To add to the above, Festool has actually made it so the wrong battery cannot be physically connected to the tool. Most people wouldn't even noticed the small ribs on the top of the batteries, or that they are different from one battery to another, but these ribs prevent the battery from being inserted into a tool not designed to handle it. This keyed system allows the batteries to be fairly universal for tools designed to use the whole range, but prevents a battery from damaging a tool that wasn't designed to accept it.