Only had to charge eneloop AAA backward once to ruin it (unintential)

I charged an eneloop backward at a low current for a few hours. That brought capacity down to 650mAh from 810mAh and I can't recover it after many cycles.

How did that happen? It was used in a 3AAA application and it got left on. They were not at exactly the same state of charge so the least charged cell depleted first and went into polarity reversal meaning that the remaining two cells were now charging the dead cell backward.

Yeah, over discharging, and especially reverse charging NiMH cells, causes some degree of permanent damage to the cell. NiCds tolerate over discharging quite well, but reverse charging is almost as bad for them, as well. At any rate, that's something you really want to try and avoid doing with either chemistry. :sigh:

Dave

What charger were you using? My 4 bay AA/AAA charger will not charge if a cell is reversed.

Bill

Bill, from what I understand, the reverse charge was applied while the cell was in use, not while it was in a charger. Sh... er, stuff happens! ;)

Dave

Yes, you are probably right. Just wasn't really clear in OP's post. Wonder what light he was using? Three cells, no doubt, with a low mode?

Bill

Bullzeyebill said:

Yes, you are probably right. Just wasn't really clear in OP's post. Wonder what light he was using? Three cells, no doubt, with a low mode?

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Bright+ said:

How did that happen? It was used in a 3AAA application and it got left on. They were not at exactly the same state of charge so the least charged cell depleted first and went into polarity reversal meaning that the remaining two cells were now charging the dead cell backward.

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I'm not sure how it could have been clearer?

Mr Happy said:

I'm not sure how it could have been clearer?

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I apologize. I should have picked that up in first post. :sigh:

Bright+ said:

How did that happen? It was used in a 3AAA application and it got left on. They were not at exactly the same state of charge so the least charged cell depleted first and went into polarity reversal meaning that the remaining two cells were now charging the dead cell backward.

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NiCd's didn't like being reversed either. In fact no cells tolerate that perhaps except lead acid under very strict conditions.

Under discharging tends not to be a very big deal - that is discharging the cell very deeply. Even as low as near 0 f.ex. in a single cell incan light - as long as it's not reversed and not stored for days or weeks afterward.
However it can be a problem getting some modern -dV chargers to recognize cells and/or get them to charge them. Just give the cells an hour in a dumb slow and/or timer based charger and switch them to a -dV one.

I don't know what application you've used to destroy the cell. But let me give you and others in same situation an idea.
I have a similar application myself with 3 NiMH cells. It's regularly run until flat so not to destroy the cells I've got the idea to use a LiIon protection circuit. It is so luckily that it also happens to cut off when 3 NiMH cells are empty but before the weakest one is pushed in reverse - even if you put in a fully depleted with two freshly charged. Those cells are 2 years old and probably has got something like 200 cycles on them - they're still like new thanks to the protection circuit.
You can even use the idea with more cells. However it has to be in multiple of 3. as you implement a circuit for each 3 cells.

You can get loose protection circuits from DX or take them out of old LiIon like cell phone and camera batteries (single cells are easier to work with). Just take care not to short the old LiIon cells as we don't want anything nasty to happen. The cell phone ones are however not very high current capable but fine for many things. Before throwing out the LiIon cell I took the circuit from I discharged the cell completely to practically 0 V with a 10 ohm resistor so it won't go :poof: in some battery container (remember it's safe to discharge LiIon to 0 V - it is however not safe to charge them again from that state).

Yep... I've done this too more times than I care to admit.

McAllan said:

It's regularly run until flat so not to destroy the cells I've got the idea to use a LiIon protection circuit. It is so luckily that it also happens to cut off when 3 NiMH cells are empty but before the weakest one is pushed in reverse - even if you put in a fully depleted with two freshly charged.

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Soulnd pretty interesting. What do I look for ? I don't have any LiON's just NiMH. I have a 3 AAA cell light too. A nickle shaped circuit would be ideal. Anything in particular to google for ?

They're perhaps most commonly known as "PCB" for protective/protection circuit board. But googling for the acronym will just get you loads of hits for the more common use printed circuit boards. Try "Li-ion PCB" or the full "protective circuit board", maybe.


A couple examples would be DX's sku.26114 or batteryspace's PCB-S1A3B.