Rechargeable batteries - NiCad and NiMh. I have a ton of the AA and AAA type. I also have adapters that will let you use the AA in the place of a C or D cell.
At some point in time these stop taking a charge. Some people erroneously call this the memory effect. What is really happening is that there is a crystalline growth inside the cell that shorts out the internal workings of the cell. With a quick large burst of current you can blow away this crystalline structure.
For batteries that will no longer charge on your charger, you can revive these with a car battery charger. (Caveat: I've been doing this for close to 20 years never had a problem, but wear a face shield and leather gloves for safety if you are paranoid). OK, so what you do is put the car battery charger on 10A manual mode. Put the red (+) clip on the table / workbench. Now hold the AA battery with the positive (+) side against the red clip. Take the black (-) clip and tap it against the negative (-) side of the battery. DON'T HOLD IT THERE! You'll see the current meter on the charger jump to 8-10A and back to 0, and you'll get some sparks. Repeat this 10-20 times, just quick little taps. If you feel the battery getting warm, STOP!
This technique also works with portable tool batteries (drills, saws, etc) provided you can take the plastic battery case apart and get to the individual cells. My original 12V batteries for my Makita drill I got in 1995 *still* take a charge.
I've read on here and other places about the de-sulphating of car batteries, and I believe this is a similar process, but I've never tried one of those units on a dead AA battery.
Buyer beware, caveat emptor, safety nazis abound, do at your own risk, yada yada yada. Like I said, 20 years, never a problem, but be careful and don't over stress the battery. If it is bulging, don't try this trick! If it is leaking, toss it. Don't try this on alkaline batteries.
