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Me again,sorry.

Temporarily I wired the accessory socket on my car with a 30a wire from car battery to the socket, so that it charged the van battery and ran the fridge. If I stopped for any time I disconnected the plug to save teh car battery. However if I tried to start the car with plug connected, there was little power which puzzled me - why? Disconnected and it was fine.

Wiring diagrams sugeset the feed should be relay controlled so only operational when engine running. On an old car, how do I do this? If I operate the relay from the ignition switch I'll get the same affect as above. Could have  a separate switch but then might as well just use the plug!

Ideas welcomed please.

Paul

Hi Paul,

The reason there would have been little power when cranking the car over would have been because a fair bit of the battery power was being used by the fridge and battery on the caravan.

Normally when you start the car, the ingnition switch very temporarily turns off all other sockets to supply maximum volts to the starter motor, it wasn't able to cut power to the van as it was a direct connection you used.

You need to fit a smart sensing relay (think that's what they are called), basically this is a relay that won't supply any power to the caravan socket until it senses the engine is running.

A car runs on a 12 volt system however when the engine is running the voltage output is between about 13.5 to 14.5 volts, the relay senses this voltage increase and then supplys power to the caravan.

They are very easy to fit apparantly - not yet fitted one as I have a trailer tent with no 12v circuit.

The smart relay is the easiest answer as you just insert this in your existing feed line. It senses the higher voltage when the car is charging and then switches the power on to the socket. Towsure do them...

http://www.ukcampsite.co.uk/redirect.asp?redir=http://www.towsure.com/product/1828-Self-Switching_Combination_Relay

I fitted one to a previous car in a matter of minutes.

Glen