A battery that is firmly sealed and equipped with valves.
This means that no liquid can be refilled, which is why these batteries are referred to as sealed maintenance-free. With AGM and HVT batteries the electrolyte is held in a non-woven mat and they are therefore leak-proof. Just like GEL batteries, where the electrolyte is in the form of a gel. Both can be installed at an angle. In some areas, leak-proof batteries are mandatory, e.g. on boats, to prevent battery fluid from escaping in the event of capsize.