3-way fridges, fooling the 12v.

[Nigel Crompton]

If I have a superabundance of Lithium and solar capacity can the dometic three-way fridge be fooled into always using 12v ?

The Dometic fridges in the previous 2 motorroams had a wire that I used to trigger 12v operation when the panels had surplus power, to reduce gas consumption.
I'm presuming the current one has similair and could therefore use 12v semipermanently. (I'd have it switched so I could get the autochangover to use gas.)

I doubt I'm the first to think of this so if anyone can point me at a thread ... ??

 

[Richard n Ann]

You need an S+ terminal on the fridge and solar controller. Never done it but someone will be along soon

 

[chaser]

If I have a superabundance of Lithium and solar capacity can the dometic three-way fridge be fooled into always using 12v ?

The Dometic fridges in the previous 2 motorroams had a wire that I used to trigger 12v operation when the panels had surplus power, to reduce gas consumption.
I'm presuming the current one has similair and could therefore use 12v semipermanently. (I'd have it switched so I could get the autochangover to use gas.)

I doubt I'm the first to think of this so if anyone can point me at a thread ... ??

Get yourself a compressor fridge then you only need 12 volt to it.

 

[Guigsy]

It can be done. I assume you're aware how hideously inefficient 3 way fridges are on electric? Roughly 200w of draw, which adds up very quickly! And yet they sip gas.

 

[Nigel Crompton]

It can be done. I assume you're aware how hideously inefficient 3 way fridges are on electric? Roughly 200w of draw, which adds up very quickly! And yet they sip gas.

Ah, thank-you Guigsy. I didn't know that, so I'll stick with gas. This current camper it's awkward to run a cable from fridge to MPPT. System worked fine on my Pilotes

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[Nigel Crompton]

You need an S+ terminal on the fridge and solar controller. Never done it but someone will be along soon

Yep, done it twice already, but not on current van - too awkward.

 

[Guigsy]

Ah, thank-you Guigsy. I didn't know that, so I'll stick with gas. This current camper it's awkward to run a cable from fridge to MPPT. System worked fine on my Pilotes

If your batteries are topped-off by lunch time from a massive array of solar panels on the roof, then running a 3-way from the otherwise wasted solar energy can be done. But given the limited use case and the increase in complexity, I'm not convinced it's worth the hassle.

 

[Barriesimpson]

My Thetford 3 way apparently will only maintain the cold when using 12v and doesn't actually cool anything. So in my case I no longer use it on auto rather I manually set it to only use gas.

 

[VXman]

I have done it by running an EHU cable from the inverter, out the window to the external EHU socket. (obviously running on 240v not 12)

 

[tonyidle]

My Thetford 3 way apparently will only maintain the cold when using 12v and doesn't actually cool anything. So in my case I no longer use it on auto rather I manually set it to only use gas.

That's a problem caused by poor installation. The wire supplying 12V to the fridge is too thin causing excessive voltage drop. Provided with full supply voltage when driving performance is the same as when on 240V.

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[Spriddler]

My 3 way Dometic performs well on gas and 230v but on 12v (whilst driving) after being precooled on gas or 230 to 5deg it can hardly maintain that temperature, even on a day with a modest midday ambient of around 20deg.

Edit.
Ah, I was posting at the same time as tonyidle 's reply above (undersized feed). I'll check the '12v' voltage at the fridge.
I've had to upgrade the feed to the main distribution panel already - I gather from various threads on here that undersized wiring is a fairly common issue with Brit m/homes.

 

[Guigsy]

That's a problem caused by poor installation. The wire supplying 12V to the fridge is too thin causing excessive voltage drop. Provided with full supply voltage when driving performance is the same as when on 240V.

My fridge on mains performs about the same as on 12v... Not great. We'll often plug the van in on the drive the day before we go away. It will take a full day for the fridge to get down to 5C. It's much faster on gas.

 

[Brains]

I have a 12v cool box (only 20 litres) that works off the cigarette lighter socket.
It would eat a leisure battery for breakfast if permitted!

If I leave it running for around 2 hrs without starting the engine, the battery is notably depleted.
I have never found out how many hours it would take to totally flatten a vehicle battery, but I suspect in the region of 6-12 hrs.
You certainly can not leave it on overnight or on a long ferry run.

That said, if you put in a couple of litres of frozen milk or water, and fill up all the remaining space with pre chilled cold food and drinks, it will keep the contents cold for ages.

 

[Barriesimpson]

That's a problem caused by poor installation. The wire supplying 12V to the fridge is too thin causing excessive voltage drop. Provided with full supply voltage when driving performance is the same as when on 240V.

I don't think that is correct, the manual states that it will only maintain the temperature.

 

[Guigsy]

I have a 12v cool box (only 20 litres) that works off the cigarette lighter socket.
It would eat a leisure battery for breakfast if permitted!

The most efficient cooling is the vapour compression cycle, which is used in home fridges and 2 way motorhome fridges.

Next is the absorption cycle, which works on heat to drive the cooling liquid around the system. In motorhomes, these use about 3 times more energy than compressor fridges. But gas heat is easy and there's no moving parts.

By far the least efficient is Peltier effect cooler technology. Low cost, compact and robust. No liquids or moving parts. Just a couple of metal plates in a sandwich. But they are about 3 times less efficient again than the absorption cycle.

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[Captain Yoghurtpot]

Get yourself a compressor fridge then you only need 12 volt to it.

Ah yes. But then I think you need either a n abundance of lithium battery power and / or a fair bit of solar.
Ours, working from a 12v new liquid lead acid battery takes a fair bit of power on start up and running from the battery, so much so that I only run it to get the fridge really cold, then switch it off, and let the insulation keep it quite cold, and the battery recover.
If on ehu I then switch on the charger to top- up the battery.
I don't have solar ( but might get some ) and the 12v compressor fridge does not seem to like being powered on 12v IF the 240v charger is is in operation at the same time, ie effectively trying to run it as if on 240v.
And the charger also does not like that arrangement as it can get quite hot.
Only my thoughts, but I am probably doing something wrong somewhere.

 

[tonyidle]

I don't think that is correct, the manual states that it will only maintain the temperature.

The heat output of the two elements is similar. As most 3 way fridges are in use in caravans with a lengthy supply cable and an outdoor plug and socket the manual is probably correct. In a MH, and correctly cabled, it will cool the fridge in much the same manner as a mains supply.

 

[Barriesimpson]

The heat output of the two elements is similar. As most 3 way fridges are in use in caravans with a lengthy supply cable and an outdoor plug and socket the manual is probably correct. In a MH, and correctly cabled, it will cool the fridge in much the same manner as a mains supply.

That's interesting. So I would need thicker cables for the 12v side?

 

[Captain Yoghurtpot]

That's interesting. So I would need thicker cables for the 12v side?

Interesting indeed.
Have had some doubts over our own 12v compressor fridge and cable suitability.
But again, I could be wrong.
And often am!

 

[peterc10]

If your batteries are topped-off by lunch time from a massive array of solar panels on the roof, then running a 3-way from the otherwise wasted solar energy can be done. But given the limited use case and the increase in complexity, I'm not convinced it's worth the hassle.

Not complex at all. All you need is a thin control wire from the S+ terminal on your solar controller to the S+ terminal on the fridge and it is all automatic. The only problem is that AFAIK Victron do not have an S+ terminal on any of their solar controllers. That is why I fitted a Votronic MPPT solar controller rather than a Victron. When it came to installing more solar and another controller I went for Victron, because the Votronic will still work as it should.

On sunny days and with 450W of solar on the roof the fridge will run on 12v for over half the time.

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[tonyidle]

That's interesting. So I would need thicker cables for the 12v side?

It depends on the length of the cable run. There are (on most modern fridges) two 12V supplies: one is to to supply the fridge electronics and size is relatively unimportant; the other supplies the 12V element and needs to be sized for 10A. Adjacent to that cable on the fridge connector is the earth connection which also needs to be sized for 10A. There are tables on various Internet sites that help with 12V cable sizes. Someone will hopefully pop up with a link.

 

[Guigsy]

Not complex at all. All you need is a thin control wire from the S+ terminal on your solar controller to the S+ terminal on the fridge and it is all automatic. The only problem is that AFAIK Victron do not have an S+ terminal on any of their solar controllers. That is why I fitted a Votronic MPPT solar controller rather than a Victron. When it came to installing more solar and another controller I went for Victron, because the Votronic will still work as it should. .

But won't that just take power from the cab battery?

 

[Nigel Crompton]

Victron do not have an S+ terminal on any of their solar controllers. That is why I fitted a Votronic MPPT solar controller rather than a Victron. When it came to installing more solar and another

I remember now, I fitted a Victron MPPT to this van, previous two had been voltronic. That's why I didn't wire it up!! (that and I would've had to crawl under the van)

 

[peterc10]

But won't that just take power from the cab battery?

Not in my Hymer. Hymer, like many other German vans, uses the hab batteries for running the fridge on 12v and use the alternator to charge the hab batteries. Every half hour it checks the hab batteries. If they are full it switches on the hab batteries charging for a half an hour and then checks again at the end of the period. If the hab batteries still have a good charge they continue to use them. If not they switch off until one of the next check shows the batteries are in good condition again, and then switches it back on.

 

[Guigsy]

Not in my Hymer. Hymer, like many other German vans, uses the hab batteries for running the fridge on 12v and use the alternator to charge the hab batteries. Every half hour it checks the hab batteries. If they are full it switches on the hab batteries charging for a half an hour and then checks again at the end of the period. If the hab batteries still have a good charge they continue to use them. If not they switch off until one of the next check shows the batteries are in good condition again, and then switches it back on.

Exactly. Just adding an S+ wire alone will leave it connected to the cab.

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[Nigel Crompton]

Exactly. Just adding an S+ wire alone will leave it connected to the cab.

I think you mis-understand Guigsy. In my two previous Pilotes the Voltronic MPPT was connected to the fridge and the hab battery. Only.
The only connection to the cab battery was via a Stirling B2B trickle charger from the hab battery. When the sun shone the hab batteries powered the fridge

 

[DBK]

The heat output of the two elements is similar. As most 3 way fridges are in use in caravans with a lengthy supply cable and an outdoor plug and socket the manual is probably correct. In a MH, and correctly cabled, it will cool the fridge in much the same manner as a mains supply.

It depends on the specific model. I just looked at one Dometic fridge and the 12 volt element was 60W the other one 175W.

 

[Guigsy]

I think you mis-understand Guigsy. In my two previous Pilotes the Voltronic MPPT was connected to the fridge and the hab battery. Only.
The only connection to the cab battery was via a Stirling B2B trickle charger from the hab battery. When the sun shone the hab batteries powered the fridge

For most vans, isn't the fridge 12v supply wired to the cab battery/alternator? Telling it to switch to 12v mode with an S+ wire when you're parked up will drain just the cab battery won't it?

 

[chaser]

Ah yes. But then I think you need either a n abundance of lithium battery power and / or a fair bit of solar.
Ours, working from a 12v new liquid lead acid battery takes a fair bit of power on start up and running from the battery, so much so that I only run it to get the fridge really cold, then switch it off, and let the insulation keep it quite cold, and the battery recover.
If on ehu I then switch on the charger to top- up the battery.
I don't have solar ( but might get some ) and the 12v compressor fridge does not seem to like being powered on 12v IF the 240v charger is is in operation at the same time, ie effectively trying to. run it as if on 240v.
And the charger also does not like that arrangement as it can get quite hot.
Only my thoughts, but I am probably doing something wrong somewhere.

Yes but he says he has got solar and plenty battery

 

[autorouter]

I don't think that is correct, the manual states that it will only maintain the temperature.

Look up the wattage of the 240V and 12V elements. If they are both about 200W then they should be equally effective in cooling. If the 12V one is 100W then as you say it will only maintain an already cooled fridge. I think some of the older models had 100W elements, which is where the story came from.

200W is about 15A at 12V, so there is also the possibility of a fridge with a 200W element and thin wiring, with a lot of voltage drop, so not getting much more than 100W of heat from the '200W' element.

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