Note: IF there is room in the battery bay you can always add a 2nd 12 volt battery, Some folks say all batteries be they in series or parallel should be Identical.. I'm not among them.
Though if you are putting batteries in series, as you do with six volt pairs, they do need to be identical, IN parallel this is not nearly as important.
So if there is room, you could "Drop" a pair of sixes in there next to that 12 volt and .. Well, best of all worlds.
NOTE: 2 six volt battreies in series = One BIG 12 volt battery (Size 8-D if I'm not mistaken) treat them as such and most all six volt questions are answered.
Home is where I park it!