We do not want to connect the added "auxiliary" battery to the vehicle charging system because:
If you connect the "auxiliary" battery to the vehicle charging system, by virtue of THAT connection, you are actually then connecting the "auxiliary" battery DIRECTLY to the MAIN battery too. As the alternator is itself connected DIRECTLY to the MAIN battery, adding a connection from the alternator to the "auxiliary" battery effectively places BOTH batteries in parallel, with NO isolation between them. Having the two batteries connected together, in parallel, is NOT what is wanted here. And that actually DEFEATS the "SWITCHING" part of the Painless Performance System. Any short or load on the MAIN battery that drains it would ALSO drain the "auxiliary" battery (since they are NOW connected together in parallel.) The idea and design of the Painless Performance Battery Switch System #40102 is to keep the two batteries
totally isolated from each other until the backup "auxiliary" battery needs to be switched in to start the vehicle from a LOW or DEAD MAIN battery.
As to an isolator, the "Painless Performance #40102" system does not provide that type of design. I have read that there are other systems that do, but I have not dealt with those nor have I any direct knowledge concerning their design and implementation. There are many dual battery systems on the market.
The Painless Performance Battery Switch System #40102 requires that it be installed and connected only as the manufacturer specifies, otherwise it will not function as intended, and that will lead to problematic electrical issues.