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Cell mismatch at high current

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Old 07-20-2015, 05:38 AM
  #1  
Jennifer Curtis
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Default 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
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Old 07-21-2015, 04:32 AM
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Jennifer Curtis
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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
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Old 07-21-2015, 04:34 AM
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Rodney
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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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Old 07-21-2015, 04:46 AM
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Jennifer Curtis
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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

Last edited by Jennifer Curtis; 07-21-2015 at 04:57 AM.
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