I put this in. Isolator and selector switch. Therefore, it charges both batteries when running, separates when not, and I have the option to put on 1,2,both,off. By selecting the battery, it chooses what one is monitored to stay full.

http://www.amazon.com/714-100A-Batte.../dp/B000NI5FAE (shop around for price as I got it cheaper).

Both batteries are deep/cycle/starter (with CCA 650 I think).

I don't have a huge stereo system, but we do listen to it while on the hook.

Positive wire from switch to both batteries. Stereo/Acc's are wired to one battery. I leave the switch turned to then monitor the other battery for the starting one and leave it set.

Advantage over the Perko alone is no longer needing to remember to switch off of all or alternating which battery it is run on which day.