I have a friend that works for a local sacramento battery distrubutor and she can get optimas and any other sort of battery for well bellow retail.
She can also awnser any questions about the amps per hour on any battery she sells.
Contact her a
talison@surewest.net here is the rating for a 6 volt battery
US2200 122 min@ 75 Amp hour draw 232 AH
again put in series and you have more aph then a 12v.