Battery Power Conference 2010, 1, Li-Ion Myth-Buster Poking holes into some common beliefs about Li-Ion cells and Li-Ion BMSs. Davide Andrea

Myths A) Cells B) Batteries C) BMS D) Balancing Battery Power Conference 2010, 2,

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 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, 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 Battery Power Conference 2010, 8, A: cell myths Yes, at high discharge a fixed cutout will stop discharge sooner

Capacity vs rate Battery Power Conference 2010, 9, A: cell myths But the rest is still there; you can still get it out, with a constant voltage load.

Capacity vs rate Battery Power Conference 2010, 10, A: cell myths But the rest is still there; you can still get it out, with a lower cutout

Capacity vs cycles Myth A3: "Cell capacity decreases with number of cycles" MIX Battery Power Conference 2010, 11, A: cell myths

Capacity vs cycles Battery Power Conference 2010, 12, A: cell myths Degradation = capacity loss Higher resistance + fixed cutoffs = unused capacity

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 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 I 2 R power It's the state of being fully charged. Battery Power Conference 2010, 14, A: cell myths

Cell swelling MUST use retention plates on prismatic or pouch cells. Battery Power Conference 2010, 15, A: cell 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" MIX vs. Battery Power Conference 2010, 17, B: Battery 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 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. 3.3 V 13.2 Vtot 2.9 V 4.5 V 2.9 V 13.2 Vtot 3.8 V 1.8 V 3.8 V 13.2 Vtot Battery Power Conference 2010, 20, C: BMS myths

No BMS: I use a cutoff Top balancing won't help 3.6 V 14.4 Vtot 3.2 V 1.6 V 3.2 V 11.2 Vtot = LV cutoff 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, 21, 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, C: BMS myths

No BMS: I use a cutoff Bottom balancing won't help 3.0 V 12.0 Vtot 3.3 V 4.5 V 3.3 V 14.4 Vtot = Charger CV 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, 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, C: BMS 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 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 No BMS & limited charge: bottom balance may work Battery Power Conference 2010, 33, D: balancing myths

Bottom balancing Otherwise, only top balance makes sense Battery Power Conference 2010, 34, D: balancing myths

Thank you "Battery Management Systems for Large Lithium Ion Battery Packs" Davide Andrea book.LiIonBMS.com elithion.com