I have to agree that the isolator is not a necessity (as long as you don't kill the batteries by running electronics for long periods without the vehicle running). Have installed many second batteries after customer has added to a system over the years. We usually have them upgrade both batteries to Optima's as part of the conversion but not every budget has allowed this. Also, think about Diesel's (Ford has no isolators on the 7.3L diesels-as a matter of fact direct connected with 0 guage running across the radiator support)-and they would only allow warranty replacement of the one battery that was failing the test-NOT the pair, or many of the RV's with dual 12v batteries (most newer ones however run dual 6V house batteries in series). I am not saying it is healthy to run two different age/condition batteries as the stronger one will mainly be working to bring the other battery up to speed (like jump-starting a vehicle with a low battery) but it can be done. Just my experience anyway...