Caravan fridge -would this work?

Submitted: Saturday, Mar 01, 2014 at 12:25
ThreadID: 106488 Views:5381 Replies:11 FollowUps:25
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I want to run my 3-way fridge on 12 volt setting. I have thought of running wiring from my car battery with ignition relay switch to Anderson plug but am wondering if what I propose below would be simpler and would work OK?
In the boot of the car there is a cig outlet that I use successfully to charge an attached auxiliary battery while driving. When the car ignition is not on the cig outlet does not work so It must be wired into the car ignition.
If I were to attach an Anderson plug to the auxiliary battery and then connect this to the caravan fridge which also had an Anderson plug, I assume the fridge would run OK while driving - the auxiliary battery would charge and the fridge would draw power from this battery. When the car ignition is turned off the fridge would continue to draw power from the auxiliary battery but the car battery would not be charging. My concern is, when the ignition is off, would the fridge also be drawing power from the car battery? Would I, in some way, have to isolate the two batteries so that when the car ignition is off there is drain only on the auxiliary battery. If so, is there a simple and cheap way to isolate the batteries?
Thanks for any advice, John.
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Reply By: The Bantam - Saturday, Mar 01, 2014 at 12:35

Saturday, Mar 01, 2014 at 12:35
The first thing you have to understand about 3 way fridges is....they are very inefficient........think about it..... they cool by heating.....hmmm.

The only reason they are viable at all is that LPG is a very high density fuel and fairly cheap......a 3 way fridge will run a long time burning gass.

On electricity......they are a joke......running on 240 volts..where someone else is paying for the power ( like in a caravan park) who cares....still some of them perform poorly on 240V.

Running them on 12 volts.....Oh hell....soo many people have problems because they do not wire their supply to these fridges anywhere near heavy enough......excessive voltage drop results and the perform poorly indeed.

The issue is they draw quite a lot of current on 12 volts...and they run continuoulsy.

So it is only viable to run a 3 way fridge on 12 volts while the vehicle motor is running.

Running 3 way fridges off battery supply is just not will run an N70 battery stone cold flat in about 4 to 6 hours...doing that your battery will be heading for an early grave.

AnswerID: 527469

Follow Up By: JohnKT - Saturday, Mar 01, 2014 at 12:41

Saturday, Mar 01, 2014 at 12:41
I appreciate they are power guzzlers and really only want a way to run it while driving. I was just thinking the workaround I have proposed would be simpler, and cheaper, than running a direct connection to the car battery.
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Follow Up By: Dave(NSW) - Saturday, Mar 01, 2014 at 12:54

Saturday, Mar 01, 2014 at 12:54
The main problem with your idea is the wiring to the cigarette plug will not be big enough to carry the current the fridge will use.

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Follow Up By: JohnKT - Saturday, Mar 01, 2014 at 13:05

Saturday, Mar 01, 2014 at 13:05
But would not the fridge, under proposed setup, be drawing power from only the auxiliary battery. The car battery would only be powering the auxiliary battery which I have successfully done in the past when I have had no drain on the auxilary battery.
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Follow Up By: garrycol - Saturday, Mar 01, 2014 at 13:21

Saturday, Mar 01, 2014 at 13:21
The wiring to the cigarette lighter socket is just not big enough. Run very heavy duty cable from your aux battery to the rear of the car and put an anderson plug on it - your van should have wiring from the van anderson plug to the fridge but this is often sub standard for a 12v on a 3 way so replace this wiring with heavy duty stuff.

My heating element in my 3 way is 120w both 240v and 12v, so cooling efficiency should be the same but it is not - I also need to upgrade the wiring in my van. On 240v it only draws .5 amp but on 12v it is drawing 10amps so heavy cable is needed.

With my current setup it will keep the fridge cold if it is already cold but will not pull the temp down. When towing it is on 12v all the time I just have to remember not to stop more than about 5 hours before switching over to gas.

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Follow Up By: The Bantam - Saturday, Mar 01, 2014 at 13:26

Saturday, Mar 01, 2014 at 13:26
Batteries are storage only.

The fridge demand will be suppied from the alternator as soon as the battery terminal voltage drops...that will be nearly straight away.

result the battery will not be charged and the voltage at the fridge terminals will be low...resulting in inefficiency all round.

The is no free lunch and there is no way arround running a heavy cable to supply the demand from you 3 way fridge.

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Follow Up By: Ross M - Saturday, Mar 01, 2014 at 13:35

Saturday, Mar 01, 2014 at 13:35
The auxiliary battery will always accept some amp flow when fully charged, perhaps not much but a little + you will have the 10 to 15 amp fridge running off the supply wire.

As mentioned above, the vehicle wiring will be a bare minimum (manufacturers don't fit big wire to anything) to run small stuff and nothing like the load of the fridge and a battery.

Therefore, the wiring will act as a resistor and lower the alt voltage to the fridge resulting in the fridge barely performing and also the aux battery becoming discharged to some degree.

The success you have had in the past may not have detected the loss of charge from the aux battery in such circumstances, so it seemed to be OK but wasn't really working well.

Unfortunately, 3way fridges take a long time to start cooling after stopped, and then have to play catch up in order to TRY and pull down internal temp.
So, effectively, for a long time after restarting the fridge on the 12v (even with and excellent 12v supply) the fridge won't be cooling for a large part of your journey and you then arrive with the fridge just beginning to do it's stuff. Hardly good for the food inside. Warm beer each day.

Been there done that, had two 3way fridges which both failed internally despite tender care and now only regard them as sort of ok if camped for long periods or at the holiday shack.
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Follow Up By: JohnKT - Saturday, Mar 01, 2014 at 13:46

Saturday, Mar 01, 2014 at 13:46
Thanks for your replies. I have worked from the assumption that the existing car wiring to the cig outlet is OK for charging a AGM battery in the boot and have found this to be the case. My hope was that if I run heavy wiring from the aux battery in the boot and to the van fridge that the draw will be on the aux battery primarily and not on the car battery but, as I read your replies, it seems the draw will be on both batteries. I can't think of a way of getting around this.
Garry, have you thought of routing you wiring via an ignition relay switch so that when the ignition is turned off there is no drain on the car battery. I am sure you have and when I have investigated this the bit where you would have wire via an ignition relay is a little daunting for me.
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Follow Up By: garrycol - Saturday, Mar 01, 2014 at 14:35

Saturday, Mar 01, 2014 at 14:35
John - there is no need on my case - in a worse case scenario I am looking at about 10 hours running on 12v before the car aux battery got low and then I have the van house battery on top of that and the starter battery (protected from discharging completely) so in reality the fridge could run for up to about 25 hours on the batterys before there is an issue.

I only use the 12v side of the fridge when towing and as soon as I stop for the day I switch to gas. If I were to stop for an extra long time during the day I would switch the fridge to gas while stopped.

Irrespective of this standard car wiring is sub standard - if you are charging an AGM from your rear socket I think that if you did some checks on loaded output voltage it will be low and it will either be taking a long time to charge your battery or not fully charging it.



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Reply By: Kevin.Hutch - Saturday, Mar 01, 2014 at 13:39

Saturday, Mar 01, 2014 at 13:39
If as you say the cig outlet socket in the back of the car is disconnected when the ignition is off (this is common) then yes the fridge iconnected to the aux battery will be running only on the aux battery.

When the aux battery gets down the fridge should shut down at a preset voltage (most do), then when the ignition is switched back on the aux will try to charge and may exceed the current capacity of the fuse protecting the cig lighter outlet (possibly 20amp should be marked on it) if it does then you will need a regulator to control the level of the charge to below the size of the fuse. A crude way would be a thermal circuit breaker of say 15amp to get the aux battery up in steps as this resets when it cools off.

But I agree with other comments that the thermal three way fridge is power hungry and the aux battery is a compromise, if it is too small it will not run for long yet if it's too big it will not get enough charge on short runs.

Kevin H
AnswerID: 527475

Follow Up By: JohnKT - Saturday, Mar 01, 2014 at 14:07

Saturday, Mar 01, 2014 at 14:07
Thanks, Kevin. Sounds like my idea is not a really good workaround.
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Reply By: JohnKT - Saturday, Mar 01, 2014 at 13:56

Saturday, Mar 01, 2014 at 13:56
As an option has anyone had experience with the Waeco RAPS power kit as a solution to wiring fridge from car battery.
AnswerID: 527478

Follow Up By: SDG - Saturday, Mar 01, 2014 at 14:10

Saturday, Mar 01, 2014 at 14:10
I had one in my last car. Worked well. No problems at all. Bit expensive though, and might have been able to make up my own.
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Follow Up By: The Bantam - Saturday, Mar 01, 2014 at 17:03

Saturday, Mar 01, 2014 at 17:03
Firtsly that power kit is designed and intended for compressor fridges, with typivally draw less than half what your 3 way does..when they are running and if they are running well and running as a fridge thay rin about 20% to 30% of the time.

John mate there sinply is no way arround it...if you wnat a 3 way fridge to run and run well all the way back there in the trailer.

You need to run heavy wire all the way back to the battery.

10mm solar twin would probaly be barely adequate.

50 amp anderson plugs are designed arround #6 B&S..that is arround 14mm2......probaly the smallest realy practical for the job.

There are plenty that would have no hesitation of running 20mm2 it it was along van.

those frdge wiring kits for compressor fridges are typically 6mm automotive that equates to about 4.5mm2...not even in the race.

As for your AGM efficiently charging from the factory fitted ciggy socket in the back of ya vehicle....sorry mate but you are kidding yourself.

FollowupID: 809888

Follow Up By: Kevin.Hutch - Saturday, Mar 01, 2014 at 18:16

Saturday, Mar 01, 2014 at 18:16
Could not agree more, my reply was specifically to answer the original question, it would work, but not well and so many valid reasons have been given to recommend against it.

If you are committed to the three way fridge, I see no real solution other than large wires from the alternator/battery.

Plus to reiterate; voltage drop on inadequate wires is the most common problem with inefficient fridge operation and confusion over wire sizes the most common reason for inadequate wires.

Pythagoras worked out the conversion from diameter to area of a circle a few years back but it's yet to be introduced to auto electrics.

I must admit I overlooked the 12v feedback from the aux battery to the other accessories, a very valid issue unless the manufacturer has been clever enough to isolate them with the ignition driven relay.

Kevin H
Kevin H
FollowupID: 809891

Reply By: Member - John and Val - Saturday, Mar 01, 2014 at 14:36

Saturday, Mar 01, 2014 at 14:36

As already said - No - for all the reasons given above, don't do it!

Another factor that hasn't yet been mentioned is that hanging a battery on the accessory circuit when that circuit is switched off can have some unintended results. All the vehicle accessories will be able to draw current from the aux battery, which may or may not be a problem. Any computers in the vehicle (and there are plenty in recent vehicles) may become very confused if they find voltage where there isn't meant to be any.

In an older vehicle I once had the problem of not being able to stop the engine. It was a freezing cold wet night and thanks to a broken earth line power was fed to the trailer lights, then back to the ignition system via the double filament stop/tail lamps. I never got fully to the bottom of it, but since then I've been sensitive to issues arising from having voltages in the wrong place! (and that was before computers were introduced to add extra confusion.)


J and V
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Reply By: Member - Chris_K - Saturday, Mar 01, 2014 at 15:38

Saturday, Mar 01, 2014 at 15:38
Agree with all the above about 3-way fridges and trying to run them on 12 volt in the car. Here's what happens if you don't like the 12v results and try to run a 3-way fridge on gas while in the really have to wonder sometimes.

AnswerID: 527485

Follow Up By: Ross M - Saturday, Mar 01, 2014 at 16:38

Saturday, Mar 01, 2014 at 16:38
Nice Camo paint job. Blends in well with Aussie terrain.
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Follow Up By: Member - bbuzz (NSW) - Saturday, Mar 01, 2014 at 21:00

Saturday, Mar 01, 2014 at 21:00
My fridge outlet in the back of the Prado is wired from the Anderson plug located at the back of the vehicle which was wired to service the van. (HD wire).
I ran short leads to a 20amp plug socket into which the fridge is plugged. No overheating of the wires and directly wired to the auxillary with a fuse in that line too.

Bill B

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Reply By: steved58 - Saturday, Mar 01, 2014 at 23:50

Saturday, Mar 01, 2014 at 23:50
My first caravan had a 3 way fridge set up to run off the start battery through a 7 pin plug ran badly and I got flat batteries if I did not disconnect the van when stopped I then bought a waeco for the back of the car because the van fridge did not work so good that I plugged in the cig plug more problems the poor battery Well to cut it short after many dollars I now have no problems with any fridge heavy wiring dual battery isolator 3 batteries in the car 1 in the van and a fridge switch on the 3 way portable solar panels and a back up generator The point I am making is there is no cheap way it will just end up costing more put in a dual battery isolator and heavy wiring at the minimum dc to dc charger even better If you must and are only travelling for a few hours a day get it cold freeze some bottles of water in the freezer put these in the fridge when travelling and have the fridge off then run on gas when stopped or 240volt at the caravan park put the bottles back in freezer and same again the next day please note donot forget to switch off fridge from gas when stopped
Hope this helps
AnswerID: 527506

Reply By: BJ & Reen - Sunday, Mar 02, 2014 at 11:10

Sunday, Mar 02, 2014 at 11:10
Hi John
I have a three way 185 lt fridge freezer it is wired with 14mm wire from the car battery(only 1 battery) through a relay down to a Anderson plug on the tow bar then on to the house batteries (2x100 ah)from there straight to the fridge same 14mm wire .
If I leave camp fridge temperature say 4 degrees travel 6 hours in summer it will be
around 8 degrees on arrival plus car keeps house batteries charged, if I stop to shop find some where flat and turn on gas while stopped.
Have had no trouble so far been on the road 2 years no trouble even in the north with the heat hope this helps.
Regards BJ

PS our fridge is tropical rated don't know if that helps might be all bull
AnswerID: 527515

Follow Up By: landseka - Tuesday, Mar 04, 2014 at 16:18

Tuesday, Mar 04, 2014 at 16:18
Hi BJ (hate to think what that stands for lol)

There is no "bull" regarding 'T' Climate Rated fridges.

I cannot understand why van manufacturers fit anything else in this country.

I would try to turn your thermostat down a tad though. The aim should be no higher than 5°c anytime. Aim for 0° - 1°c in the morning then late afternoon you should see 4.5° - 5°c before it starts coming down again.

The 12v cable from my car goes directly to the fridge. The house battery is Solar charged only.

Cheers Neil
FollowupID: 810071

Reply By: Member - Bruce C (NSW) - Sunday, Mar 02, 2014 at 11:11

Sunday, Mar 02, 2014 at 11:11
You are on a hiding for nothing running a three way fridge on 12 volts.

My advice is "Don't do it". It will flatten batteries PDQ.

If you absolutely positively must do it then run a dedicated heavy circuit from main battery, via a battery isolator, to an Anderson plug on the back of your vehicle and another dedicated heavy circuit on the van to the Aux battery. Then power the fridge from that.

With regard to disconnecting one battery from the other, use a battery isolator. very simple to fit Like this one

Fit as much solar as you can afford on the roof of the van to keep the battery charged that way as well. Also think about updating the fridge to a more efficient modern three way fridge. Then you wont need to power it while traveling unless well north of Alice Springs.

When touring etc. we never run our three way fridge on 12 volts and just turn it off and keep the access to a minimum during stops. We run the fridge on gas when stopped during lunch breaks ensuring it is turned off again before resuming travels and we have traveled all day like this many times with no adverse effects. The more things you have in the fridge the better it will keep cool. The fridge in our van is about a 2006 manufactured unit and is very efficient and works very well providing nice cold beer at the end of a days travel in the manner I have stated.

Cheers, Bruce.
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Follow Up By: garrycol - Sunday, Mar 02, 2014 at 12:35

Sunday, Mar 02, 2014 at 12:35
3 ways are marginal at the best of times - why would you turn it off when driving when there is a perfectly good 12v supply from the engine when it is running. For sure the fridges run worst on 12v but it will at least maintain temperature or only marginally increase when running on 12v - the temp will certainly rise when it is off.

Doesn't make sense to me - a bit like my old uncle who would only turn his mobile phone on the afternoon supposedly to safe the battery but it was normally on charge in the afternoon - doesn't make sense.
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Follow Up By: Member - Bruce C (NSW) - Sunday, Mar 02, 2014 at 16:23

Sunday, Mar 02, 2014 at 16:23
Many of us have found that these later 3 way fridges work fine if left unpowered and access is kept to the minimum for a days travel. We have not had any food spoil nor warm beer, in fact the beer is as cold as if the fridge was on all day.

While you make a lot of logic with your comments and I agree, on face value, the fact is that it works well the way I have said and therefore I have found no need to run the fridge on 12 volts while traveling, trust me, my fridge works exactly as I have said.

It surprised me the first time I can assure you. It would not have worked that way with the older 3 way fridges from my experiences and I have owned near new caravans since 1973.

As I have said, many others have recently found this out for themselves also and have stated same on these pages from time to time. So I am far from the only one that does this. In fact it is probably what prompted me to try it the first time.

Not only that but I think these later models are slightly better on gas consumption also. Perhaps it is just that it is a near new fridge.

Cheers, Bruce.
At home and at ease on a track that I know not and
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Reply By: MEMBER - Darian, SA - Sunday, Mar 02, 2014 at 12:05

Sunday, Mar 02, 2014 at 12:05
As an aside to the 12V supply issue, the 'camping strategy' is always a factor...we ran a 3 way for some years in our Jackaroo..12v while mobile via a decent fused cable direct from the aux battery...soon as we stopped it went onto gas (while we were with the car that is) ...this became impractical after we retired because we wanted to leave the car unattended more often... goodbye to the 3 way and got a. compressor version...camping profile changed...setup had to change. Then there is the batteries and solar panels...but enough said :-)
AnswerID: 527517

Follow Up By: landseka - Tuesday, Mar 04, 2014 at 16:05

Tuesday, Mar 04, 2014 at 16:05
I can't imagine that running a 3 Way Fridge on gas inside a car would be legal anywhere. Certainly not very smart IMHO.
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Follow Up By: MEMBER - Darian, SA - Tuesday, Mar 04, 2014 at 20:28

Tuesday, Mar 04, 2014 at 20:28
I certainly may not be smart, but the fridge only ran on gas when we were camped, the car was open and the cylinder was outside on the ground..... as soon as the car had to be closed or we were mobile, it was back to 12V. It was a practical solution for the time and no more dangerous in my view than many other calculated risks associated with camping.
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Follow Up By: landseka - Tuesday, Mar 04, 2014 at 20:44

Tuesday, Mar 04, 2014 at 20:44
Sorry Darian, I didn't read your post thoroughly, my head saw you were driving with it running on gas, re-reading put me straight on that. lol
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Reply By: JohnKT - Sunday, Mar 02, 2014 at 16:39

Sunday, Mar 02, 2014 at 16:39
Below is an attempt to summarise all the information that has been posted in this thread. Thank you all.
- the 3 way fridge should ONLY be run on 12 volt when the car ignition is on and motor is running
- a dedicated line from the car battery should be used for running the fridge
- the line from the car battery to fridge should be heavy duty (min 6mm up to 12mm) and fused
- 50 amp Anderson plug to be used for connection between car and van
- if an auxilary battery is used a battery isolator or relay (alternatively disconnect when ignition is off) and fuse should be incorporated
- have fridge cooled to desired temperature before setting out and then best expectation is that fridge temperature will be maintained rather than temperature being pulled down.

Taking this all into account my present thinking is as follows. Run dedicated heavy duty wiring from main car battery to aux battery in car boot - isolator and fuse inline. From aux battery connect, again with heavy duty wiring, via Anderson plug to fridge in van.
Hope this has covered all bases.
Thanks again, John.
AnswerID: 527524

Follow Up By: JohnKT - Monday, Mar 03, 2014 at 07:50

Monday, Mar 03, 2014 at 07:50
On further reading, minimum cable size would be #6B&S (13mm2).
FollowupID: 809975

Follow Up By: steved58 - Monday, Mar 03, 2014 at 11:51

Monday, Mar 03, 2014 at 11:51
All looks good however as I found out the aux battery can still be flattened by the fridge when stopped I have installed a fridge switch that switches the fridge off when stopped it is a motion controlled switch easy to install takes 5 minutes and not too expensive Fridge switch click here your system will then run perfectly set and forget
FollowupID: 809983

Follow Up By: JohnKT - Monday, Mar 03, 2014 at 15:41

Monday, Mar 03, 2014 at 15:41
You are right, Steve. I've been so focused on protecting the batteries that I was not remembering that the fridge will still continue to drain the aux battery. It is all quite exhausting to think about.
I discovered another complication this morning when researching VSR's. There are three types of alternators used in cars: fixed voltage,temperature compensating and variable voltage alternators. Depending on which type you have, you may need to include in your setup an in-vehicle battery charger.
FollowupID: 809997

Reply By: landseka - Tuesday, Mar 04, 2014 at 16:10

Tuesday, Mar 04, 2014 at 16:10
I have my 3 Way Fridge set up such that it runs on 12v via an Andersen Plug direct from my car battery.

My In House is charged from Solar only unless we are in a van park and there is days of no sun, then I will switch on the 240v charger.

I cannot understand those that refuse to run their fridge from 12v. We all (those with 3 ways) know that the fridge needs an initial run on, preferably 240v overnight to get the boiler running to temp and get the cabinet cool. Why then turn it of to do several hour driving and allow it to cool down again?

Correctly fitted and correctly wired 3 ways are very efficient and great for weeks at a time off the grid at free camps.
AnswerID: 527637

Follow Up By: JohnKT - Tuesday, Mar 04, 2014 at 16:27

Tuesday, Mar 04, 2014 at 16:27
Couple of questions landseka.
What size cable have you used from battery to Anderson to fridge, #6B&S?
How do you isolate your fridge from battery when car motor is not running?
Thanks, John.
FollowupID: 810072

Follow Up By: landseka - Tuesday, Mar 04, 2014 at 16:33

Tuesday, Mar 04, 2014 at 16:33
To be honest John I am not sure but it was wired by a specialist and I made sure he knew what I wanted. It does the job admirably so must be good enough.

My fridge isolation is done by me switching the fridge to gas if we plan to stop more than 15 minutes. Many will 'pooh pooh' this method but, having been caught once I will never forget again!

It would not be hard to put an ignition powered relay in the circuit or a Fridge Switch but I have not found the need. (perhaps if I did forget again.....)

Cheers Neil
FollowupID: 810074

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