Trailer Power project

I bought a trailer which has it’s own power supply, so I’m trying to configure a power management system.

The trailer has room for a battery, it has LED lighting, 12v outlets and a car radio. I wanted to run power from the car as an alternative power source and to charge the battery. Several iterations and 3 trips to Jaycar later I have a system which works, not finished, but works.

CAM00619In order to avoid soldering in all the parts at the trailer, I attached a terminal block. This makes it upgradable and removable. You can just see the solar charger poking up at the top, has an LED so I put it there.

10 Oct. 2014

Wired a line from the rear 12v power jack in my car to the trailer plug and also one from the reversing light to the trailer plug which wasn’t wired

11 Oct. 2014

Things to power:

  • LED lights in the trailer
  • 12v outlets in the trailer for accessories
  • Stereo in the trailer
  • possibly an inverter down the road to create 240v for tyre warmers amongst other mains voltage uses

12v Power supplies in trailer:

  • 12v battery for power storage (previous ley a boat battery)
  • Low power solar charger
  • Power line to car battery through the trailer car connector, on a 15A fuse.


  • Not sure if small solar panel can over charge battery. Solved – it can’t
  • Suitable control of the power source from the car. Partially solved. Wire resistance alone is suitable. Used this and this. Haven’t solved over draining car battery yet…
  • Displaying voltages and currents. Waiting for ebay current voltage meter – which may require calibration.

14 Oct.

Used the trailer as ground as much as possible to avoid voltage drops over the cable, also did the same for the car.

CAM0060915 Oct.

This diagram is close to what is currently installed, it’s one of many schematic diagrams I’ve come up with over the past week.

CAM00621The current shunt is from an ebay current/voltage meter I’m waiting for from ebay. This will allow me to read current from the car. I was planning on monitoring 2 separate currents, as well as 3 different voltages (open circuit car, battery and solar) with a micro controller, but I’ve since abandoned this in favour of simplicity. S1 and S2 are not installed yet, which leads me to my next problem, draining the car battery. I planned on using a voltage sensitive relay to only close S1 when the car voltage was sufficiently high, this would prevent the trailer from flattening the car battery. Voltage sensitive relays were recommended not to be used, also I calculated that if I had the relay in the trailer, I would have the problem that when the relay switches closed that the voltage would drop to the trailer battery voltage… I was going to get around this by putting the voltage sensing behind a current limiter, which was another problem.. current limiting. I calculated that with a flat trailer battery and the alternator running (biggest difference in voltages) I would be drawing ~15A which I figured was about the maximum for the cable I used. So there has been many iterations over the last few days and the above schematic diagram is the one I’ve basically gone with currently.

camimgAttempted voltage sensitive relay, triggered by a Darlington pair, behind a 100W lamp as a current regulator (all scrapped)


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