If you charge a 6-volt battery with a 12-volt charger, then it would result in overcharging which could cause the battery to blow up. For example, if you have an 8-volt golf cart battery then just use an 8-volt charger on that instead of using a 12 volt one. This will ensure that you don't overcharge the battery and blow it up. Also, you should never have any trouble if charged correctly so always make sure to charge your batteries without overcharging them to ensure that they last as long as possible. If you do experience troubles then consult a professional since this is not something an amateur usually has problems with.