There are currently three types of batteries commonly used for portable computers. The memory effect is caused by crystallization of the battery's substances and can permanently reduce your battery's lifetime, even make it useless. To avoid it, you should completely discharge the battery and then fully recharge it again at least once every few weeks. He says that this will break the whiskers that have formed in the battery and that are the cause of the memory effect if your battery is already affected by it. Cadmium is a very hazardous poison, but if returned to your dealer, the material can almost be fully recycled.
NiMh batteries are the current standard used in most low price laptops to date.
They can be made smaller and are less affected by the memory effect than NiCd. However, they have problems at very high or low room temperatures. The new high performance batteries use LiIon technology. In theory, there is no memory effect at all with these batteries, but on occasion, they seem to have similar problems. Their substances are non-hazardous to the enviroment, but they should be returned for recycling as well.