Parallel batteries discussion

Most vehicles have an isolator on 2 battery systems... one or both generally powers the starter, but then, there's one battery for the lights or accessories, and the computer.

I know that the Ford diesels will not run properly with less than 14V average voltage...

As for getting a high voltage starter setup... You need a few relays and/or solenoids, and problem solved...

I know for towing a trailer, my dad had a continuous duty starter solenoid (85 amp maximum current) so the truck's alternator would charge the trailer, but as soon as the key was shut off, the trailer was effectively disconnected from the trucks electrical system without unplugging it.

Those relays work great for high power lighting too...