i put 2nd battery's in cars at work like every day and you dont need an issolater and if you uesd a dioed it would need to be a big one you can use a big realy like a 100 -200 amp realy but you dont need that. and i find it is best to use a deepcycel battery, the stinger is good but we have 5 in one of the vans we put a system in and they dont like to stay at 12 volts nop matter how much you charge them. a adding a battery wont drain your old one just dont play your system with the car off. any system with more than 100 amps of curent draw needs an extra battery,