My experience has generally been that when it gets to the point that it's not holding a charge, a battery charger doesn't help you much except in starting your car to go buy a new battery.

However, I've also noticed that maintenance-free batteries, well, aren't. Once or twice a year I pry up the tops with a screwdriver and look in the cells. They sometimes need to be filled up with a little bit of distilled water, because if the cells get up out of the water they're likely to short out and stop working. The water levels tend to gradually go down over time, probably due to how horrendously hot it gets around here.