The battery does not have to be on or off to charge. There are some variations as to what the battery does while charging. Some turn off. Others stay on. Manufacturing differences.
Most of the problems associated with charging come from second rate chargers and batteries. You get what you pay for generally speaking. There have been a lot of threads on problem chargers and batteries and most have been cheap knock offs. That being said, not all knock offs are bad.
Best result is with a compatable charger. I believe the rating is 420ma output. I think the max is 500ma. The rest are mechanical problems; dirty contacts on battery or charger, cheap materials that dont conduct well, misalignment of posts, faulty wall plug.