NiMH accepting more charge than it should

[Fallingwater]

A few months ago I got a pack of eight off-brand NiMH AAs from a random seller on eBay. I didn't need them, but they were so very, very cheap I got them anyway. Yeah, I'm a batteryholic.

I charged them a few times with a normal AA charger I had and used them in random gadgets.

Today I took a discharged one from a gadget that hadn't been used in a while and charged it on my new RC charger because I wanted to know how much energy it held.
They are supposedly 2600mah cells, but for the price I paid for them I thought they had the usual lie factor of cheap cells and could hold maybe 2000 or 2200 mah.

Surprise surprise, the charger managed to put 2800mah in it, and was still charging at full blast (800ma, manually set). I wussed out at that point and shut it off manually.
The cell was warm but not hot, and everything seems ok.

My question is: is there any kind of failure mode that would make a NiMH accept more charge than it should? Because I have trouble believing these $0.80 each cells can hold as much charge as the most expensive Sanyo AAs.

 

[SilverFox]

Hello Fallingwater,

First of all, keep in mind that the discharge capacity is what counts. The amount you put in is interesting, but what comes out is what really matters.

You are charging at too low a charge rate. Your charger probably missed the end of termination signal and you ended up overcharging your cell. Most hobby chargers are set up to respond to a 1C charge rate.

Try charging at 2.5 amps and see if there is a difference in the amount that goes in.

Tom

 

[Fallingwater]

Huh. That's weird, it detected end-of-charge correctly in various AAA and AA NiMHs I charged with an even lower charge rate.

Besides, it's an expensive charger that's supposed to be unable to make the sort of mistakes cheapo chargers do, such as missing charge termination.

Will try what you suggest, though.

 

[SilverFox]

Hello Fallingwater,

It is not impossible to pick up the end of charge termination signal at lower rates, it's just not reliable. Sometimes it will detect it and sometimes you end up in an overcharging situation.

I have a Schulze charger, and I can set it up to miss (or at least delay) terminations. I have to work at it, but it always involves low charging rates.

Tom