A properly set up and wired 6 volt system with a good battery should not need a second battery. You've either got cables that are too small, dirty connections, or a battery that does not have enough reserve capacity. Start out by finding someone who can test the battery properly to be sure you're not dealing with one that just can't produce. Even "new" batteries and/or parts can be defective.
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!