You need a circuit like the one on:
http://www.ncws.com/rcrock/lowbat.htm

If you replace the LM336 and its 3K resistor with a 10K little
thumbwheel potentiometer (volume control), hooking the wiper arm to
the minus input to the LM311 voltage comparator and the two ends
between battery + and - terminals, you can adjust the trip point all
over the place to suit your needs.

Replace the little alarm beeper with a small 12V relay that will turn
your big control relay on and off remotely mounted at the inverter's
DC power source. The little relay contacts, being isolated, will
prevent the big relay's coil from kicking high voltage into the IC
destroying it.

The LM311 is a voltage comparator IC, made for this service. Its
output (hooked to the piezo buzzer in the schematic) switches from off
to full on whenever the voltage at the + input crosses the voltage set
on the - input. This will click the relay ON any time the voltage
drops below your set point. If you power the big control relay from
normally closed contacts on the little relay powered by the IC, any
time the little relay is energized, the big relay will be
de-energized, turning off the fridge. I like this because if this
circuit fails for some reason, the failure mode is to leave the fridge
ON, not off, continuously....protecting your food in an emergency
scenario.

As this circuit draws virtually whatever the tiny control relay draws,
a few milliamps, there's no reason to ever turn it off. Just put its
source to the breaker the inverter is running from.

Once set by the variable control, it'll stay that way for years.

To adjust it is simple. Turn the charging on, and adjust the control
so the inverter runs. Turn the charging off and adjust the control
back until the inverter shuts off at the desired voltage. You can
adjust the shut-down point to anything you like.....leaving the
inverter on until the battery voltage drops to X volts.



On Sat, 25 Oct 2003 22:13:20 -0700, "R. A. Piziali"
wrote:

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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Larry W4CSC

"Very funny, Scotty! Now, BEAM ME MY CLOTHES! KIRK OUT!"