I fitted one of these type switches and as said before b1 is the vehicle b2 is the auxiliary or camper. On mine the two were not connected unless the 1+2 option is selected. so it does not matter which one is selected when stopped. it is advisable to only start on battery 1 which should be the vehicle battery. I never had a problem with this system, very simple and reliable many boats use them. Just start on 1 and switch to 2 when driving or after a few kilometers. You have to remember that it is a manual switch and as such all the power from the alternator is going to one battery or the other unless 1+2 is selected. This is not recommended unless all batteries are the same and same age (I was told).
You will not run down your vehicle battery but you must remember to keep it topped up. Never drive in the off position!
Graeme
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