You should not mix battery types. Either use 6-volt batteries in series or 12-volt batteries in parallel. To get the equivalent of two 12-volt batteries in parallel, you would need two sets of 6-volt batteries connected in series (+ post of one to - post of the other = total 12 volts) and then connect the two sets of 6-volt batteries in parallel with each other (+ post to + post and - post to - post).
Mismatched battery types will oft times not hold a charge.
Orv
|