Installing a 12V power socket in your car for a portable fridge.
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Article by Val Rigoli ©, free advice freely given, my personal thoughts and advice gathered from in-excess of 50 years of practical hands-on experience, learned skills, and industry knowledge.
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Wiring, or to be more precise wire sizing is without doubt one of the biggest problems that cause people grief, I have load tested the standard cig/lighter type outlet fitted to the rear of many 4x4’s, and all have come up short so far.

Most of these outlets state "120 watts Max", but when I put just a 100 watt (8 Amps) load on them they immediately drop about 2 volts, i.e. battery reads say 12.6 volts but the outlet is only receiving 10.6 volts.

The wiring is simply not heavy enough to supply a reasonable current at the required voltage, and this is crucial.

Now some people do get away with it ok because when the vehicle is running the alternator lifts the voltage up to around 13.9 - 14.5 volts, so at the plug you have around 12 volts, but as soon as you turn the motor off the voltage drops way low.

The simple answer is to fit a much heavier cable (wire) to a new and better outlet, as the cig/lighter outlet is far from the best for good electrical contact anyway, with these outlets it only takes a small bump to disconnect the plug, and when you arrive at destination ‘B’ you find the fridge has been off for hours.

I’ll get to the plug/outlet options in a minute, but the cable is our first priority, now wires ain’t wires, here in Australia we have a weird way of grading cable, the standard cable "Auto Cable" is not what it seems, like if it is marked 6 mm then you will find that it’s only 4.59 mm square, and that’s what really matters, how much copper core it has in square mm², not how thick the overall cable is.

I won’t bog us down with all the tech ramblings on cable here, but if you want to know a bit more about this problem please visit my easy to understand article on wire sizing and voltage drop problems, >>Click here>>cable-voltage-drop<<

You can do this simple rewire yourself, and it should cost you under $50 all up, and then you will never have a problem.

My recommendation here is to run what is called 8 B&S cable, it’s a single cable of 7.91 mm², run from you battery to where you are going to place the new plug socket, if it is in the back of your car it will be about 4.5 metres (about $5 per metre from ebay, JayCar, SCA etc), you should also put at the battery a 30 Amp Midi fuse (my preference) or a 'short stop' 30A circuit breaker to protect this cable.

You can run this cable through the body and under sill covers etc, or strap it on top of the chassis rails, the main thing of course is to make sure it can’t rub or chafe through anywhere, and is well protected from sharp corners etc.

The earth return can be just a short length of cable, attached to the vehicle body fitted close to the socket using a nut and bolt with a sand papered ‘bright steel’ contact (don’t forget to paint/protect afterwards), just sticking it under a nearby screw is very bad practice and will not give you the long term sound electrical contact that you need, and of course its worth taking particular care when crimping and fitting all connections and terminations, get it right now, and then you can forget about it , and not continually chase up problems like you may have seen your mates do.

Updated 2022 update
Because the price of Anderson plugs found on the likes of eBay have reduced so much in price (about $5 - $10 for a set of two) it is now far less expensive to use these fantastic robust plugs that offer great electrical contact, and won't come loose, even better than the plugs and sockets that I have recommended in the past. So while you may still like to fit a standard socket for other accessories, I think that you should seriously consider fitting an Anderson plug as well for your fridge.

This sentence is sort of obsolete now considering what I just said above, but still may suit some people .....................Now for the plug/socket etc, if you going to run a compressor type fridge (Waeco, Dometic, evaKool, Engel, etc) one of the best fittings I have found for this job is the HELLA type socket, and the universal plug, these sockets are well made and provide good electrical contact and a positive click in. and allow you to retain the standard plug on your fridge.

Acc Sockets (41708 bytes)

The universal plug (fitted standard to Waeco/Dometic and many other fridges) can be used in a standard cig/lighter socket, and will also fit the HELLA type socket.

If however you are using a Finch/Chescold adsorption type fridge, these fridges draw a constant 10.5 amps and need a much heavier connection, here I would suggest the "Anderson" type plug (now really cheep on eBay) rated at 50 Amps it’s a sure, safe and easy way to transfer heavy currents.

Even with the best quality plugs and sockets you can still run into problems (except with the Anderson plugs), this is because the fridge draw current is somewhere between 3.5 and 6.5 amps (depending on the size of the fridge) and sometimes for long lengths of time, often the plugs and sockets get so hot that the plugs start to melt, and the fuse inside it sometimes looses contact too, anyway the bottom line is you can end up with poor fridge operation and efficiency.


So here is a hot tip for anyone that likes to use their fridge mostly in the one spot (i.e. back of the car/ute/caravan/camper), spend an extra $25 or there abouts and buy a spare lead for your fridge, then chop off the cig plug from one of the two leads that you now have,  add an in-line fuse and either hard wire it directly into your nice heavy duty cabling that joins to the battery, or fit Anderson plugs.
So how is that, no more problems with loose and falling out plugs, melting and losing contact etc, and if you do want to take your fridge out, like put in the wife's car, or the mates boat etc etc, you just unplug the lead at the fridge, grab the spare lead and plug it into the mates 4x4 etc, how cool is that ?

Big trap, please don't get caught....Note on wire & cable....

A real trap for the unaware is that auto cable is often marketed as 10-amp, 30-amp, 50-amp and so on, this can be very misleading, the rating is simply the current the wire can carry before the cable gets so hot that the plastic cover starts to melt, it has no other real meaning of any consequence!!

It cannot for a moment be assumed that a cable described as '30-amps' can carry even half that current over any distance without a totally unacceptable voltage drop, so it pays to be very aware of the mm² size of the copper in the cable, and disregard almost all else, bigger is always better, it is a resistance over distance thing, the further the electrical current has to travel, the larger the cable must be, or the voltage drops to an unacceptable unusable level.

All of the items I have talked about can be purchased from any of the better auto electrical shops.

Well that’s about it, sorry if this has become a little long winded, but I hope this helps clear up some of the camp-fire mythology around wire and cable etc.

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