not exactly sure what you are trying to do, but if you are adding another battery, use a split charge system.
it basically senses when your main (starting) battery is fully charged and switches to charge the reserve battery aswell. also when the engine is switched off it isolates the 2 batterys so you can't flatten them both.
used on landys etc. for winching and campervans, ive got this setup in my vw t4.