2 batteries

 To Isolate, or not to isolate..

 It all depends on what you're trying to accomplish. Full sized diesel pickups can come with dual batteries, wired in parallel. This is because one battery doesn't have the cranking amps to start the engine.. some of these engines spec cranking amps as 'not more than 500 amps'. In this situation, both batteries are matched and located in the engine compartment. They are subjected to as equal as possible operating conditions. Heavy equipment can also run dual battery systems; but I've seen those in both parallel config, as well as series config. Only in very rare situations would you _not_ run a matched set of batteries.

 On the other hand, in a situation where you'd have a cranking battery and a 'house' battery, like a motorhome or camper, matching is not necessary as long as the batteries are not parallelled together unless for charging. In the audio, trunk mounted second battery situation, I'd just get a good battery isolator and call it good. With all the computer controlled alternators out there now, the easiest way to isolate is just use a decent battery isolator.

 Just my 0.02

 J