how big of gauge wire to 2nd battery?

i have a 8 gauge wire that i wanted to use to run a 2nd battery for my amp when my car is off. i got all of this for free and figured i might as well use it. i only have a small amp that pulls 30amps max. so could i put a 30 or 40 amp fuse on the 8 gauge wire going to the 2nd battery? would this blow from the car simply trying to charge the 2nd battery? i also have a isolater 

Update:

i have a 130 amp alternator.

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  • 2 months ago

    The location of the battery will be a problem.  You'll probably have to mount it in the trunk then wire it in "series" to the car's battery.  Then add a marine "on/off" (or similiar) switch to isolate the battery when the car is off.  Charging won't be a problem.  I have 6 batteries on my boat and a 90 amp charger takes care of them.

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