Fitting a leisure battery to trailer tent
 

Not having much idea on towing electrics, I thought I'd ask here:

I'm thinking of fitting a small leisure battery to my trailer tent, mainly to charge ipads/phones etc, and possibly run LED lights and the tap for the sink. We don't tend to use EHU when camping.

The trailer is currently fitted with a 7 pin plug, but it needs replacing as it is falling apart, is this just a case of fitting a 13 pin plug and running the relevant cables to the battery terminals to charge it? My car has a 13 pin socket which currently has an adapter to 7pin on it.

Cheers

Most caravans and probably trailer tents with leisure batteries fitted will have what's called a habitation relay fitted it connects the leisure battery to the charging circuit on the car only when the car is running/charging, and at all other times it connects the battery to the devices requiring power.

This is a typical diagram of the circuits using the fridge supply circuit from the car side to switch the relay.

https://i.imgur.com/JXIflGK.jpg

.....aka split charge relay.

If you are just charging phones, led lights etc a decent battery should last a few weeks.

The logic behind a habitation relay is that it only comes into operation to provide charge to the battery once the engine is running and the alternator is charging. The relay is activated by pin 10 becoming live and the relay then connects the permanent live pin 9 to the battery for charging. However, having researched this in some detail for my V50 it is apparent that most current Volvos simply provide switched power to pin 10, i.e. it becomes live as soon as the ignition is switched on and not just when the engine is running. This 'could' result in the starter motor in the tow car drawing on the auxiliary battery in the trailer. The solution would seem to be to install a voltage sensitive relay on pin 10 that will only operate once the alternator is producing somewhere around 14 volts, i.e. the engine is running.

Whether or not it is a problem in practice is another matter and as I said, as far as I can tell this is the way all current Volvos operate so it’s perhaps more a theoretical problem than a practical one.

A split charge relay is a different thing altogether. Its purpose is, as its name suggests, to split the charge from the main battery, once it’s fully charged, to a second battery, and back again if the main battery gets depleted. This dates from the days when dynamos would have struggled to charge two batteries but modern alternators can cope with ease. With a regular trailer/caravan setup, the auxiliary battery is charged together with the tow car battery - it’s not an either/or situation.

Sorry but I disagree on your view of split charge relays - there's no system that disconnects the cars' main battery in favor if the caravan battery which is connected by maybe a 1.5mm conductor through the socket... that's a fire waiting to happen.

This explains the systems well:

https://www.12voltplanet.co.uk/split-charging.html

And if I agreed with you, then we’d both be wrong!

What is being described on the page you linked to is in effect the use of a voltage sensitive relay to avoid drawing on the auxiliary battery when starting. That is often referred to erroneously as split charging but the term originally derived from the need to split the relatively modest output of a dynamo between two batteries - a main and an auxiliary. If the load on the main battery increases, say, because the headlights are switched on, then the charging switches back to it. But this doesn’t mean that current is also drawn from the auxiliary battery along the light gauge charging circuit. The circuits are ‘split’. Split charging is more usually found in motor homes with two or more batteries rather than in caravan/trailer combinations.

The voltage sensing relay is option 3 - if you look at option 2 that is a split charge relay operated by the signal from the alternator (i.e. when ignition light goes out...)

Option 2 may be called a split charge relay by that vendor, but it’s not. It’s just an alternative way of connecting two batteries in parallel for charging once the tow car's engine is running, i.e. when the ignition light goes out. A split charge system charges one or the other, but not both, and is superfluous nowadays with high output alternators.

But to go back to the original question, the standard way of charging an additional battery in a trailer is to connect a relay - the 'habitation relay' - to pin 10 and have this relay connect pin 9 to the battery. But given that as far as I could determine when sorting this out on my V50, pin 10 is live once the ignition is switched on and not just once the engine is running, it may be advisable to fit a voltage sensing relay to pin 10 so that power to the habitation relay, and thus to the auxiliary battery, only comes on once the alternator is charging.

In my case, the battery in the trailer is only for powering a winch and there was no habitation relay so I just connected the trailer battery to pin 10 via a voltage sensing relayed mounted on the trailer rather than in the car. This ensures that if I use another car to tow the trailer it will still charge the trailer battery without risk of trying to draw power from it when starting.

I will confess I've not looked to the exact point of contention here, but you appear to have it about right. Split-charge in most instances is a mis-noma. It's done via a voltage-sensing relay.

And to the OP, a battery is a battery is a battery. If you're the muppet that buys ahem... 'Patio gas', and 'shaving-foam' don't get hoodwinked into buying a 'Leisure battery'.

Soap is soap.

Gee, thanks!

A 'Leisure Battery' is a deep cycle battery that is better able to withstand high levels of discharge than a 'Starting Battery'. So more useful when the current draw will not be regularly topped up by the alternator as would be the case with a regular battery.