All makes sense thank you. When I get back I will get an extra single panel, get a BCDC1225 reg for the car to replace my SBI12 ( I think it can be done) so then I have the option of using the single panel for the car or add to the trailer. But if I were to run a lead from the car to the trailer for triple batts I'd have to have a switch of some sort to shut off the reg in the car wouldn't I?