I have a cordless drill which I take the battery out of every time I finish using it. I had been putting it on charge and just leaving it there until the next time I used it, but I got to thinking and wondered if that was the best idea. It made me wonder about all the other batteries I leave on charge, like me laptop, cell phone and batteries for my digital camera.
Is it ok to leave a battery on charge for hours/days at a time or is that a bad idea?
Generally, it’s OK. Battery chargers usually fall into two categories, trickle-charge and fast-charge. The current on a trickle-charge charger is low enough to not damage the battery, even if it’s fully charged. Fast-charge chargers have circuits to detect when the battery is fully charged, then they switch to trickle-charge. With NiCD batteries, it’s a good idea to occasionally use them until the low-battery indicator turns on, and then give them a full charge. This prevents some electrochemical problems in the battery. You don’t want to discharge the battery completely, this can damage some of the cells.
Different things happen to different types.
NiMHs and to some extent NiCads can exhibit “voltage depression” when overcharged. The device ends up thinking the battery is discharged when in fact it has a lot of oomph left. So you run it “down” (but definitely not out) a few times and then recharge it.
There is no benefit to keeping such a battery in a charger and can cause problems. Leave it out of the charger. If unused for a long period of time, put it in, charge it up, take it out. Less problems, better charges, longer lifetime, etc.
LiIons are even more dangerous than NiCads and can do Really Bad Things if overcharged to dangerous/explosive levels. The circuitry for LiIon chargers are much, much more complicated than for the others. So as long as you are using a working charger for the corresponding LiIon pack, you should be okay. This makes overcharging LiIons unlikely.
LiIons’ lifetimes are dictated mainly by plain old aging, but proper use helps some.