I'm finally getting around to installing a stereo system in the boat and I want to install another battery to power it seperate from my 2 starting batteries. My question is, how do I go about wiring the battery so that I can charge it off of one of my engines altenators? Secondly should I increase the amp output of that engines alt? Right now they are both 100amp and each engine is on it's own 4 position battery switch, off / 1 / 2 / both. Obviously I only have my existing batteries hooked up to terminal 1 of each switch. Is it assimple as connecting the third battery up to terminal 2 of the engine I want to have charge the third battery and increase the amp output of that engine ?

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Yes