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

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1 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,

2 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,

3 EXTRA! EXTRA! The same BMS with work equally well with both formats. BMS cell boards BMS cell boards Small cells FALSE Large cells Battery Power Conference 2010, 3,

4 Myths A) Cells B) Batteries C) BMS D) Balancing Battery Power Conference 2010, 4,

5 A: cell myths A: Cell myths Group A: Myths about Li-Ion cells - Cell capacity myths - Cell swelling myths Battery Power Conference 2010, 5,

6 A: cell myths Charge efficiency Myth A1: "A cell puts out less charge than you put into it" FALSE Battery Power Conference 2010, 6,

7 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, 7,

8 A: cell myths Charge efficiency Current 1 A 0 A -1 A DISCHARGE 1 hr CHARGE Time 2 hr 1 A in for 1 hour, 1 A out for 1 hour SOC 100 % 0 % 1 Ah cell Time 1 hr 2 hr SOC: 100 %, to 0 %, back to 100 %: charge efficiency is 100 % Voltage 4 V 0 V Open Circuit Terminal Terminal Open Circuit Time 1 hr 2 hr Charge voltage is higher than OCV, lower during discharge Power 4 W Ideal 0 V -4 W Actual Time 1 hr 2 hr Ideal Charge power is more than discharge power Energy Actual 3.5 Wh 0 Wh 1 hr Time Energy doesn't go back down to 0: energy efficiency is < 100 % Battery Power Conference 2010, 8,

9 A: cell myths Capacity vs rate Myth A2: "Cell capacity depends on rate of discharge" FALSE Battery Power Conference 2010, 9,

10 A: cell myths Capacity vs rate 4 V 3 V 10 C 2 V Cell voltage 0.1 C 2.0 V cutoff Yes, at high discharge a fixed cutout will stop discharge sooner 1 V 0 V 0 % DOD 22 % 40 % 65 % 90 % 100 % Battery Power Conference 2010, 10,

11 A: cell myths Capacity vs rate 4 V 3 V Cell voltage 0.1 C But the rest is still there; you can still get it out, with a constant voltage load. 2 V 10 C 02V constant voltage load 1 V 0 V 0 % DOD 22 % 40 % 65 % 90 % 100 % Battery Power Conference 2010, 11,

12 A: cell myths Capacity vs rate 4 V 3 V 10 C 2 V Cell voltage 0.1 C But the rest is still there; you can still get it out, with a lower cutout 1 V 0 V 0 % 0 V cutoff DOD 22 % 40 % 65 % 90 % 100 % Battery Power Conference 2010, 12,

13 A: cell myths Capacity vs cycles Myth A3: "Cell capacity decreases with number of cycles" MIX Battery Power Conference 2010, 13,

14 A: cell myths Capacity vs cycles 100 Ah Cell capacity Degradation = capacity loss Cycles Cell voltage 4 V 4.2 V cutoff OCV Term vtg, new cell (high R) Term vtg, new cell (low R) Higher resistance + fixed cutoffs = 0 V SOC unused capacity 85 % 100 % Battery Power Conference 2010, 14,

15 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,

16 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, 16,

17 C: BMS myths No BMS: I use a cutoff There's no way of knowing the state of individual cells from the pack voltage Vtot 3.3 V 13.2 Vtot 3.8 V 13.2 Vtot 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, 17,

18 C: BMS myths No BMS: I use a cutoff Top balancing won't help Fully charged, Top balanced 14.4 Vtot 3.6 V Discharged. All same capacity 11.2 Vtot = LV cutoff 2.8 V Discharged. One has low capacity 11.2 Vtot = LV cutoff 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, 18,

19 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, 19,

20 C: BMS myths No BMS: I use a cutoff Bottom balancing won't help Fully discharged, Bottom balanced 12.0 Vtot 3.0 V Charged. All same capacity 14.4 Vtot = Charger CV 3.6 V Charged. 1 has low capacity 14.4 Vtot = Charger CV 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, 20,

21 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, 21,

22 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, 22,

23 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, 23,

24 C: BMS myths More cells: no BMS Myth #C4: "Extra cells are cheaper than a BMS" FALSE Battery Power Conference 2010, 24,

25 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, 25,

26 D: balancing myths D: Balancing myths Group D: Myths about cell balancing - Balance purpose myths - Balance point myths Battery Power Conference 2010, 26,

27 D: balancing myths Balancing purpose D1: "Balancing protects a battery" D2: "Balancing compensates for variations in cell capacity" FALSE FALSE Battery Power Conference 2010, 27,

28 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, 28,

29 D: balancing myths Bottom balancing Myth D4: "Bottom balancing protects the cells" MIX Battery Power Conference 2010, 29,

30 D: balancing myths Bottom balancing ٤ V ٢.٥ V ٠ V Cell voltage Manually balance all cells at ٠ % SOC ٠ % ٧٠ % Stop charging at ٧٠ % SOC to stay away from overcharging SOC No BMS & limited charge: bottom balance may work Batt voltage Cell voltage ٣٥ ٢٥ ٤ V Turn off load ٢.٥ V Turn off charger SOC ٧٠ % ٠ V Charge Discharge Charge ٠ % Time Battery Power Conference 2010, 30,

31 D: balancing myths Bottom balancing Cell voltage ٤ V BMS balances all cells at Vhi (١٠٠ % SOC) Otherwise, only top balance ٢.٥ V ٠ V DOD ٠ % ١٠٠ % Stop discharging when voltage of lowest capacitance cell reaches Vlow makes sense Cell voltage ٤ V Turn off charger ٢.٥ V Turn off load SOC ١٠٠ % ٠ V Discharge Charge Discharge ٠ % Time Battery Power Conference 2010, 31,

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