There is a very simple way to do this; you connect a relay to the oil light
switch on the engine[s] that drives the alternator[s].
The positive side of the relay coil goes to the ignition circuit.
This is the standard setup for separating a house battery from the start
battery with a relay, so avoiding the voltage drop from diodes.
The advantage of using the oil light instead of the ignition, is that the
engine will [normally] start before the load is added.
Not as clever as those comparator circuits, but sure is easy!
Regards, Mark Holden



"R. A. Piziali" wrote in message
...
I have need of the following:

A relay that will handle a 15amp load and close at about 13.0-13.4vdc
and
open at, say, about 12.7vdc. Is such a device available - if so where?
OR what
can I put together to operate in such a manner??

The application: I want to AC power an RV refrigerator with an
Inverter from
the vehicle DC charging system but automatically disconnect when RV
battery is
not being charged, ie, to prevent powering Inverter from RV battery.

Ray
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