Dual Batteries

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I have a 1985 Ford f150 and i was wondering if i could run a dual battery setup. it doesnt say anything in the owners manual. there is another spot for a battery and the previous owner just mounted a PA to it and there is also another sylinoid that has a positive cable coming from it. if i can setup dual batteries how could i go through with doing it?

 

[MIL1ION]

Is this for an RV application or just for another power source ?

Solenoid ...... not sylinoid also known as a starter relay

 

[[email protected]]

it is for an extra power source