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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:

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  • 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
Myths

  • A) Cells

  • B) Batteries

  • C) BMS

  • D) Balancing

Battery Power Conference 2010, 2,


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, 3,


Charge efficiency

A: cell myths

Charge efficiency

  • Myth A1:

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

FALSE

Battery Power Conference 2010, 4,


Charge efficiency1

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,


Charge efficiency2

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, 6,


Capacity vs rate

A: cell myths

Capacity vs rate

  • Myth A2:

  • "Cell capacity depends on rate of discharge"

FALSE

Battery Power Conference 2010, 7,


Capacity vs rate1

A: cell myths

Capacity vs rate

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

Battery Power Conference 2010, 8,


Capacity vs rate2

A: cell myths

Capacity vs rate

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

Battery Power Conference 2010, 9,


Capacity vs rate3

A: cell myths

Capacity vs rate

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

Battery Power Conference 2010, 10,


Capacity vs cycles

A: cell myths

Capacity vs cycles

  • Myth A3:

  • "Cell capacity decreases with number of cycles"

MIX

Battery Power Conference 2010, 11,


Capacity vs cycles1

A: cell myths

Capacity vs cycles

Degradation =

capacity loss

Higher resistance

+ fixed cutoffs =

unused capacity

Battery Power Conference 2010, 12,


Cell swelling

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,


Cell swelling1

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.

Battery Power Conference 2010, 14,


Cell swelling2

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

B: Battery myths

  • Group B:

  • Myths about batteries

  • - Reliability myths

Battery Power Conference 2010, 16,


Paral strings are better

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

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,


No bms i use a cutoff

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, 19,


No bms i use a cutoff1

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

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, 20,


No bms i use a cutoff2

C: BMS myths

No BMS: I use a cutoff

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, 21,


No bms i use a cutoff3

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, 22,


No bms i use a cutoff4

C: BMS myths

No BMS: I use a cutoff

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, 23,


No bms i use a cutoff5

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,


No bms i mind the soc

C: BMS myths

No BMS: I mind the SOC

  • Myth #C3:

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

FALSE

Battery Power Conference 2010, 25,


Nice and easy no bms

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,


More cells no bms

C: BMS myths

More cells: no BMS

  • Myth #C4:

  • "Extra cellsare cheaperthan a BMS"

FALSE

Battery Power Conference 2010, 27,


More cells no bms1

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

D: Balancing myths

  • Group D:

  • Myths about cell balancing

  • - Balance purpose myths

  • - Balance point myths

Battery Power Conference 2010, 29,


Balancing purpose

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,


Balancing purpose1

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,


Bottom balancing

D: balancing myths

Bottom balancing

  • Myth D4:

  • "Bottom balancing protects the cells"

MIX

Battery Power Conference 2010, 32,


Bottom balancing1

D: balancing myths

Bottom balancing

  • No BMS & limited charge:

  • bottom balancemay work

Battery Power Conference 2010, 33,


Bottom balancing2

D: balancing myths

Bottom balancing

  • Otherwise, only top balancemakes sense

Battery Power Conference 2010, 34,


"Battery Management Systems for Large Lithium Ion Battery Packs"

Davide Andrea

book.LiIonBMS.com

elithion.com

Thank you


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