NiMH batteries

[133MHz]

Hi everyone!

Recently, my AAA NiMHs won't hold a charge enough to play a single song on my MP3 player, even after charging overnight in normal mode. I think they are about two and a half years old. As far as I know, NiMH technology doesn't suffer from the same problems as NiCd, like the memory effect (it is recommended to discharge your NiCds periodically to maintain them in top shape). But NiMHs? Is there any 'reconditioning' techniques for them? I didn't tried to discharge them completely, because I don't know if that would make them more harm than good (like SLA batteries which die if discharged completely). Any ideas?

[manuel]

I don't know if that applies here... back then we put batteries on the heater for some minutes. The warming up made them "come back".

[133MHz]

I think that's for alkalines, and I remember doing that once or twice in my life (also forcing 12V from a car battery through a whole stack of dead alkaline batteries in series a couple of seconds seems to give them 'life' again). But rechargeables are much more delicate and tend to explode rather violently if you do that kind of stuff to them .

[manuel]

Oops. 
Better not try that...

[kezja]

lol, I haven't seen an NiMH explode yet from heat, but I have seen it from an alkaline.  Ages ago, one of my neighbor started a barrel fire and started chucking in alkaline batteries...  It was a weird experience and only half of them blew up.

Sooo uhhh...  Here's something from your good friend WikiPedia.

Care must also be taken during discharge to ensure that one or more cells in a series-connected battery pack, like the common arrangement of four AA cells in series in a digital camera, do not become completely discharged and go into polarity reversal. Cells are never absolutely identical, and inevitably one will be completely discharged before the others. When this happens, the "good" cells will start to "drive" the discharged cell in reverse, which can cause permanent damage to that cell. Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and shut themselves down, but devices like flashlights and some toys do not. Once noticeable dimming or slowing of the device is noticed, it should be turned off immediately to avoid polarity reversal. A single cell driving a load won't suffer from polarity reversal, because there are no other cells to reverse-charge it when it becomes discharged.

[133MHz]

So that means to discharge them you should do it individually (discharge each battery separately). Found some info on batteryuniversity.com:

Nickel-metal-hydride (NiMH)

Charging:
Do run the battery fully down once every 3 months. Over-cycling is not advised.

Do not leave battery in charger for more than 2 days because of memory.

Avoid getting battery too hot during charge.

Charge methods:
Constant current, followed by trickle charge when full. Slow charge not recommended.
Battery will get warm towards full charge.
Rapid charge = 3h
Fast charge = 1h+

Discharging:
Avoid too many full cycles because of wear. Use 80% depth-of-discharge.
NiMH has higher energy density than NiCd at the expense of shorter cycle life.

Service needs:
Discharge to 1V/cell every 3 months to prevent memory.
Do not discharge before each charge

So... gotta try that