Camper Trailers Tech Tips

DIY 12volt wiring for CuB Supamatic




12volt power
for our cub supermatic  

    Arguably one of the most written about and discussed topics relating to camper trailers. Here is the solution which I came up with to satisfy my ‘power on the go’ needs, as well as a few of my creative ones, after purchasing a new Cub Supamatic Escape Off Road Camper in May of 2010.

    It would be adaptable to other types of campers with some extra thought and planning and I will apologise in advance if there is too much information provided, but as with all articles, I have no idea of the knowledge or ability of any readers. If in doubt ask an auto electrician or someone who has done it before. It does cater for the ‘first timer’ so those that have been down this path before will no doubt ‘skim’.

    My personal experience with magazine articles on this subject, whilst coming up with great ideas and pictures, they do tend to leave you scratching your head at the end of the article as to what has actually been done in between some steps to achieve the end product, my aim is that does not happen here.


    The Cub is our second camper trailer. In late 2003 we purchased a 2nd hand Adventure which had been returned to the manufacturer in Adelaide for reselling. This was the trailer that my wife Francine and I and our two children, Jessica (14) and Andrew (12), were to spend three months travelling through South and Western Australia and the Northern Territory in. We did pretty well every iconic destination possible, I won’t list them all here and it was the trip of a lifetime. There are not many travel shows highlighting these areas that I can’t watch and say “I’ve been there” and settle back into the lounge with a contented smile and start reminiscing, and planning again !

    The Adventure suited us as it was the early model with two double beds (closer the ¾ beds actually) and plenty of storage. It also came ‘12volt-less’ and was my first attempt at a 12volt electrical fit out.

    After the trip we kept the Adventure for another twelve months or so and as the kids got older and showed less interest in holidaying with us we sold it as the bed setup was not ideal for two. I had added home made boat/wood racks and done the electrical fit out, including a 90 amp hour GEL battery and got what I paid for it after owning it for nearly two years. I would call that a good investment.

    When ‘the itch’ couldn't be satisfied by no other means (and doing a delivery run on a Cavalier for my brother-in-law to Victoria over a week didn’t help either) and after doing much research and working within budgetary constrains, we headed off to Cub in Sydney to look a demo trailer, which sold before we got there, we ended up with a great deal and four months build time to save up some more ‘hard earned’ and ended up with a new camper for the same price instead.

    The Escape model (read ‘poverty pack’) came with some basic wiring i.e. 1 x internal and 1 x external 12volt ‘merit’ type socket wired through the trailer plug to run off the starter/aux battery of the tow vehicle, along with a 15 amp 240volt inlet on the off side, 1 x double GPO and a C/B RCD combo inside and 1 x ext GPO on the near side. No battery or charging or lighting is provided. Of course these things can be added as options, but even Cub will admit that whilst their trailers are good value for money, their options come with a premium price tag.

tow vehicle set up

    I still own the same tow vehicle used for the last big jaunt, a ’98 Holden Jackaroo S TD manual which I had purchased specifically for the trip 2nd hand in 2005. It already had a dual battery system fitted which consisted of a manually operated charging solenoid, a home made battery cradle which was just large enough for a 50 amp hour battery and a rear mounted cigarette socket and dash switch. A basic set up which worked fine, so long as you remembered to push the ‘under dash’ mounted button to connect the auxilary battery to the starter battery to charge it up ! This didn’t last long and the solenoid was replaced with a 90 amp Matson voltage sensing relay. These are a fully sealed unit which isolate the main and auxilary batteries, sense when the main is fully charged and then automatically sends power to charge the auxilary battery. I also ran a pair 6 B&S gauge cables from the auxilary battery through the firewall and via the interior of the vehicle to the tow bar and mounted a 50 amp Anderson type socket. I still have the same set up six years later and has never failed. Packing the back of the Anderson plug, where the cables enter, with silicone to keep the dust and water out is not a bad idea either.

    There are better ways to charge an auxilary battery these days, such as a multi stage C Tek, Redarc, ABR Sidewinder or Projecta, to name a few, 12v/12v charger, these generally cost more but will probably make your auxilary battery last a lot longer. For under bonnet applications the Redarc is probably the pick due to its compact size as well as many other dual battery charging systems on the market from many manufacturers. Redarc makes a cracker but you will need deep pockets! My next pick would be one of the Piranha units.

what I wanted to achieve

    My goal was to be able to recharge batteries and/or provide power, regardless of the location, the time of day or the weather conditions and also to make use of the equipment that I had accumulated over the years. I also wanted to be able to monitor the condition of the batteries and how much load was being placed upon them. Sound familiar ?

what I already had

    Apart from the modifications to the vehicle I also had a 80 watt solar panel fitted with a Projecta regulator and a Honda Eu10i inverter genset. I wanted to incorporate both of these into the project. Both are not required, but at least one would certainly make life easier when it comes to recharging batteries for a long stay. If I did not already own the genset I would have at least another solar panel of the same size or bigger.

my first attempt

    As previously mentioned the Adventure had no 12volt electrics or lights either.

    My first go at wiring a trailer was attempted with minimal research and doing it as cheap as possible. It consisted of, from the tow hitch, a 50 amp Anderson plug, 2 x 6B&S cables to a 2nd hand 90 amp hour sealed GEL battery and a series of cigarette sockets scattered around the trailer and that was about it. Lighting was provided by a couple of fluro camping lights from ARB. Coupled with the 50 amp hour battery in the Jackaroo this gave me a total of 140 amp hours of power – or so I thought at the time. I was running an 80 litre Waeco fridge freezer as well to cater for the four of us over an extended period of time. This fridge served us well and has never missed a beat since we bought it but, when using it as a freezer, it does like it’s quota of electrickery !

    What I didn’t realise, due to lack of research, was that because I had the 50 amp hour battery wired parallel with the 90 amp hour and the 50 was receiving it’s charge first from the VSR, the 90 was never getting its full charge. Adding to the problem was the voltage drop over the 12 metres or so of cable that ran between the two batteries from the engine bay to the back of the trailer. My true amp hour capacity may have been less than 100 amp hours. During our time north of Broome, in the Kimberly and the Pilbara, I was lucky to get two days out of the batteries even with the solar panel attached. The mistake I made with the solar panel was using wire that was barley heavier than automotive speaker wire, so I was electrically ‘strangling’ it as well. Another really annoying problem that occurred was after one day of ‘sitting still’ I had great difficulty getting the fluro lights to ‘fire’ because of the voltage drop in the batteries. I had to start the car and get the auxiliary batteries into a state of charging to increase the voltage and get the lights to come on. So you can see that not much had gone right in this project.

the cub project

Parts list Supplier
Anderson plugs (10 pack) ABR Sidewinder Ebay Shop (Derek Bester)
Anderson plug dust boot ABR Sidewinder Ebay Shop (Derek Bester)
6 B&S cable x 6 metres black & red ABR Sidewinder website
heat shrink to suit ABR Sidewinder website
cable lugs to suit ABR Sidewinder website
battery switch ABR Sidewinder website
large electrical box ABR Sidewinder website
1.5 metres 16mm split flex conduit auto accessory store
50 amp C/B “ “ “ auto accessory store
12volt switches with LED - Narva 62059 BL auto accessory store
spade connectors auto accessory store
15 amp wire red & black auto accessory store
50 amp wire red & black auto accessory store
fuse block - Narva 54430 – 35. Depending on needs. auto accessory store
20 amp Ctek 12v/12v battery charger - $345 inc p&h 4WD Extreme Ebay store
25 amp Ctek 240 v/12v battery charger - $385inc p&h. This item optional. This was the biggest charger my genset would handle.) 4WD Extreme Ebay store
battery - min 80 amp\hr) Shop around for this one. Must be sealed so no gasses when charging. I chose AGM type. Battery World
Matson battery tray - coated steel Battery World
battery hold down - Matson Battery World
digital volt meter Ebay various
digital amp meter Ebay various
merit & cigarette sockets Ebay various

where's it all going

    As with any project like this remember; check everything at least three times; measure at least twice; cut or drill just once.

    Inside the trailer decide where you will fit the battery and charger. I chose to put them under the bed as there was a space between the wheel arch and the rear bed bulkhead on the near side of the trailer (same side as the cable run through the draw bar) which was a perfect fit for the battery and will allow for future upsizing if needed and it was an easy space to work in. This also kept the charger nice and close to the battery and for close mounting of the control box on the opposite side of the bulkhead.

existing wiring modification

    There is a piece of panelling at the back of this space which needs to come off to get to the wiring behind it so remove it now, there is also one directly opposite on the other wall about the same size, it needs to come off as well to get to the wiring behind it. Behind this panel (on the drivers side) you will find a black wire (this is the 12volt supply from the trailer plug that we no longer need) and two white wires (12volt supply to the internal and external merit sockets) all terminated together. Remove the terminator and separate the three wires. Rejoin the two white wires and refit the terminator and cut the black wire off flush and just leave it, you can wrap the end in electrical tape if you wish.

    On the opposite side behind where the battery will go, you will find two white wires terminated together. This is the feed to the out side merit socket and the 12volt supply from the opposite side that you just changed. Remove the terminator and add length of 10 amp red wire, enough to reach the control box, and refit the terminator. This red wire will become the new fused 12volt supply to the two merit sockets. The panel can be refitted now if you wish, I didn’t bother because the battery will fill this area up later. If you plan to fit and annexe light on the side of the trailer fit it now so you can run the wires to the control box before the battery goes into place.

    The only variable here may be the colour of the white wires; I don’t know if Cub uses the same colours on each trailer, just have a look at the rear of each merit sockets’ centre connection to check. Each socket will also have a yellow/green earth wire attached as well, don’t alter them.

    The 6 B&S cables will come up through the floor right next to the bulkhead so fit the battery tray very close to the wheel arch wall. The charger was mounted on the inside of the bed bulkhead, very near to the battery and the control box on the outside of the bulkhead. This kept the cable runs to a minimum and avoided voltage drop, even if only a little.

fitting the charging cables

    Starting from the tow hitch. Fit a 50 amp Anderson plug to the end of a pair of 6 B&S cables about 6 metre long each. I also fitted a dust cover over the back of the Anderson plug. Both these, and many other items, came from the ABR Sidewinder’s Ebay shop. A ten pack of Anderson plugs cost me $30 + $5 p&h, that’s $3.50 per plug at your door, I couldn’t find them any cheaper.

    Cub drill holes in the ends of their draw bars for wiring connections. The other side has the trailer plug wiring in it. Feed both power cables into the draw bar until there is enough still hanging out to reach your vehicle mounted Anderson plug, usually, but not always, the same length as your trailer plug lead, and secure with electrical tape so it stays at the right length. As the cables poke out the other end of the draw bar they have to turned back towards the tow hitch a short distance and fed into the main lateral member of the chassis heading towards the rear of the trailer. Where the cables are exposed here fit a short length of split conduit and secure with a cable tie or two. When the cables come out of the end of the lateral member they will be exposed to stone damage so time to add some more split conduit. I secured the conduit to the timber floor using single sided conduit saddles (3) till I reached the point where the cable and conduit will enter the interior via a hole you will drill, just a bit larger than the conduit you have used, through the floor.

    Use a small drill bit for a pilot hole as this will puncture the vinyl flooring material inside and you can cut out a neat hole with a Stanley knife for the cables to enter. It will also show you if the two 6 B&S cables are going to come up in the right spot in relation to the battery tray and the location of the charger. A small pilot hole is much easier to hide than an 18 mm ooops ! If your trailer chassis is the same as mine you will be very close to a piece of steel angle, running across the chassis, as you bore through the floor. Run the cables through the hole and push the conduit 20mm or so into the hole as well. Seal around the conduit with silicone sealant. If all has gone well you should have a meter or so of cable coiled up under the bed next to the battery tray.

the charger

    This needs to be fitted in its final location now so you can accurately measure the length to cut the cables. When cutting the cables allow a bit extra, so should the need arise in the future, if you change chargers, the terminals may be in a different location. Fit lugs to ends of the cables by crimping or soldering. Crimpers for a job like this will set you back anywhere from $40 to $100. I had the equipment and knowhow to solder, so that is how I did it. Whether crimping or soldering, fit the appropriate sized heat shrink over the fitted lug(s) to support the connection. The heat shrink should cover the round cable receptacle of the lug and at least 20 mm of the cable.

    Place the new auxiliary battery in the tray and clamp it into place.

    Fit another pair of lugs to the leftover cables, these will become the leads from the charger to the battery. Fit these cables to the charger and run them to the correct terminals of the battery and cut to length, allowing a bit extra for future changes. Fit either lugs or terminals, whatever is appropriate for the type of connections your new battery has, to the other end of these cables.

    At this point your new auxiliary battery is able to be charged from the tow vehicle. If it is not already installed a circuit breaker should be fitted to the positive lead on the charging cable of the tow vehicle as close to the auxiliary battery or charging source on the tow vehicle as possible. This breaker should be rated at 50 amps to match the rating of the Anderson plug. A lower rating may be required depending on the input rating of your chosen charger.

    Now that we have all this power on hand it’s time to send it out and do stuff !

the control box

    These can be as simple or as complicated as either your ability, patience or wallet permit. There are a few ‘must haves’ to call it a control box, switches and some fuses at least, as it’s job is to send power out to the various power sockets, lights and other appliances on your needs list. This brings us to the next step – what are your needs ? I’m not going into power consumption calculations relating to amp hours (battery capacity) but you will need to know how much current (amps) your individual items draw when running. This is to make sure that none of your electrical items exceeds the rating of your control box components. This is unlikely for the average camper. DC ‘camping’ fluro lights are generally around an amp with LED’s about 200 mili/amps. An 80 litre Waeco with a BD50 compressor will draw 7 amps for a couple of seconds when starting up and then drop back to about a 5 amp draw when running. My ARB fridge draws just 1.87 amps when running and about 4.2 amps on start up, according to my multi meter. Working out what you want, or need, will also help you with the number of switches and sockets you need to purchase and ultimately how big your control box or panel will need to be.

    So, work out your needs, buy your components and then work out how big a box or panel you will need.

    I built my control box to suit my requirements as well as trying a few things because I could ! I have an amp meter to see what I am using, a volt meter to see how my batteries are doing. Rough rule of thumb, 12.8 v = full, 12.6 v = ½ full, 12.4 v = ¼ full, 12.0 v = flat. I did say rough ! Also four Narva 62059 BL switches with red LED indicators, 1 x cigarette & 2 x merit sockets, a panel mount 1 amp fuse for the volt meter display (you need to use separate 12volt supplies for the digital displays otherwise your volt meter may read low. Why, I have no idea !) and an isolation switch to disconnect the batteries from everything else (except the charger).

    If the box building thing seems a bit too time consuming or daunting ABR Sidewinder has an excellent ready made panel for about $95 which can be surface mounted or is a perfect fit for the large box he sells as well. Saves a lot of time and probably will be sufficient for the average punter. Have a look at 

    This panel could be surface mounted in the same location as I have installed my control box, but would need a large hole to be cut into the bulkhead which I just couldn’t bring myself to do.

    Whilst the internals of my box resembles a fist full of spaghetti, it is a fairly straight forward wiring job if you follow the diagram. This was my first go at using MS paint for a wiring diagram so its pretty average.

    This system has been installed now for a couple of months and works well. The volt meter seems to be reading a little low, about 0.2 of a volt, and I’m not sure if it’s my wiring that is at fault or if it’s that the meter only cost me $22 on Ebay !

    If there is a down side I can find at this stage it is that the auxiliary battery in my truck powers the Ctek 12 volt/12 volt charger in the trailer. If the charging of the trailer battery(s) doesn’t reach the ‘float’ stage i.e. fully charged, then you can be left with a partially charged truck auxiliary battery if it hasn’t had sufficient time to fully charge. This happened recently when we re-located from the Callala Beach weekend to Greenwell Point for an ‘extended’ break.

    This is only a 20 minute drive and because I had been running my fridge from the trailer battery for four days it was fairly low. I had moved the fridge to its usual travel place in the truck and plugged it in for the short drive and it fired up ok. I connected up the charging plug for the trailer at the hitch which started up the charger in the trailer and away we went. After arriving at our next stop I switched the truck off and after about 10 minutes noticed the fault light on the fridge flashing. I checked and found that it had stopped altogether ! A quick move inside and connection to mains power and all was good with the fridge, must be a problem with the truck. With the trailer (and charger) still connected I put a multi meter onto the truck auxiliary battery – 10.6 volts ! Not so good. I disconnected the trailer charger and it rose to about 11.5 volts. The combination of powering the fridge and the trailer charger and the short run had left me with an all but flat auxiliary battery. Apart from avoiding this scenario in the future I’m open to suggestions for a remedy to this one.




internal panels removed for foot light wiriing inside the control box completed control box foot lights


battery and CTEK 12v to 12v charger LED clearance lamps fitted under floor Internal wiring for clearance lamps


50 LED light on 20mm conduit 80 LED light hitch detail camper diagram



 thanks to Gary Laker for sharing this idea


november 2010