Whats the reason behind this?
isolator cuts the rear battery off from getting charged from the alt. so the voltage is going to be at like 12.8 and drop from there. once the voltage drops enough, something like in the 10's and low 11's, the isolator will switch so that the rear battery will be charging now. alts HATE charging dead batteries. it puts a shit ton of stress on them, since they're made to keep an already charged battery charged.
I agree there, I didn't realize the isolator switch's only once the second battery is discharged? I thought it isolates them when the vehicle isn't running, so they are always in parallel when the vehicle is running.