Leisure battery charging via tow socket
 

Hi all,

I’ve recently installed a 12v battery in my trailer tent to power lights, USB sockets etc. The electrics I’ve added are physically disconnected from the battery once the trailer is in configuration for towing.

If I connect the leisure battery to pin 9 and 11 of a 13 pin trailer socket appropriately, will the car electrics look after the charging current or do I need something in line (DC - DC charger) to charge the battery correctly?

I’m aware caravans have a habilitation relay, I don’t really need this as the loads in the trailer will be physically disconnected, but if it’s a legality will fit one.

I’m likely to be changing the car soon (possibly) from a EU3 engine to a EU5 V70 if this makes any difference (possible smart alternator?)

Thanks

What you need to do is install something that prevents the battery in the caravan "back-feeding" into the cars electrical systems. When the alternator is running and the car's electrical system is running at circa 14.1v - 14.3v then current will always flow into the leisure battery - i.e. charge it. But when you turn the engine off, depending on how the live connections are wired in the socket, there is the potential of the leisure battery supplying power to things on the car - this is not a good idea. The other problem is the caravan can also draw power from the car - meaning that car battery could be flat when you go to start it.

This can be done easily with a simple relay, which only connects the caravan battery into the car live system when the engine is running and the voltage is high enough to charge it, see this: https://www.vanlifeadventure.com/cam...harging-guide/

You should also check if your car is fitted with one already. If the pins are only live when the engine is running (opposed to just the ignition being on) then there probably is such a device.

Great, thanks.

So worst case I need a normally open relay with the coil connected to an ignition switched live to connect the permanent live feed to the leisure battery.

Think that’s me shopping for a small project box and relay.

Cheers

Yep that would be 100x better than nothing, but ideally you would want a voltage sensing relay which would prevent your leisure battery trying to power the starter motor when the engine is cranking...

As Tannaton says, a relay by itself is a start, but a split charge relay will only allow the leisure battery to charge once the car battery voltage is up to normal and, if the battery voltage falls, it turns the leisure battery charge circuit off. So the car always has priority (because that's what gets you home).

This will do the job. It starts charging the leisure battery when teh bar battery is at or above 13.5 v and turns it off when the car is at or below 12.9v

The usual way is to put the relay coil to the white wire (it's white all my V40s) on the alternator. This is the wire which does the IGN lamp on your dashboard. It extinguishes the lamp when it goes high (12v), which is enough to throw your relay, connecting both batteries for charging.

This method will keep the caravan battery disconnected until the engine is ACTUALLY RUNNING (not just the key turned on) and also ensures the batteries are isolated from each other for starting the engine.