Hi BFL13,
There are two relays.
In my E-450 the "house" solenoid is below the battery. It is fed from a 60 amp fuse in the "engine" fuse box. I replaced the fuse with a 40 amp automatic circuit breaker, after the 3rd one failed. I added a second charging path fed from the starter battery, also with a 40 amp automatic breaker.
Each path has its own 200 amp continuous duty solenoid to which I added switches to allow me to manually control the flow of power.
They are, unfortunately still controlled by the ignition switch. But I can do a sort of "poor man's" boost from the house batteries.
I added a trik-l-start to keep the "engine starting battery" charged from the solar panel.
