Wiring vehicle to camper trailer

Submitted: Wednesday, May 14, 2003 at 14:35
ThreadID: 4910 Views:8119 Replies:8 FollowUps:14
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Well we've finally bitten the bullet and after much research put a deposit on a new camper trailer. In October we take delivery of a 4.2m Coromal Silhouette Offroader.

The deciding factor in our choice was the quality of fittings both internally and externally in comparison to the alternative brands.

But now the fun starts. As part of the deal Coromal supply a Hayes Lemmerz Energize III pendulum type brake controller and a 12v trickle charge kit in the camper all fully fitted. Apparently their auto electrician will contact me in the next couple of weeks to arrange a time to wire my Triton dual cab appropriately.

My question for those in the know is; Is there anything I should be requesting, or even demanding, from the sparky with regard to minimum wire weights, Anderson plugs etc, to facilitate the optimum charging of the auxiliary battery in the camper from the car alternator? I think the plan is to run the wiring through the two unused pins on the existing seven point plug. Is this the correct procedure?

Thanks in advanceLive today as if there may be no tomorrow
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Reply By: Member - Topcat - Wednesday, May 14, 2003 at 15:52

Wednesday, May 14, 2003 at 15:52
Hi Wombat, I would make sure that the wiring on your vehicle is able to carry the current load on the two unused auxiliary pin connections on your trailer plug (if they are going to be used). Some Trailer plug wirings are only light duty just sufficient enough for lights only.
Not knowing the current draw rating on the trickle charger I can only suggest that the wiring is able to carry a load in excess of that required to be on the safe side.
Hope this helps. Cheers.Have Wheels Will Travel
AnswerID: 20033

Reply By: Member -BJ (Sydney) - Wednesday, May 14, 2003 at 16:17

Wednesday, May 14, 2003 at 16:17
Wombat it should be wired to 50amp anderson plug direct from main battery via circuit breaker as close as possible to battery with min 6mm twin cable but any sparkie worth his salt should know this.Wish i was still here / Gulf in July
AnswerID: 20035

Follow Up By: Dozer - Wednesday, May 14, 2003 at 19:24

Wednesday, May 14, 2003 at 19:24
Do u know where this saying originated? (worth his salt)
Same place as to get the sack, and field of expertise :-)
A clue :-
Andy sang Andy watched, Andy waited till his billy boiled
FollowupID: 12772

Follow Up By: Autolec - Thursday, May 15, 2003 at 07:47

Thursday, May 15, 2003 at 07:47
That needs to read 2 x "min 6mm twin cable".
The cable size needs to be a minimum of 10mm2 and preferably 12mm2.
Running 2 x 6mm2 twin sheathed is easier and more practical than running one pair of 10mm2 or 12mm2
It is important to realise the difference between cable sizes. The actual copper conductor size needs to be 6mm2 not the outside diameter of the cable including the plastic insulation
FollowupID: 12815

Reply By: Member - Melissa - Wednesday, May 14, 2003 at 16:57

Wednesday, May 14, 2003 at 16:57
Hi Wombat,

Recently been through a similar exercise with our camper. After plenty of research this is what we did. Ran 10mm wire to trailer battery via an anderson plug and 4mm cable through 7 pin plug for electric brakes. Reason we went with anderson plug is it is more reliable, less voltage drop and you really need a large 7 pin plug, not the standard smaller sized 7 pin plug. Also, if for any reason you don't want to be charging the trailer battery, its simply a matter of unplugging the a/plug.

Hope this helps.

:o) MelissaPetrol 4.5L GU Patrol &
Camprite TL8 offroad camper
AnswerID: 20038

Reply By: herkman - Wednesday, May 14, 2003 at 17:31

Wednesday, May 14, 2003 at 17:31
What has been suggested is all good stuff.

The wire for the brake controller, from the unit to the seven pin plug at the bar, should be as heavy as the plug will take. Whilst light wire will work, the heavy wire will allow the brake magnets to get full voltage.

Also I suggest that you need a battery load sharing device, there are various types, but we used the Pirana electronic one, what it will do is give you car battery for starting, and once the car is started, it will allow charging of the battery in your trailer. The battery in your trailer should be fitted with a low voltage cut out, which will shut off the trailer battery when it get down to 11.6 volts. If this is not fitted, then your battery in the trailer will be quickly destroyed.

The Pirana will also stop you drawing current from the cars battery, when the car is shut down. This system is fully auto, and is better than manual switches and relays.


Col Tigwell
AnswerID: 20045

Reply By: raybates - Wednesday, May 14, 2003 at 18:33

Wednesday, May 14, 2003 at 18:33
Hi Wombat,
I have just completed rewiring up my car and caravan.
I have a Trojan deep cycle battery mounted on the draw bar of the caravan. This battery is fed indipendantly via an Anderson 5o amp plug. Not not try and run these cables through a normal 7 pin plug be it the large or small size as they are not capable of carrying sufficiant current.
Do not use anything smaller than 10mm cable and take both the line and the return back to the vehicle battery. You will need a good quality circuit breaker installed on the posotive cable in the engine bay.
Using this method I am getting 13.7volts at the secondary battery.
I would also suggest that you put an isolating switch on the posotive cable as it exits the secondary battery as caravan manufacturers are not too generous with earth cables and spurfy currents are not too uncommon.
AnswerID: 20050

Follow Up By: GOB vic - Wednesday, May 14, 2003 at 19:41

Wednesday, May 14, 2003 at 19:41
hi people
can some one tell me is there a lot of trust going on with batteries on the draw bar or has somebody found a good lockable container for the battery as i have looked around but not found a sturdy lockable battery holder for drawbar i saw some at caravan show in melb but most would open with a pocket knife and as a very untrusting person with some places we travel i would like a nice solid container

steve89 nissan looking foward to august and more travelling
FollowupID: 12776

Follow Up By: Ray M (Vic) - Wednesday, May 14, 2003 at 20:09

Wednesday, May 14, 2003 at 20:09
Good idea Gob. Some mongrel nicked off with one of mine from the driveway a few months ago, cut the flaming wires so he came equiped for a fast job.Hooroo
FollowupID: 12782

Reply By: Member - Russell - Wednesday, May 14, 2003 at 19:33

Wednesday, May 14, 2003 at 19:33
Hi wombat,
Will join the chorus of people giving advice which will probably make you even more confused than you were! I have an Aussie Swag camper which has large battery and battery management device built in. It also has an Explorer fridge on the drawbar. The fridge supply can come from one of two sources - the vehicle or the trailer - and so there are two anderson plugs (one above the other) on the trailer. When I'm driving, it makes little sense to have the fridge drawing power from the trailer battery, and the car charging the trailer battery, so the idea is that you plug into the vehicle supply. When unhitched or stationary for a while, you don't want to drain your vehicle batteries unduly, so you swap to the trailer supply. I am getting to the point.......
On the vehicle, therefore, the Aussie Swag solution is to make use of the two vacant wires, one for trailer electric brakes, and the other for powering the fridge while driving. Provided you use 6mm wire to a large round plug for the fridge supply, I can't see a problem. In my truck, that supply comes from the second battery, which disconnects from the main battery when the key is turned off. The main charging wires come from the main battery, and these are big suckers - forget the name, but the core is 6 mm or so in diameter (as opposed to what some people call 6mm, which is more like 6 sq mm). These run to a separate anderson plug, and on the trailer side run directly back to the battery management device. So I have two plugs from vehicle to trailer. I don't have any fancy electronic management/isolator devices for the charging wires, and haven't had any problems, but fill your boots!
Just quickly on the brake controller front, you might consider a Prodigy proportional brake controller instead of the pendulum device. They are all electronic, which gives flexibility in mounting, and they are VERY good. Cost a bit though....

Enough rambling. Enjoy your trailer!

Russell S.Russell S
Prado RV6
AnswerID: 20055

Follow Up By: Autolec - Thursday, May 15, 2003 at 07:55

Thursday, May 15, 2003 at 07:55
1 6mm2 wire is insufficient size. You need a minimum of 10mm2 and preferably 12mm2
You have a clumsy round about system when you could have installed a Redarc battery isolater for less money than all the work you have stuffed around with. Powering a fridge through a trailer plug is asking for fridge problems.
Quote "it makes little sense to have the fridge drawing power from the trailer battery, and the car charging the trailer battery". The amount of power the fridge takes is not from the battery but from the alternator supply which is charging the battery and running the fridge.
FollowupID: 12816

Follow Up By: Member - Russell - Saturday, May 17, 2003 at 20:36

Saturday, May 17, 2003 at 20:36
Your name suggests more electrical knowledge, but I think you suffer from the 'more is better' problem in the area of wire diameter. I simply make three observations: 1. The Explorer fridge draws 3 amps, and both the compressor and fridge manufacturer recommend a 6mm supply wire. 2. The wire diameter and configuration were recommended by Aussie Swag - I was simply describing them. 3. There are a million ways to do things - this one works, as I'm sure yours does. I'm not even going to start about isolators......Russell S
Prado RV6
FollowupID: 13014

Follow Up By: Autolec - Saturday, May 17, 2003 at 21:51

Saturday, May 17, 2003 at 21:51
Russell pity they did not take a basic course in electrics and learn to work out Ohms law and not give incorrect and poor advice. It is not what I recommended, it is what is correct and needed. If you take the time to read and learn you will get things running correctly the way they should. When you find out your battery is not charging to capacity you may well remember this post.
Refrigerator manufacturers normally state a maximum voltage drop of 3% which is .35 volts Voltage drop equates to the length of the copper conductor in metres by the current in amps x .017 divided by cross section area is sq mm. Now armed with that information you can go here http://webhome.idirect.com/~jadams/electronics/ohm.htm and it will show you how to work out the basics of Ohms law and see that one 6mm2 wire is insufficient for your requirements.
It is not a case of more is better, it is a case of being correct. I see vehicles everyday supposedly wired by people who know what they should be doing, but in reality they have little idea as they lack to the knowledge to pick up a calculator and work out Ohms law.. Thankfully they keep our workshop busy as we are recommended around Australia to fix up their auxiliary battery and isolator issues.

This is a good primer for the terminology and meanings of vehicle electrics
FollowupID: 13018

Follow Up By: Member - Russell - Saturday, May 17, 2003 at 23:43

Saturday, May 17, 2003 at 23:43
Apologies for the 'more is better' comment - but I see it so often in many areas of life. Happy to eat humble pie whenever required, and will change my setup if necessary. Have looked at both sites and gained some info, but not sure if I have your formula right. Ohms law is V=IR, so I think this means that a .35 volt voltage drop = 3 amps x the allowable resistance of the wire used. If I know the resistance per metre of the cable I'm using, then the formula will tell me how many metres of a particular wire I can use without exceeding the allowable voltage drop. A vehicle wiring website I found said that 5mm2 core wire commonly referred to as '6mm' has a resistance of .0034 ohms/metre, which means that I have .35v = 3 amps x .0034 ohms/meter x allowable cable length. Solving that gives me 34.3 m of cable before I have a problem - ?
If I use your info above, I have voltage drop = length of cable (say 5m) x .017 x 3 amps /6mm2 = .042 V. This is not far from the same answer in terms of allowable cable length as before. Have I got it wrong? Maybe I'll go and check a metre of my 6mm cable with my multimeter...

Russell S.Russell S
Prado RV6
FollowupID: 13026

Follow Up By: Autolec - Sunday, May 18, 2003 at 19:28

Sunday, May 18, 2003 at 19:28
Congratulations Russell you are getting on the track. You will need to check to see if your cable is true 6mm2 or something else. If it was installed by an autolec it will more than likely be 6mm2. The only cable sold that is not mm2 cable is by auto accessory shops like Autobahn SuperCheap Strathfield etc. I doubt you would find lousy undersize wire at an autolec. While some autolecs do not do their sums properly they do use proper cable which the majority of the time is Australian made as imported cable is generally lousy. Australian cable is some of the best in the world.
Russell what factor you did leave out if I am correct you are also charging your battery in your camper trailer. Any voltage drop when charging a battery is significant.
Flooded absorption charge 14.2v to 14.5v (voltage required to charge the battery)
Float charge 13.2v to 13.5v
When you get a voltage drop below the 14.2v you start to reduce the charge of your battery exponentially. For every .2 volt loss on your cable to your camper you will get approximately 10% less battery capacity. So if you have a 80aH battery and you have a .2 volt loss you now have a 72aH battery. With your voltage loss if you have an 80aH battery you are already getting down towards a 65aH battery. People just do not realise the incredible difference and importance of the correct size wiring.
FollowupID: 13050

Follow Up By: Member - Wombat (Vic) - Tuesday, May 20, 2003 at 11:52

Tuesday, May 20, 2003 at 11:52

Thanks for the great advice. If the assigned tradesperson is not up to the job where can you be contacted?Live today as if there may be no tomorrow
FollowupID: 13171

Follow Up By: Autolec - Tuesday, May 20, 2003 at 21:15

Tuesday, May 20, 2003 at 21:15
Wombat just post here and in a day or two I will reply.

Keep a copy of the posts and show the person doing the job how to do the basic sums.

It is difficul to find competence in any profession. You need to acquire enough knowledge to know that you are not being bluffed by the person doing the job for you. I see poor quality workmanship every week that needs redoing.
FollowupID: 13234

Follow Up By: Member - Wombat (Vic) - Wednesday, May 21, 2003 at 11:22

Wednesday, May 21, 2003 at 11:22
It will be interesting to see how long it takes him to ring and make an appointment. Once again thanks for the infoLive today as if there may be no tomorrow
FollowupID: 13273

Reply By: Chris from North West Camper Trailers Hire & Sales - Tuesday, May 20, 2003 at 00:01

Tuesday, May 20, 2003 at 00:01
Hi Wombat I know you are having your trailer wired but thought I would add this anyway, recently I bought a Waeco Thumper battery {around $650} to add to one of our trailers as this allowed me to make it portable and ended up ringing Waeco to discuss this very problem and as it turns out they actually make and sell a kit that supplies power from your car battery all the way through to your trailer, it boasts true 6mm cable 50amp anderson plugs at the tow bar, it has a isolation switch at the car battery end with a wire that is connected to an ignition switch so that it {if you set it this way or can be bypassed} only allows power to flow to the trailer when the car is running, it has extra twin wires from the car anderson plug to allow another power source to be added to the back of your vehicle, it has the special connection trailer end for the Thumper battery but they may be able to delete that from the pack and also extra wires at the trailer end for another power source.
It all comes in a pack complete with cable ties and directions, there are a couple of tricky bits, ie:finding the ignition wire to attatch to but all in all it was very impressive, cost around $150 I think but there isnt anything else to buy and from what I can find out it doesnt allow power to flow backwards back to the car.
I am still in the middle of installing so will let you know how it ends up.

AnswerID: 20498

Follow Up By: William - Tuesday, May 20, 2003 at 21:19

Tuesday, May 20, 2003 at 21:19
For $150 it cannot do a good and proper job.

6mm2 cable is undersize cable for the job.
FollowupID: 13235

Follow Up By: Chris from North West Camper Trailers Hire & Sales - Tuesday, May 20, 2003 at 21:44

Tuesday, May 20, 2003 at 21:44
I guess battery cable would be better but to run that all the way to the back of a trailer is a bit much, the cable supplied by Waeco is identicle to the extra that I bought from the auto electrician so assumed it was the right gear, will look into it more.


FollowupID: 13241

Reply By: Member - Wombat (Vic) - Tuesday, May 20, 2003 at 15:51

Tuesday, May 20, 2003 at 15:51
Thanks everyone for the advice and tips. I think I've got my head around what I need to specify. Now I just have to convince the auto electrician that I "really do" know what I want!Live today as if there may be no tomorrow
AnswerID: 20559

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