It will explode if you seal a battery in a container and charge it time after time, it is true, havnt you noticed that on every battery pack the instructions say 'do not charge in a sealed container'

do you mean a Sealed lead acid battery, they are fine but look at the back of them they have a pressure nipple to let gas out,
if the container its in is open when you are charging the battery you have nothing to worry about

Paris