3 Way Fridges

dundreamin · Veteran Member

3 Way Fridges

Whenarewethere · Guru

What is the actual voltage while running on 12v at the fridge, NOT at the battery.

If the wiring is too long & undersized you will have too much of a voltage drop.

Are We Lost · Guru

dundreamin wrote:
If I stop for lunch & open & close the fridge door a few times the temp shoots up to around 15°.

I have a Dometic but imagine the Thetford would be similar. If you open the door you can feel all the cold air falling out onto your feet. It does not take much to lose a lot of that air. When you close the fridge the air inside gradually gets cooled by the food and drinks inside and by the fridge actively cooling. So it is quite normal to see a fast increase in temperature when the door is open.

Then comes the 12v issue. The 12v element will typically have less cooling power than 240v. Mine is 275 watts for 12v and 325 watts for 240v. So, by design it will not work as well on 12v. Even the smaller element consumes 25 amps continuously on mine .... far too much for the typical house battery. Or worse, for the car battery.

This is where the next part comes into play. When wiring it to be run from the car while driving it needs a cutout facility to turn off the 12v supply if the engine is not running. So when stopped for lunch, the fridge won't run.

It's possible yours is set up differently but that is the norm. Then there are possible 12v faults that could also be affecting it. The power provided through a 12 pin trailer socket is limited, and not suitable for a large fridge. Apart from overloading the circuit there may be a large voltage drop by the time it reaches the fridge.

Have you checked the fridge specs to see how many watts the 12v and 240 elements draw? Providing such details and the way yours is wired may get some more accurate answers.

As for running on LPG while driving I used to do that all the time. After reading warnings on here about risks if there is a petrol spill in a service station I have revised my thinking. The chances of there being a problem are exceedingly slim, but still there.

dundreamin · Veteran Member

No idea what the voltage would be, same for wiring size but thanks so much for the quick replies. Might look at getting the 12v checked.

Greg 1 · Guru

I have a Thetford and if you read the instruction book it says that 12v is really only to maintain temperature whilst you are travelling and the fridge should be run on either 240v or gas when stopped. I have found with mine that if I am quick getting stuff out of the fridge at lunchtime, the fridge will keep cold for the time I am travelling, but as soon as I am stopped for the night I get it swapped to 240v or gas and I have never had an issue.

Peter_n_Margaret · Senior Member

The only time we have owned a 3 way fridge was the time it took to choose and install a compressor fridge.
Cheers,
Peter

Whenarewethere · Guru

Are We Lost wrote:
Then comes the 12v issue. The 12v element will typically have less cooling power than 240v. Mine is 275 watts for 12v and 325 watts for 240v. So, by design it will not work as well on 12v. Even the smaller element consumes 25 amps continuously on mine .... far too much for the typical house battery. Or worse, for the car battery.

For 25 amps you probably need 50mm² cable or more depending on length of run.

For OP:

Not to forget wire resistance is the total length of both positive & negative. & underrated cable connectors are also an issue.

A DC wire resistance calculator will quickly provide the wire's cross sectional area required.

On top of that you can also put a DC-DC converter next to the fridge to up the voltage that bit more, but not to forget its going to draw even a few more amps accordingly.

dundreamin · Veteran Member

Yes Greg I was aware that it's only to maintain the temp not to actually cool but as I said I can't even open the door a few times to make lunch. As I stated previously it doesn't even maintain the temp after about 4hrs even if I don't open the door at all and of course when stopped I use 240 or gas & it's fine. I've had it serviced recently so nothing wrong with the door seal. I'm beginning to think maybe it's just age & not working as well as it should because of that.

Thanks

Di D

dundreamin · Veteran Member

Yes I had a compressor fridge in my old motorhome & that was the opposite - it often froze stuff.

Are We Lost · Guru

dundreamin wrote:
No idea what the voltage would be, same for wiring size but thanks so much for the quick replies. Might look at getting the 12v checked.

Before spending money getting someone to look at it when there may be no problem, why don't you look at the fridge, get the model and look up the specs.

Without that and how it gets its power (via car or house battery? via 12 pin connector or separate cable?) we can only guess.

dundreamin · Veteran Member

Yes I might do that. I looked in the manual but there's no specs in it, just a wiring diagram but I know it gets power from 2 x 130AH AGM house batteries.

Thanks

-- Edited by dundreamin on Monday 3rd of June 2024 02:22:32 PM

Peter_n_Margaret · Senior Member

VOTLAGE DROP =
[total cable length (in metres) X current (in amps) X 0.0164] divided by cable cross-section in mm.sq.
Cheers,
Peter

dundreamin · Veteran Member

Dick0 · Guru

First of all the 12V supply from within the vehicle should be wired from the battery through a 100A capacity fuse relay and a 100A capacity Ignition relay to the rear of the vehicle to an Anderson plug (not the 12Pin trailer connector) using 6B&S cabling.

From the car rear Anderson plug there should be another Anderson plug connector from the drawbar with 6B&S or 8B&S cabling to the fridge 12V terminals.

If you don't have this setup or similar the 12V cooling will lose efficiency.

Most 3Way fridges have a preset temp cooling setting (non adjustable) for 12V operation and are not relative to the fridge manual cooling settings for Gas/240V operation.

Some 3Way fridges draw 25A-30A so cabling is very important to maintain high Amp cable efficiency.

Joda · Senior Member

Just be aware of running fridge off vehicle if it has electric power steering, reversing a van on a hot day with a lot of steering inputs can damage the torque dc motor in the steering rack, usually have to replace steering rack as motor is not a serviceable part.

Whenarewethere · Guru

Dick0 wrote:
From the car rear Anderson plug there should be another Anderson plug connector from the drawbar with 6B&S or 8B&S cabling to the fridge 12V terminals....
....25A-30A so cabling is very important to maintain high Amp cable efficiency.

8awg/B&S is only 8.3mm², 6awg is not much better at 13.3mm².

You need to add up all resistance figures for each different length of cable cross section size.

25 amps from the engine bay all the way to the fridge in a caravan is going to need a "bit" more cross sectional area than 6awg.

Whenarewethere · Guru

Apart from running 50mm² - 70mm² cable from tug to anchor. Using 120 or 175 amp Anderson plugs (I haven't tried 350 amp plugs to date) are far smoother to plug & release than 50 amp plugs.

I installed a 175 amp Anderson plug from the battery (also another one on auxiliary batteries to jump start myself or others) to make life easier as I was sick & tired of alligator clamps, at least at my end!:

50 amp plug hot plugging (120 amps but cold plugging)

120 amp plug hot plugging (240 amps but cold plugging)

175 amp plug hot plugging (280 amps but cold plugging)

350 amp plug hot plugging (500 amps but cold plugging)

All with maximum cable size for lug per plug size.

dundreamin · Veteran Member

Thank you everybody for your very technical advice/suggestions of which I understand absolutely nothing. I have no idea about checking amps or size of wiring etc & in fact have no idea where the wiring is, probably somewhere in behind the vent I assume. Also I neglected to point out that my fridge is in a motorhome not a van, don't know if that makes any difference or not. I'm also aware that more than likely all the wiring is probably lighter than it should be as I know manufacturers do anything they can to keep weight down so they can sell motorhomes that can be driven on a car licence no matter how big they are.

Again Thanks to everyone.

Di D

Whenarewethere · Guru

dundreamin wrote:
Also I neglected to point out that my fridge is in a motorhome not a van, don't know if that makes any difference or not.

It does make a difference. Wiring length will be shorter, thus a bit less cross sectional area of cables.

Also makes answers more useful if people state the products involved. Less stabbing around in the dark & going around in circles wasting everyone's time.

Something or probably two problems are pointlessly bringing down a system that would otherwise work well if set up properly.

But from what you last said, get someone to look at the whole set-up to get it working properly.

Are We Lost · Guru

Whenarewethere wrote:
But from what you last said, get someone to look at the whole set-up to get it working properly.

I agree. Not everyone wants to get involved in the technical aspects and Di D has made that clear. But perhaps it is just normal behaviour. It would be a common design feature that it will not cool while on 12v without the engine running.

-- Edited by Are We Lost on Tuesday 4th of June 2024 09:49:12 AM

Whenarewethere · Guru

I can't see why one (or get someone else) can't get a gas fridge running off 12v for short periods, at least off an auxiliary battery.

The correct size wiring & a DC-DC converter next to the fridge, wound up to say 14 or 15 volts.

Otherwise the voltage at the fridge is probably down to 11 volts or worse with the undersized cable at >40°C due to resistance.

Are We Lost · Guru

Yes it would be easy to do that but I am guessing the motorhome has protection to avoid the battery being drawn down when not driving. Good design suggests it should have that feature. Perhaps a device like this:

Fridge auto cut off

rgren2 · Guru

I wouldnt be surprised if the fridge ran off one of these.

Dick0 · Guru

Whenarewethere wrote:
Dick0 wrote:
From the car rear Anderson plug there should be another Anderson plug connector from the drawbar with 6B&S or 8B&S cabling to the fridge 12V terminals....
....25A-30A so cabling is very important to maintain high Amp cable efficiency.
8awg/B&S is only 8.3mm², 6awg is not much better at 13.3mm².

You need to add up all resistance figures for each different length of cable cross section size.

25 amps from the engine bay all the way to the fridge in a caravan is going to need a "bit" more cross sectional area than 6awg.

I would love to hear from other Nomads who have cabled their tugs and dens with cabling greater than 6B&S. I don't think it would be the norm and would be overkill for normal caravanning.

I may be wrong, of course.

Cupie · Guru

Some really good advice in the previous posts.

I struggled with the efficiency of my 3 way for years & did all the usual mods like internal & external fans along with ignition controlled 12 v supply via larger cables direct to the fridge via Andersen plugs, insulation and air flow baffles etc.

(In my old GQ Patrol, I use a relay that operates off the ignition switch accessory position to connect 12 towards the van via an Andersen plug using large cables ALL the way. Inside the van I have the option of connecting that feed direct to the fridge .. via more Anderson plugs & large cabling .., or to charge the van battery if I choose that option. Usually I just charge the van battery when traveling from the solar panel.)

I find that the fridge more than holds its temperature when driving in temperate weather, since I made these mods.

In the past I always ran the fridge on gas when traveling but was very nervous about the practice & never do it these days.

To reduce the loss of cold air when the door is opened, we have established a system of labelled plastic containers for most of the items inside the fridge. This way only the cool air outside the containers may fall out the bottom. I chill down the containers & their contents in the Engel when filling the fidge initially or after a shopping expedition. An added benefit of this method is that I never freeze the lettuce or tomatoes, which are wrapped in foil & paper towels inside their containers.

Whenever I stop for a cuppa, or to put the van on site, or go sight seeing etc, I always switch to gas. It's an automatic thing to do after a while.

If we were just starting out, I suppose I would go to the trouble of changing to a modern 12/240v compressor fridge supported by more solar & batteries.

NOTE .. The Engle keeps the white wine & beer cold so that's the most important thing covered.

Warren-Pat_01 · Guru

Hi Di,
Before we head off with the van, we connect the fridge to the mains power & cool it down. That gives our food a head start. For lunch we carry a small Esky, esky bag with one or two frozen blocks to keep it cool.

When we had our Jayco Eagle camper with a 90L 3way fridge, we would cool it down on the power, cover the exterior vents & put some frozen water bottles within the fridge & open it when we arrived in Rockhampton (7 hours from home). It would still be cold within the main fridge section, frozen in the crisper. I made the covers to keep dust out when travelling on dirt roads.

Greg 1 · Guru

Never really had a problem with any of the 3 way fridges I have had to be honest. The current Thetford is now nearly 9 years old and has proven to be a very good fridge even in high temperatures.

dundreamin · Veteran Member

Hi Warren

Good idea. I might have to organise a small esky for myself too. I usually either buy something or try to have something ready that doesn't involve opening the fridge door because I've found that by opening the door 2 or 3 times to get stuff out then again to put it back the temp goes up really quickly. Add another couple of hours travel time onto that & the fridge is up to about 10-15° by the time I stop for the night. Might also try the frozen water bottles in the fridge as well, another good idea. Thank You.

Di D

-- Edited by dundreamin on Tuesday 4th of June 2024 04:05:20 PM

dundreamin · Veteran Member

I'm trying to remember to switch the gas on when I stop but sometimes if it's just for a 20min m/tea stop I don't bother, thinking it's not going to be of much benefit in 20mins but perhaps I'm wrong about that. If I'm going shopping or doing a tour or something, I always put the gas on. I do think my compressor fridge in my old motorhome was better but it would on occasion freeze stuff.

Thanks

Di D

-- Edited by dundreamin on Tuesday 4th of June 2024 04:13:54 PM

Cupie · Guru

The frozen water bottles is a good idea but I prefer to empty a few wine casks to do the same job. As a Van Park regular I use the camp fridge/freezers to freeze the part filled cask bladders. If I forget to pick them up before leaving then no real loss, I just have to drink a bit more to get a replacement.

Our Engel is used instead of an Esky for the milk & butter ... if I remember to load it up, assuming also that there is room amongst the wine & beer. I think that I'll adopt the Esky & frozen bricks option in future.