Caravan 12volt fridge hookup to thermostat option.

Submitted: Monday, May 21, 2018 at 18:41
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I have just been reviewing notes I printed off ten years ago and came across a comment that went unnoticed back then and wondered if anyone has actually tried this and how effective it may be: Oziexplorer explained the various operations of how the thermostat operates for the differing powering options of 240AC / gas / 12v and added if using 12v continually - it is easy to hook up a relay to utilise the thermostat.

As I had flattened my one and only vehicle battery a couple of times by pulling up for coffee and forgetting to unplug the Anderson plug, and having a relay installed to prevent this, I hadn't given that much thought to the matter, but now consider that could be a worthy consideration for free camping now my tug has a second battery.

Just curious as to how much punishment the battery would take parked overnight.

I know there are many variables, but door opening would be minimal (meal prep), with essentials (beer) in an Engel and the fridge is a Thetford 164 litre unit with specs inside the door showing 14.2 amps 170 watts operating on 12 volt.

The mysteries of volts,amps and watts are still black magic to me, despite past attempts to fathom them.

Thanks & regards to all

Phil
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Reply By: RMD - Monday, May 21, 2018 at 22:05

Monday, May 21, 2018 at 22:05
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.
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Reply By: Allan B (Sunshine Coast) - Monday, May 21, 2018 at 22:10

Monday, May 21, 2018 at 22:10
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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.

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Reply By: Member - Phil 'n Jill (WA) - Monday, May 21, 2018 at 23:23

Monday, May 21, 2018 at 23:23
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

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Follow Up By: Allan B (Sunshine Coast) - Tuesday, May 22, 2018 at 08:20

Tuesday, May 22, 2018 at 08:20
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Ah Phil, I didn't fully perceive what you were proposing.... to 'use the thermostat on 12v duty to reduce consumption'.

However, the nub of it is that the 12v element is only just able to drive the fridge when it is constantly energised, hence no thermostat control is employed. In order to reduce 12v consumption the thermostat would have to be set to a temperature which would cause modulation and that would not be sufficient to keep the fridge at an acceptable temperature. Yes, it may save power, but would ruin the food!
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Follow Up By: RMD - Tuesday, May 22, 2018 at 10:28

Tuesday, May 22, 2018 at 10:28
Phil
It seems your gas system didn't fail but just the sparker for the lighting of the gas. Most use a wire from a Piezzo electric impulse spark unit and send a high voltage down the wire to the spark gap for pilot light ignition.
If you can get to the piezzo sparker end, the button end, any piezzo sparker from a BBQ or similar will provide a spark when it is attached to the wire and the body of the piezzo unit contacting the metal frame of the fridge. That would get you out of trouble if it happened again.
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Reply By: Member -Pinko (NSW) - Tuesday, May 22, 2018 at 08:00

Tuesday, May 22, 2018 at 08:00
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.
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Follow Up By: RMD - Tuesday, May 22, 2018 at 16:44

Tuesday, May 22, 2018 at 16:44
A good idea to have the fridge work out it's heat source. But I wonder how reliable they will prove to be. They have to be much more complicated.
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Reply By: Member - DOZER - Tuesday, May 22, 2018 at 14:31

Tuesday, May 22, 2018 at 14:31
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
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Follow Up By: Allan B (Sunshine Coast) - Tuesday, May 22, 2018 at 14:39

Tuesday, May 22, 2018 at 14:39
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The only reason that the 240v element "works better than 12v" is because it is a higher wattage element. Add in some inverter losses and the power consumption will be significantly higher.

Definitely not "similar amps". But yes, it will cool better, for a while. lol

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Follow Up By: HKB Electronics - Wednesday, May 23, 2018 at 14:12

Wednesday, May 23, 2018 at 14:12
Don't know about that Allan,

I know the waeco I had in my camper had the same wattage for both, I just looked at a Dometic 80Ltr and it stated 240V/125W and 12V/120W.

In my old camper the original 80Ltr Waeco fridges 12V was controlled by the thermostate, when that died I found the replacements was not.

I would hazard a guess and say any difference in cooling is due to either the way the 12V element is located compared to the 240V element or more likely due to the fact the fridge is most likely moving around when running on 12V, same would apply to 240V mode.
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Reply By: Member - Phil 'n Jill (WA) - Tuesday, May 22, 2018 at 17:01

Tuesday, May 22, 2018 at 17:01
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
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Follow Up By: RMD - Wednesday, May 23, 2018 at 09:09

Wednesday, May 23, 2018 at 09:09
Phil.
I know you mentioned the electric power issue as black magic, it isn't really, but the fact remains, no thermostat is used in the 12 heater circuit because the 12v heater supplies far less heat value than the other two sources. It doesn't cause the fridge to get to the lower temp to need any control because it is always striving to create enough cool in the system.
It never reaches what the other two sources can achieve so no regulation is used there. ie flat out all the time.
The other two sources supply more than sufficient heat and so when the temp gets down to what you want, it turns off the 240v ac heater or turns down the gas to pilot light level so it doesn't get too cold. 12v never gets there.
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Reply By: Member - DOZER - Tuesday, May 22, 2018 at 20:47

Tuesday, May 22, 2018 at 20:47
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.
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