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Overdischarging a Cell

By 22/01/2013 June 4th, 2020 Battery Expert Q&A

The Question:

I have had similar experiences with two packs. Both were healthy packs (TX & RX) that were cycling to their factory rated capacity….until I left the switch on overnight. They wouldn’t even take a charge after that!

Red Scholefield replies:

Let’s have a little battery talk. Bill’s comment “wouldn’t take a charge” is used commonly to describe a battery problem. Let’s examine what it really says. It says essentially, “after I pass current through the battery for a period of time the battery could not deliver current at a usable voltage level”. In order for the battery “not to take a charge” it would have to be open circuit (broken lead or so dried out that there is a very high resistance) so that no current is flowing. Neither of which could have been caused by leaving the switch on for a day or so.

In the case mentioned where the packs were left on a load for some time the battery terminal voltage approached zero volts. This would mean that one or more cells were probably reversed since the pack voltage = 0 and not every cell has identical capacity, some would read 1.1 volts and others might read -.5 volts to get to the zero volts seen. Reversed cells have a rather poor charge acceptance on the next recharge and require a longer time to reach a full state of charge. Since cells cannot store any significant energy when reversed they are at essentially zero state of charge, a point where if a short is going to manifest itself it does.

With these suspect packs, you will find, after they have been allowed to stand for a few days, that most of the cells will have a voltage greater than 0.9 volts. One will be at a very low voltage, or zero. If the low cell doesn’t recover and hold a voltage after passing current (normal charge rate C/10)through it in the charge direction this cell is shorted.

Driving a pack to zero volts by leaving it on a load isn’t a good practice. But these things happen. I left one on for six months once and had to replace the whole wiring harness as the cells leaked. A day or two on the other hand will not significantly damage a pack in most cases. A good 48 hour slow charge will usually bring them back.
OK, class dismissed. Lets go flying.