strange NiMH battery behavior

[adict2jane]

Hi all!
Love these forums, great source of info from lots of very knowledgeable people!

I have a few second-rate NiMH AA/AAA batteries that exhibit some strange behavior. I was wondering if anyone had an idea what the problem with the cells are.

My charger is a La Crossee BC700

Some of these damaged cells will charge fine but then have a very difficult time retaining the charge for more than a week. After which the cells will have only about 50% of their charge remaining.
Other cells will have a very difficult time charging. For instance, I will place them in the BC700 and it will charge them to over 3K mAh. However, their actual capacity is quite low.

What is the problem with NiMH cells that exhibit these two phenomenons? Any ideas welcome, simply curious

 

[espresso]

I noticed the same thing with IPC-1L (it's the same charger). I think it gets confused after some time and it stops charging but mAh counter keeps on going. Notice the batteries are cool.
It could be a firmware bug where they didn't handle scenario with bad batteries correctly. C9000 won't even touch those batteries (impendance too high).
And while those batteries can be used in some extremely low consumption devices, I always ended up throwing them away. They're just unusable for me.

 

[WalkIntoTheLight]

Yes, crap cells will confuse some chargers. Those batteries are dying. They might still be okay for some low-discharge uses, although if the self-discharge fast, they're not even good for that. Throw them away, and get some new decent cells.

 

[Power Me Up]

Some of these damaged cells will charge fine but then have a very difficult time retaining the charge for more than a week. After which the cells will have only about 50% of their charge remaining.
Other cells will have a very difficult time charging. For instance, I will place them in the BC700 and it will charge them to over 3K mAh. However, their actual capacity is quite low.

What is the problem with NiMH cells that exhibit these two phenomenons? Any ideas welcome, simply curious

I think that the most common cause of cells that self discharge quickly is when the separator between the anode and cathode is damaged, tiny whiskers of material can grow through the holes and cause current to flow internally. Unfortunately, there isn't anything that can be done once this happens other than to dispose of them.

Old cells often exhibit a poor -dV signal at the end of charge (-dV is when the cell voltage drops slightly after the cell is full) The voltage drop is associated with the cell warming up as it starts to overcharge. When a cell has high internal resistance (like a lot of old cells end up having) they tend to heat up a lot during the main part of the charge and the temperature rise at the end when they're starting to overcharge becomes insignificant which I think is why they don't show the normal -dV signal. Without a -dV signal, most chargers will just keep charging them until they reach either a capacity or time limit.