Why is my NiMH AAA so hot?

June 30, 2005 11:01 AM

I have a question about strange rechargeable battery behavior.

I use rechargeable NiMH AAA batteries for my mp3 player, it only takes one, so I usually keep two extra, charged in a pocket I've sewn in my purse, so they don't rattle around. I was in the movies the other night with my purse in my lap and it suddenly felt like something was burning me, and one of the batteries that I wasnt using had gotten really hot, so I put it on the floor under my seat and picked it up after the movie and it was cool again.

I'm assuming it's not reuseable, but what happened? How can I keep future batteries from doing this, and is it safe to use, cause those things aren't cheap. I'm pretty sure it didnt get banged around any, and there's no visible damage to the battery.

posted by nile_red to technology (5 comments total)

You shorted it. If you read the fine print, batteries tend to come with a warning not to throw them loose into pockets, purses, etc. Keep it in a non-conductive case of some kind and you'll be fine.

My guess is that you could still recharge and use that battery (but I'll be checking back here to see if some smarter MeFite has a good reason not to.)
posted by Zed_Lopez at 11:13 AM on June 30, 2005

Yeah, it shorted against something metal in your purse. Keep inside something plastic/paper in future (My minidisc player and camera both came with little plastic cases for the rechargeable batteries).

I'd say there aren't really any dangers in using it again. It might have lower capacity, but unless there's any physical damage visible, it's fine.
posted by cillit bang at 12:02 PM on June 30, 2005

Batteries will sometimes short internally (either a manufacturing defect or rough handling) in which case you probably won't be able to charge it. This is what causes those laptop fires that pop up every year or so.
posted by Mitheral at 2:17 PM on June 30, 2005

Strikes or blows are really bad for most recharchables, but especially AA and AAAs.

The thin plastic shrinkwrap around the battery, sometimes along with a small plastic disc around the + terminal-button, and the fine gap between the positive and negative side, is the only thing that's really protecting it from shorting out.

The positive side can become dented, especially around the edges of the cylinder, and make contact with the negative side, which in some - or a lot - of batteries is the whole of the metal case underneath the shrink wrap. (basically, all of the metal that is not the positive end.)

And battery explosions can be pretty nasty. You can get chemical and heat burns, even if it doesn't explode or catch fire, and just wants to get really hot and leak everywhere.

A good source for battery cases is those multipacks of disposable batteries that actually come in a little plastic recloseable clamshell. (Example, Rayovac alkalines from WalMart.) I save those for keeping rechargables and alkalines in, and have one for AAs and one for AAAs. I flip discharged batteries upside down to keep track of which is which.
posted by loquacious at 3:55 PM on June 30, 2005

Once I had put a button battery in my pocket, with change. LOL! Hot pants! Pity no one was filming.
posted by Goofyy at 12:24 AM on July 1, 2005