Gentlemen,
I am considering putting 3-12v automotive batteries in one of my show tractors. It is used very sparingly, I.e. for 100 yd distances (to and from trailer). What I do not know is whether I should expect the on-board charger to handle the charging? Would I set the timer for different (less) amounts of time compared to the robust bank of 6-6volt batteries?
As an aside, a couple years ago I rigged 4-18v drill batteries into a parallel and series configuration to deliver 36 volts. They moved the tractor nicely but one battery shorted out 
and I gave up on that project.l
and I gave up on that project.lThank you for any insight.
Mike