A little more detail about what you're doing would be good. The extra battery is either a good idea, or totally unnecessary and useless, depending on what you're doing with it. If whatever accessory you're planning to power is only going to be used when the engine is running, you don't need another battery. If you're worried about running the starting battery down when the engine is off, you MUST install an isolation solenoid.
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!