An 8 volt battery is a solution to a nonexistent problem. Find someone who understands electricity, and get him to show you how to get the 6V system running properly. It's mostly done by using the proper size conductors, having clean, tight connections, and adjusting the voltage regulator properly. 8V batteries cause a host of problems and are mostly used by people who don't want to go to the effort to make a 6V system work right.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!