Here's a really simple analogy for the memory effect, although it doesn't match up exactly. Don't try to read too much into it. Also I made this up myself (the sugar part), so it could be totally wrong.
Imagine you have a full container of sugar. Over time (a long time) the sugar tends to clump into a solid mass. If you take a full bottle of fresh sugar, you can spoon out the sugar without a problem. Say you take the container down to half full, and then fill the container up again, then start using it again. And then you keep doing that (stopping at about the same level in the sugar bottle). Also assume you don't shake the bottle, because that screws up the analogy. After a long time the sugar at the bottom has sort of hardened into a cake. The next time you need to get sugar below the halfway mark it'll be a lot harder to spoon it out, because you have to break up the cake that formed. If you then use the sugar all the way down to the bottom, it'll be a pain until you get to the bottom, but you can refill it with fresh sugar and your problem will be solved.
This is for the most part what happens in NiCd batteries (ONLY NiCd). If you only take the charge to a certain point every time, then the cadmium crystals at the "bottom" of the battery grow and form larger crystals. The larger crystals have a smaller surface area and react slower. So when it comes time to discharge in that region you get a depressed voltage. But it's not permanent just like the sugar, if you take the battery down most of the way and bring it back up then it'll breakup the large crystals and they will reform as smaller ones. If the crystal growth gets really really bad, then the battery might be toast because the large crystals just can't be broken down again. The solution is to occasionally cycle the battery all the way, but not too far (below 1 V/cell), because you can 'reverse' the cells.
Once again, this only affects nickel cadmium (NiCd) batteries. NOT lithium ion (Li-Ion) or nickel metal hydride (NiMH). If you buy a new cell phone or laptop it will almost surely use Li-Ion and not NiCd because of the power to weight ratio. Old camcorder batteries or old AA rechargables are probably the batteries you have to worry about in terms of the memory effect.
Good short links on the topic:
Sci.Electronics FAQ: NiCd Battery FAQ
Battery Reviews
Fine analogy. Made it easier to understand.
Posted by: Dan at March 7, 2005 12:40 PMYou're a genius Ram.
Posted by: Nick at March 7, 2005 04:22 PMthat's a really good analogy, most people don't know you're supposed to discharge cell phone batteries all the way every once and a while.
and i never really knew exactly why...