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12v caravan charging system.


New Member
Hello all.
First post so please be gentle.

I'm looking for some advice for my 12v caravan system.
Ok the caravan has a 12v system powering lights, water pump and heating control.
When I'm not hooked up to the mains, these are powered by the leisure battery.

When I'm hooked up to the car without the engine running power can be taken from the car battery. When the engine is running voltage on the fridge circuit closes the habitation relay and powers the 12v fridge and charges the caravan battery at the alternator output.

When I'm hooked up the mains. The caravan has a split load charger rated at 10amps. This powers the 12v circuit at 13.4v and also charges the battery to 13.4. it is a simple single phase charger as far as I know.

The positive and negative from the outlet of the PSU go to a series of block connectors that route to the other circuits. There are two wires that lead off to the battery, these are the wires that charge the battery from the PSU or the car, but then also supplies 12v from the battery to the caravan circuit when it isn't hooked up to the mains.

What I want to do is change the battery for a LifeP04 purpose built leisure battery. The battery I'm looking at has BMS and an inbuilt charger.
I have a b-b charger that has a lithium cycle so charging from the car is no problem.
The issue is the 13.4 v from the onboard charger when we are hooked up to the mains. I need to have the charger on so that it powers the 12v circuits but stop the voltage getting to the battery.
But should the mains voltage go off, I need to be able to draw the 12v from the battery to power the 12v circuit as this runs the lights and fridge control circuit.

I know I could do this with an automotive NC relay on the live that leads to the battery. My problem is the trigger for that relay.

I can't take it from the output of the PSU as that always has 12v at it from the battery or from the car charging circuit.
I have looked at a 230v coil relay, I could wire this into the positive and negative for the battery charger but the only relays I have seen do not have a safe way to shield the terminals and look like they are designed to be used within a larger powerbox.
So my other idea was to add a one way diode rated at 15amp to the positive outlet of the PSU. I would the I stall the relay trigger on the PSU side of the diode so when mains is on the relay is closed preventing the charge reaching the battery, but when mains goes off, the relay opens and 12v is returned to the caravan from the battery.

To keep things neat I was going to use MC4 connectors as it would be easy for me to cut the PSU 12v live and clip on the mc4 male and female terminals and then add the relay in-between, making it easier to remove later and connect the original wires up if I chose to sell the caravan.

Can anyone with knowledge recommend a better way?
Will a Bosch relay work ok with voltage passing in either direction via pins 30 and 87a.
When on hookup we can be for two weeks sometimes, will a relay hold closed ok for this amount of time?

Any safe suggestions using a mains powered relay coil.
The mains connection for the charger is a three terminal mains connector, I haven't opened it but would imagine it is just terminal blocks within wherethe supply from the fuse board meets thePSU mains lead.

i was hoping I would be able to automate this, so if the mains goes off in the middle of the night, I don't have to go out to the battery box to switch the battery back on.


Well-Known Member
Find a small, mains input, 12 V output power supply, and run that from the mains, and connect a 12 V relay coil to be powered by that power supply. You then just plug the small power supply into the same source as the mains charger. Normal mains wiring precaution will be needed, but what you do there won't need to be any where near the 12 V stuff.


A relay will work for current in either direction between 30 and 87a.

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