I have an enclosed car trailer which has two interior 12 volt lights which are presently only available when the trailer is attached to the tow vehicle (4 litre Jeep Cherokee).

I have replaced the exterior running lights with LED's, I would like to use the electrical power saved by the LED's to charge an onboard 12 volt battery in the trailer while it is being towed to the circuit.

This would allow the lights in the trailer to be used when the tow vehicle is not attached, as in when working on the car to solve problems in the evening, or for shelter from the elements while at the track.

I need some method which will keep the onboard battery from powering the trailer running lights when the tow vehicle lights are turned off, or while the trailer is on its own short of going in the trailer and throwing a switch.

The interior lights would be on a seperate circuit switched on from inside the trailer.

Would a diode or some othe mechanical switch allow this function?

Thanks in advance, please don't make it too technical, wiring is not my forte.

Check this out, http://homepage.ntlworld.com/mark1968/Towing%20&%20Wiring.htm#WiringInfo

the locost solution is a diode, and a current limiting resistor (read light bulb).

take +12v from the trailer socket to the battery through a spare brake light bulb and then a 10amp diode to the +12v on the battery. this will charge the battery when connected to the car, and limit the current, very important.

connect the -ve side to the common -ve on the lights.

wire the interior lights and a switch across the battery.

if the +12v from the sidelight circuit is used then when the trailer board lights are on it will charge the battery, and when they are off there is no chance of flattening the cae battery.

hth