Cell mismatch at high current
I discovered something that should have been
obvious, but I never saw it written anywhere. When drawing high current from a Lipo, like 25C from a 30C battery, the mismatch between cells is magnified, and can be detrimental as the battery is depleted. After a couple of short flights the battery's three cells showed 3.80 to 3.83 volts, . still enough for another short flight. The next flight I got a bit wild with the throttle. Once I landed two cells showed 3.70 volts and one cell showed 3.48 volts. Still above minimum voltage OK? To see what the issue was, I ran the motor up to full throttle with the voltage tester connected. As the speed increased the one cell voltage dropped off faster than the others, and went down to 2.56 volts. So it appears that getting wild with the throttle early in the flight might not be so bad, since there is plenty of voltage, but doing it later in the flight, even when there seems to be plenty of volts, is a bit more risky. I'm not sure if this battery is toast yet. It might go into the Cessna instead of the Corsair from now on. Jenny |
Follow up:
Yep the battery definitely lost some life. The one cell is down in capacity between 10% and 20% Its internal resistance is up and C rating is down. It goes in the Cessna now. The battery is not puffed in any way, so I'm not yet afraid to use it. I'm guessing its on its way to soon becoming a 2S pack. Jenny |
Your tests simply show that the internal impedance of the cells is not matched. One cell definitely has a higher internal resistance.
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My test shows that one cell accepts 10-20% less
mAh than the other two cells when charging before reaching 4.2 volts and tapering off to near zero current. It also give that much less back before reaching cut off voltage, and that the voltage drops off quicker. For a slow charge (less than .5C) and slow discharge (5-10C gentle flying) the energy stored is 10% less than the other cells. For a standard 1C charge and regular discharge (aerobatic flying, about 10-20C) it is down about 20%. Jenny |
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