split charge relay or not

[warbulator]

after trying to figure out why car wouldnt charge battery/run fridge i found my used cars towbar has a 12s socket but it wasnt wired up!!. got the stuff to do it but caravan booklet says if relay fitted bypass it, modern batteries dont need em, is this right? just straight wired (with a fuse!) from car battery. the power does however run into a zig unit in the van first.

advise please.

 

[DavidW1]

The short answer is that a split charge relay is essential. Reason, your caravan battery will drain your car battery if it is in a lower state of charge(likley)as long as it is connected through the 12S with the engine not running. Result, severe risk of a flat car battery just when you need it!

The split charge relay isolates the van from the car until there is enough voltage flowing from the alternator to supply the van circuits; between 13.5 and 14 volts but different makers relays will vary marginally around this value.

An essential test if you are fitting a relay or doubt it's serviceability is test the output at the 12S with the engine off and then with the engine running.

Obviously there should be no output with engine off but if all is well, a few seconds after starting, you will here a click from the relay and see circa 14 volts on your meter, it should also click and fall to zero again at switch off.

Hope this helps but I would recomend a visit to Ryders website for a fuller description and details of all the various relays and their function. It is probably the best site of it's kind for towing electrics with very informative Adobe downloads written in textbook format and excellent diagrams. Just put in car towbar electrics on Google and click on Ryder Towing