important facts about batteries

i was doing some research on rechargeable batteries a few days ago, and i uncovered a few nuggets of info (gleaned from many online sources, but unfortunately i didn't save the links at the time)

first of all, knowing which battery type you're using is important. most devices use NiMH batteries these days, but NiCd is still used in cordless phones, toys etc that don't need a lot of power. all cameras, laptops, cellphones, mp3 players etc. use NiMH batteries. batteries are usually the first (and most expensive) part of a laptop to fail. read on and learn:

• NiMH batteries cannot be charged in a NiCd charger. they will simply explode. NiCd batteries can be charged in a NiMH charger though. Look carefully for NiMH written on a charger before trying to charge NiMH batteries.
  
• NiCd and NiMH batteries can be overcharged by keeping them on the charger for too much time after they've finished charging. which means, using your laptop while keeping in plugged into the charger even when the battery is full (a very common practice) causes it to heat and reduce its life.
• you can tell a battery is fully charged by it's temperature. when it's charging, it will be slightly warm, but when it's fully charged the temperature increases significantly. if the charger automatically switches off when the battery is full (look for an indicator light), the temperature will subsequently drop, but if not, temperature is the best gauge of when to switch off the charger.
• do not drain a NiMH or NiCd battery below 10%. if you do, the battery goes dead and will refuse to be subsequently charged. you'll have to just throw it away. most electronic devices are intelligent enough to detect when the battery is down to 10% - they call that 10% as "battery empty" and switch off. analog devices like bulbs, motors, speakers, toys etc don't, so you need to switch them off the moment their brightness/speed/volume *starts* dropping (rechargeable batteries keep a constant voltage for the top 90% of their charge, and drop dramatically at the last 10%).
• NiCd batteries *do* have the memory effect i.e. if they're repeatedly charged before they're drained to 10%, they lose that capacity, and take many rounds of full draining/charging before they can regain it. NiMH batteries don't have the memory effect, so they can be charged at any time, except when they're already full.