NiMH batteries aren't quite as bad as NiCD with the dreaded "memory effect," but they do suffer from it. Usually, totally draining the battery until the computer sleeps on its own and then plugging it in overnight to completely recharge will help to reset the cells so that they "remember" their full charge state. You may have to do it once or twice initially to get it back to normal, then about once per month or so afterwards as maintenance. Batteries don't last forever, though, so if it keeps going downhill, you'll probably need to buy a new battery or re-cell your old one.
As for the PRAM battery, on the PowerBook 100 series (except the 100 and 190), it's a little rechargeable button cell roughly the size of a quarter. It's got little tabs spot-welded onto it, which are soldered to the intermediary board mounted in the top half of the case (the board that interfaces the keyboard and display to the motherboard via that large IDE-looking ribbon cable). It's not too hard to replace, so long as you find the proper battery to replace it with. I don't have any 100-series machines to play with now, so one of the other forum members can probably help you with the specifics.
