Caravan 12volt fridge hookup to thermostat option.

Phil 'n Jill (WA)
There is no way on earth you would beable to use a 12v 3way fridge on 12v overnight. If it flattens the battery just on coffee stops then overnight is going to kill the battery very soon after and then the frodge won't be cool AND the battery will be dead, ie, unable to be recharged again because it has been taken below it's minimum.
A 14.2 amp draw will take a 100AH battery, (only 50% of that is usable to retain any battery life) to flat in 3 1/2 hours even if the battery is a very good deep cycle battery. Any further discharge sees the voltage so low it isn't sufficient to create enough heat to keep the fridge cooling much at all.
So, in reality the battery will be flat to the point of almost nil performance of the fridge before you get to bed. Forget the rest of the night, it isn't going to happen.
Iff also running and Engel on the same battery at the same time, then the battery will give up.

Punishment of a battery is one thing, abuse is another. the battery will run an Engel overnight quite well but never a 3 way fridge. 12v use means a large recharge amount is required each day to charge the battery enough to be able to run the Engel reliably.
If used on 12v the 3 way fridge isn't in the thermostat circuitry or control and runs it's 12v heater all the time. A 12v heater makes less heat than the other two sources of heat to run the fridge and is usually not thermostat controlled.

12v use is only an option with good heavy wiring from engine bay, through an ignition controlled heavy relay (prevents coffee stop discharge) to Anderson plug dedicated to the fridge circuit. Most fridges do not have sufficient size of wiring to run a 12v fridge very well. You need 13v at the fridge at least while engine running, hopefully much more.
Often to run a 12v fridge while travelling (the only option) the voltage at the fridge input terminals requires additional wiring size or doubling of wiring Neg and Positive lines, so voltage drop is at a minimum and not a maximum as vans usually are.

.
Phil,

A 3-way fridge will draw as much as 15 Amps from the 12v supply, so if you have a fully charged 100Ah battery, which will offer you no more than 45 useable Amp hours then it will run the fridge for about 3 hours.

Thanks RMD and Allan - I gather that hooking into the thermostat doesn't really make a huge difference.

I do appreciate the time estimates - both of you have indicated around 3-1/2 hours.

I am happy to stick with the gas as the preferred supply, but was curious about the 12 volt option through a thermostat as a backup.

Our first visit into Cape Keraudren ended up a failure when the gas option failed and we had to pack up and head to Sandfire for a power supply overnight. The ignition unit failed and had to be replaced at Broome. Just another of those challenges we come across as we tour this great country.

Thanks again for your support - all the best in your travels.

Regards - Phil

Dometic three way fridges have come a long way in ten years.
Having a 12v relay to cut power when the ignition is turned off is mandatory.
The new dometics search for another power supply automatically once the 12v. supply cuts out.
When you stop for a cuppa or go shopping just turn on the gas and it will automatically ignite and a 240v connection will be the default energy source.
Also the 90lt cabinet has gone to 95lt and has the same dimensions and is far more efficient.

Best way is to use the thermistat in the fridge by inverting 240 volts. Probably similar amps used but 240 coil gets fridge colder/works better than 12v. Otherwise, relay to cut power when motor stops is best, then manually select gas. I ran 6mm wire to my fridge from second battery in car via anderson, and it worked better, byut still was unable to lower temp, just keep it constant

Once again - thank you each for your feedback.

I will stick to the tried and true methods - 12v whilst travelling, 240v in parks and gas out in the wilderness.

It seems no-one has actually used the '12 volt into thermostat concept' and that probably speaks for itself - not a great idea.

Greatly appreciated.

Regards - Phil

From memory 120 watts for 240 volts,(so 0.5 amps, plus losses in inverter.) I think the 12 volt usage was 6 amps, so its coil was 72 watts.