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

[Dr. Gerald N. Johnson, electrical engineer]

The circuit is simple, but Radio Shack doesn't sell large enough diodes
or heatsink.

The circuit is simply two diodes, anodes common to the alternator
output, one cathode to the vehicle circuits, the other cathode to the
trailer battery charging line. The diodes need to be good for at least
the current rating of the alternator which can be as much as 140
amperes. RV catalogs and JC Whitney have them. Its usually in the form
of an aluminum heatsink with near radial fins 3 to 4 inches square and 6
to 10 inches long with three terminals on top of an epoxy lump. There's
benefit to its survival to add a fuse or circuit breaker in the wire to
the trailer, close to the isolator, so that when that wire gets shorted,
it doesn't kill the isolator or the alternator. Unfortunately the
resistance of that fuse or circuit breaker will limit the charging
current to the trailer battery.

Since the alternator voltage regulator (if its a separate regulator)
senses voltage from the vehicle wiring, it compensates for the extra
diode drop. The diode isolator is not as great an idea with the internal
regulator alternator with one wire unless that regulator can be set up
about 0.6 volts.

Gerald J.



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