Thought about adding a leisure battery to your van? Wondering how to install a split charge relay to your T25 – let’s have a look at that in this article.

If you have a van or motorhome and you want to add a leisure battery then you will need some way of charging that battery. There are typically 3 ways to charge your precious leisure battery in a camper van.

  1.  via a battery charger when on electrical hook up – when you plug into a campsite electrical supply your onboard battery charger starts to supply a charge current to either your leisure battery or both your leisure and starter battery.
  2. via solar panels – a great option for those who want to get away from traditional camping and spend more time off-grid. The solar panels capture energy from the sun and this is converted to a charge current by your solar controller in the van.
  3. via the alternator as you drive along. Vehicles have an alternator which charges the battery as you drive so that your battery that starts the vehicle is always charged. With the addition of a leisure battery, the alternator can charge both.

The perils of joining the starter and leisure battery

I’ve seen leisure battery installs where the two batteries have simply been joined together, in this case, the alternator will treat them as one big battery and charge both accordingly. The issue comes when you park up and all your internal van electrics also see them as one big battery and drain the two equally as you consume power. The result is often a discharged starter battery and an inability to start the van.

The voltage-sensitive relay or split charge system

For both batteries to be charged by the alternator they do need ‘joining’ together but a virtual gate needs to be put in place to stop the starter battery being drained when you stop the van on the campsite and start enjoying your holiday.

This is what a voltage-sensitive relay does for you. It measures the voltage at the starter battery and when this reaches a certain threshold the relay opens the gate to allow the charging current through to both batteries. When you start your vehicle and the alternator starts charging the starter battery, the voltage will rise to a charge level and it is this rise that the relay measures.

Equally, when you turn off the ignition and the voltage at the starter battery drops to its normal level the split charge relay detects this and closes the gate thus separating the two batteries so you can safely consume power in the van without worrying about draining your starter battery.

Darwin note: A voltage-sensitive relay is not a fool-proof system, for those of you that leave your lights on without realising or run the radio in the front of the van will still be draining your starter battery. Equally, be aware of USB chargers in the front of vans, these are often powered by the starter battery. So leaving your teenage tech nerd in the front all day with his iPad plugged in will probably mean you can’t start the van.

Wiring a split charge relay to your campervan

So how do you wire one of these relays into your campervan? They are fairly straight forward to do and here is a video showing one wired into a VW T25 T3 camper.

Pay attention to the cable sizing and fuses that are needed to protect the install. I have listed below a full list of the equipment required to install the relay into a typical VW T25 camper, for the later VW campers like the T4, T5 etc then you may need a longer run of cable as your starter battery will be in the engine bay and the leisure battery in the rear somewhere. The principle is the same though.

I have also created a pdf wiring diagram for you to follow and you can download this using the form below:

Equipment list for installing a split charge system in your campervan

Here is a list of the equipment that you will need (I am assuming here that you have your leisure battery installed)

If this is part of the leisure battery install then here are some additional items you might need.

The alternative approach

Split charge systems have been around for a good while and technology has moved on and so has the methods for charging your leisure battery.

The problem with a split charge system is that all it does is open the gate to allow the alternator to charge the battery, there is no battery management – for that, you need a battery to battery charger or sometimes referred to as DC to DC chargers.

What these do is manage the power from the alternator and off you a range of charging profiles as you would find on a good quality 240v battery charger. These include Bulk charge, Absorption charge and float charge options. All managed automatically by the unit – keeping your battery in better health that with just a simple split charge system. The costs of this marvel? Typically the battery to battery charger is 4 or 5 times the price of a voltage-sensitive relay.

If you have questions about these types of systems or installing a leisure battery in your van then just drop me a line or give me a call.