it out, with a lower cutout4 V

0 V

Cell voltage

0 %

3 V

2 V

1 V

10 C

DOD

0.1 C

22 % 65 % 90 % 100 %40 %

0 V cutoff

Capacity vs cycles

Myth A3:"Cell capacity decreases

with number of cycles"

MIX


A: cell myths

Capacity vs cycles


A: cell myths

Degradation = capacity loss

Higher resistance+ fixed cutoffs = unused capacity

Cell capacity100 Ah

Cycles

4 V

0 V

Cell voltage

SOC

100 %

4.2 V cutoff

85 %

OCV

Term vtg,new cell(low R)

Term vtg,new cell(high R)

C: BMS myths

Group C:"I don't need a BMS" myths

- Cutoff myth- Small SOC range myth- Extra cells myth


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


C: BMS myths


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


C: BMS myths


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


C: BMS myths


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.


C: BMS myths


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


C: BMS myths


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!


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


C: BMS myths

Nice and easy: no BMS

The assumptions are that:1) The pack SOC is known2) All the cells are at that same SOCNot so. Without a BMS, the pack SOC is not known (and even a BMS doesn't always know the SOC)


C: BMS myths

More cells: no BMS

Myth #C4:"Extra cellsare cheaperthan a BMS"

FALSE


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.


C: BMS myths

D: Balancing myths

Group D:Myths about cell balancing

- Balance purpose myths- Balance point myths


D: balancing myths

Balancing purpose

D1: "Balancing protects a battery"D2: "Balancing compensates for

variations in cell capacity"

FALSE FALSE


D: balancing myths

Balancing purposeThe 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


D: balancing myths

Bottom balancing

No BMS & limited charge:

bottom balancemay work


D: balancing myths

٤ V

٠ V

Cell voltage

SOC

٠ % ٧٠ %

Manually balanceall cells at ٠ % SOC

Stop charging at ٧٠ % SOCto stay away fromovercharging

٢.٥ V

٤ V

٠ V

Cellvoltage

٧٠ %

٢.٥ V

TimeCharge Discharge Charge

SOC

Battvoltage

٣٥

٢٥

Turn off charger

Turn off load

٠ %

Bottom balancing

Otherwise, only top balance

makes sense


D: balancing myths

٤ V

٠ V

Cellvoltage

DOD٠ % ١٠٠ %

BMS balancesall cells at Vhi(١٠٠ % SOC)

Stop discharging when voltage of lowest capacitancecell reaches Vlow

٢.٥ V

٤ V

٠ V

Cellvoltage

١٠٠ %

٢.٥ V

TimeChargeDischarge Discharge

SOC

Turn off charger

Turn off load

٠ %

Thank you

"Battery Management Systems for Large Lithium Ion Battery Packs"

Davide Andrea

book.LiIonBMS.com

elithion.com

li-ion myth-buster - battery management systems (bms)...

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