[VAC] Re: Why Car & Trailer Battery to Univolt?

["Dr. Gerald N. Johnson, electrical engineer" <geraldj@xxxxxxxx>]

Marc, I suspect the Chrysler tow car may have used the wire from
alternator to battery as the shunt. Adding a diode to that wire would
add a whole lot of voltage drop to fry the panel meter. That could have
been avoided had the little wire on the alternator been moved to the
diode isolator along with the original alternator to battery wire.

When the ignition was on the second battery and that battery was dead,
the car wouldn't start until the second battery was charged enough to
run the ignition. 

I don't consider that the starting battery should be isolated from the
ignition battery in the tow vehicle. What's needed is that the trailer
battery be isolated from the car starting and running battery so that
the trailer use won't run the car starting and running battery down.
Also since the wire to the trailer is not capable of handling starting
the diode isolator keeps the trailer battery from trying to charge the
starting battery while cranking.

My '98 F-150 does use a relay, but that relay may be controlled by the
computer to prevent it being on unless the engine is actually running.
Not just with the ignition switch on.

Your relay proposal does allow using both batteries for cranking but I
don't think the tow car benefits from isolating the second battery. If
it needs all that cranking power, I'd be sure the starter circuit cables
were enlarged a size or two (maybe to 1 or 2 gauge if not 0 gauge) and
the starter brushes, commutator and bearings were not worn out and I'd
just hook the two tow car batteries in parallel (as in a diesel). I'd
use the diode isolator for the trailer battery and wiring protection.

P&B makes a relay like the starter relay, for continuos service, but
since the pressure on the contacts is smaller (lower coil power) its not
rated for starting current either.

I've found some aircraft relays that are continuous duty and rated for
200 amps continuos, but they have 24 volt coils.

Gerald J.