Powering fridge in 4wd with no aux battery

“3 way fridges do not as a rule work very well on 12v,”

That generally only applies when insufficient wiring is involved. That myth was propagated in the 1990s with the Electrolux fridges of the day. Those of us who upgraded the cabling in the vans and tugs proved that furphy to be wrong.

With right size wiring they can work fine..BUT ..they are just too power hungry to run off 12 volts for even short amount of time . Pulling 20 amps could see an AGM of 100a/h flat in 4 hours.

From the charts I have seen, an absorption fridge will draw something between 15 to 20 amps per hour when run on 12 volts. This amount of current draw is likely to see a cranking battery below the voltage/amperage required to start a large Diesel engine (3.0+ Ltrs.) within a couple of hours.

Macca.

Your right McLaren3030...I forgot that he want using an AGM. 2 hours would be risky ..he may only have a smallish start battery as well.

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Snoopy, I will address your questions, which were.....

1. I do have a Redarc SB12 isolator that he could use which should?? prevent draining the battery if/when he stops and forgets to disconnect fridge/s.

2. He would need 11AWG wire for one fridge and 8AWG for two assuming both pulling 10 amps.

3. Can an alternator on a 2004 Patrol handle a load of 10 or 20 amps to run fridges?

So Snoopy, my response is.....

1. Yes, a Redarc SB12 isolator should be OK to prevent draining your battery. It will connect the fridges at 13.2v as the alternator begins to charge and disconnect them as the voltage falls below 12.7v. There is a 5 second delay on connecting which is good as the alternative of a ignition-operated relay allows the fridge load to be applied during cranking.

2. Cable sizing may be OK but depends on cable length and fridge current which you have not specified.

3. Possibly, but I do not know the specs of a 2004 Patrol alternator. Someone else may? However, I believe that your two fridges together will draw more than you estimate, possibly as much as 30-35A.

In general, the arrangement you describe should work OK and certainly protects the cranking battery from being depleted by the fridges which is an absolute necessity. To operate the fridges without an auto-disconnect device would be a problem waiting to happen!

You would be wise to consider the advice given by responders above re the statutory requirements and hazards of gas appliances within a vehicle. It is a real danger that has cost lives.

EDIT: I have edited my answer re the isolator operating voltages. Heavens only knows how I expressed it so wrong before!

On recent Gibb River trip, we had problems with the 12v system in our caravan, and as a consequence, had a very reluctant 3-way fridge.
On speaking with an experienced fridgie in Kununurra, he mentioned that these fridges lose 1°C for each hour they are running on 12v. This would agree with Bazooka’s observations.

Bob

Interesting Bob. Ime 3-ways were excellent on 240V, too efficient on gas (had to be dialled back to stop things freezing - don't know how or why), and marginal at best on 12V. Great for long stays.

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Looks good to me Snoopy.
I would definitely use 6 B&S for each fridge from the isolator.... A 3-way fridge needs all the help it can get!

" Distance from battery to fridges is 5 metres so I have calculated distance as 6m. "
From a voltage drop perspective, the cable distance is 11m.
Cheers,
Peter
OKA196 motorhome

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Peter, I think that Snoopy took the return circuit into consideration in his calculation.

From Snoopy's post: "I have used numerous online calculators for cable size and the consensus seems to be that 8 b&s (7.7mm sq) over 5m will see 4.1%voltage drop."

So I don't think he took the return circuit into consideration. If so, the voltage drop is going to be more like 10% using 8B&S over 11m.

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Kier & Marg and Peter,

You are quite right about the volt drop.
Somehow I got the impression that Snoopy would be using separate runs from the crank battery to each fridge. On re-reading, it is clear from his totalling to "22.5 Amps" that I made a wrong assumption. He has not allowed for the return circuit.
My most humble apologies to you both.

My recommendation to use 6 B&S to EACH fridge @ 12A would see about 0.32 volt-drop which would be reasonable for a 3-way fridge as they need all the juice they can get. The thing about 3-way fridges is that they rely on a resistive element for energy. If the voltage drops, so proportionaly does the current and as the watts of power produced is a function of the square of the current (W=I*I/R) the loss in energy is also squared. So as an example, a loss of say 0.5 volt (4%) causes a loss in energy of 16% which is very significant in the fridge's performance. Scrimping on cable size is an exercise in futility.

Hi Allan,
Great advice and I'd do the same as you and go for 6B&S for the best result. We run 6B&S via Anderson plugs directly to the fridge (via a Fridgeswitch) and can quite comfortably carry ice cream in the freezer whilst on DC on a long drive; the grandkids love us for it.

Just a minor pedantic correction on the power formulae:

It's W=I*I x R, or even easier, W=V*V/R

Cheers, Keir

I use this....
VOTLAGE DROP =
[cable length (in metres) X current (in amps) X 0.0164] divided by cable cross-section in mm.sq.
Cheers,
Peter
OKA196 motorhome

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Yes, of course it is Keir. Thanks
I wish I could select superscript on this thing.

Hi Allan,
Yes, superscript would really help. On some platforms, you can use Alt+numbers for alternate keyboard characters, but not this one it seems.

Hi Peter n Margaret.
I like your formula for the voltage drop which includes the wiring cross-section and cable length. Presumably the cable length is the cable run x 2????

Cheers, Keir

"Presumably the cable length is the cable run x 2????"
Correct.
Cheers,
Peter
OKA196 motorhome

In my experience, absorption fridges struggle more with high humidity along with high ambient temperatures. High temperatures alone are one thing, but it is the high humidity that really puts a strain on the condensing capability.

Macca.

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You are quite right Macca.
The 'condenser' on the fridge must transfer heat to the atmosphere but heat transfer decreases as relative humidity increases due to lower air thermal conductivity. The same applies to compressor fridges.
Here is one article that deals with that.