Hi!
I bought an E350 and it has the second battery under the side doors. It is a 5.4 gas.

The previous owner did not know it had the second battery and it was the original and not in very good shape. I found it when I was looking at the undercarriage before buying it.

My van has the trailer kit, which included the second battery. I know its not wired in parallel as when I used the factory trailer wiring my trailer lights were very dim or not working at all until I put a new second battery in.

Now for my question....

I have searched all around and not really found a good answer as to just how the second battery is wired and what is running off of it? Some posts just says its parallel, but I know that is not true. Some say its fully isolated but can be used to start the van if needed.

Can anyone point me to a thread or site that has a good diagram or explanation of the secondary battery on the gas E350s?

Thanks,
Scott