Li ion myth buster
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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.

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Li ion myth buster

Li-Ion Myth-Buster

Poking holes intosome common beliefs

about Li-Ion cells and Li-Ion BMSs.

Davide Andrea

Elithion - Electronics for Lithium Ion

Battery Power Conference 2010, 1,


Extra extra

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 extra1

EXTRA! EXTRA!

The same BMS with work equally well with both formats.

FALSE

Battery Power Conference 2010, 3,


Myths

Myths

A) Cells

B) Batteries

C) BMS

D) Balancing

Battery Power Conference 2010, 4,


A cell myths

A: Cell myths

A: cell myths

Group A:

Myths about Li-Ion cells

- Cell capacity myths

- Cell swelling myths

Battery Power Conference 2010, 5,


Charge efficiency

Charge efficiency

A: cell myths

Myth A1:

"A cell puts out less charge than you put into it"

FALSE

Battery Power Conference 2010, 6,


Charge efficiency1

Charge efficiency

A: cell myths

Li-Ion charge efficiency = 100 %:

Every electron that goes in

can come back out

(Though, energy efficiency is < 100 %)‏

Battery Power Conference 2010, 7,


Charge efficiency2

Charge efficiency

A: cell myths

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,


Capacity vs rate

Capacity vs rate

A: cell myths

Myth A2:

"Cell capacity depends on rate of discharge"

FALSE

Battery Power Conference 2010, 9,


Capacity vs rate1

Capacity vs rate

A: cell myths

Yes, at high discharge a fixed cutout will stop discharge sooner

Battery Power Conference 2010, 10,


Capacity vs rate2

Capacity vs rate

A: cell myths

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

Battery Power Conference 2010, 11,


Capacity vs rate3

Capacity vs rate

A: cell myths

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

Battery Power Conference 2010, 12,


Capacity vs cycles

Capacity vs cycles

A: cell myths

Myth A3:

"Cell capacity decreases with number of cycles"

MIX

Battery Power Conference 2010, 13,


Capacity vs cycles1

Capacity vs cycles

A: cell myths

Degradation =

capacity loss

Higher resistance

+ fixed cutoffs =

unused capacity

Battery Power Conference 2010, 14,


C bms myths

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, 15,


No bms i use a cutoff

No BMS: I use a cutoff

C: BMS myths

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,


No bms i use a cutoff1

No BMS: I use a cutoff

C: BMS myths

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

2.9 V

3.8 V

3.3 V

4.5 V

1.8 V

3.3 V

2.9 V

3.8 V

3.3 V

2.9 V

3.8 V

Battery Power Conference 2010, 17,


No bms i use a cutoff2

No BMS: I use a cutoff

C: BMS myths

Top balancing won't help

Fully charged,

Top balanced

Discharged.

One has low capacity

Discharged.

All same capacity

14.4 Vtot

11.2 Vtot = LV cutoff

11.2 Vtot = LV cutoff

3.6 V

3.2 V

2.8 V

3.6 V

1.6 V

2.8 V

3.6 V

3.2 V

2.8 V

3.6 V

3.2 V

2.8 V

Battery Power Conference 2010, 18,


No bms i use a cutoff3

No BMS: I use a cutoff

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.

Battery Power Conference 2010, 19,


No bms i use a cutoff4

No BMS: I use a cutoff

C: BMS myths

Bottom balancing won't help

Fully discharged,

Bottom balanced

Charged.

1 has low capacity

Charged.

All same capacity

12.0 Vtot

14.4 Vtot = Charger CV

14.4 Vtot = Charger CV

3.0 V

3.3 V

3.6 V

3.0 V

4.5 V

3.6 V

3.0 V

3.3 V

3.6 V

3.0 V

3.3 V

3.6 V

Battery Power Conference 2010, 20,


No bms i use a cutoff5

No BMS: I use a cutoff

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!

Battery Power Conference 2010, 21,


No bms i mind the soc

No BMS: I mind the SOC

C: BMS myths

Myth #C3:

"I'll won't usethe entire SOC range,so I don't need a BMS"

FALSE

Battery Power Conference 2010, 22,


Nice and easy no bms

Nice and easy: no BMS

C: BMS myths

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,


More cells no bms

More cells: no BMS

C: BMS myths

Myth #C4:

"Extra cellsare cheaperthan a BMS"

FALSE

Battery Power Conference 2010, 24,


More cells no bms1

More cells: no BMS

C: BMS myths

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,


D balancing myths

D: Balancing myths

D: balancing myths

Group D:

Myths about cell balancing

- Balance purpose myths

- Balance point myths

Battery Power Conference 2010, 26,


Balancing purpose

Balancing purpose

D: balancing myths

D1: "Balancing protects a battery"

D2: "Balancing compensates for variations in cell capacity"

FALSE

FALSE

Battery Power Conference 2010, 27,


Balancing purpose1

Balancing purpose

D: balancing myths

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,


Bottom balancing

Bottom balancing

D: balancing myths

Myth D4:

"Bottom balancing protects the cells"

MIX

Battery Power Conference 2010, 29,


Bottom balancing1

Bottom balancing

D: balancing myths

No BMS & limited charge:

bottom balancemay work

Battery Power Conference 2010, 30,


Bottom balancing2

Bottom balancing

D: balancing myths

Otherwise, only top balancemakes sense

Battery Power Conference 2010, 31,


Thank you

Thank you

"Battery Management Systems for Large Lithium Ion Battery Packs"

Davide Andrea

book.LiIonBMS.com

elithion.com


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