Battery voltage

Compressors draw a large current and will give the cutout a false impression of battery state. Have the engine running when using the compressor. At 12 volts with a very light load the battery is 50% depleted. If running a fridge overnight I'd say set it to 11.5 - 11.8

I would set it slightly lower around the voltage that most frigs will cut out, just under 11 volts. Most vehicles (especially petrols) will start at this point. Being adjustable you can easily reset it.
Peter

An ARLEC battery load tester would say your battery is OK under load at anthing over 8 volts, which i reckons a bit low but its under load remember. I am with Mad Dog, 11.5v is enough for a prolonged load on the battery.

That kit interests me. Not sure when I'd use it. Having an alarm go off in the middle of the night when the fridge draws down on the battery may not be popular. Any ideas??

I think you'd be better off with a low voltage cutout. The kits or premade versions that Jaycar and Dick Smiths stock are good, provided you're happy for the fridge to stop working until the morning. I used the relay type kit for a long time, and last year made up the transistorised kit. I used a potentiometer to make the cutout voltage adjustable.

Thanks to everyone for your help.
I'll do some experimenting around the 11-11.5 volts and see how I go
Camper

Just for information sake, i have a Prado Turbo Diesel which has (2) 6month old batteries (exide extreme 620cca) to run fridge/lights etc and a (marshall 600cca) which was a warranty replacement for the main battery. I inadvertently left the interior light on for @ 24hrs, at that point the main battery voltage had dropped to
11.8v, the engine would not even kick over to start,i jumped started the car off the auxilary battery (handy), drove the car to work @ 20mins, checked the voltage again that afternoon before returning home and it had risen to 12.2volts the engine started but it was a bit slower than usual. Shows that a second battery is a great investment if your forgetful like me or if your fridge used just a bit more power than you expected and you only have the 1 battery.