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An Illustrated Guide to Wiring a Split Charge Relay

Want to charge your leisure batteries while you drive? Well, you're going to need a split charge relay kit! In this article we will learn all about wiring a split charge relay and explore the devices you could use, including: a manual switch, split charge relay, voltage sensitive relay, split charge diode, and DC to DC battery chargers. We will also look at several split charge relay wiring diagrams. By the end, a large part of your van electrical system will be complete! I'm Shane, I've been teaching people to convert campervans for many years, I'm the author of The Van Conversion Newsletter, the van conversion instructor at Udemy, and the proud owner of a beautiful self-build campervan called Beans. So let's jump in and have a look at wiring a split charge relay!

A useful guide to split charging your van conversion

This post contains affiliate links, which means that if you click a product link and buy anything from the merchant (Amazon, eBay, etc.) we will receive a commission fee. The price you pay remains the same, affiliate link or not. By using these links, you are helping me to continue writing free educational content!



Note: Before we hop in, you might want to grab yourself a wiring diagram which you can get for free by signing up to The Van Conversion Newsletter (the wiring for split charging is included in the diagram 🙂 - wiring diagram gets sent out to you straight away).


What is a split charging?

Split charging means charging both the starter batteries and leisure batteries at the same time while you drive. People often think of 'split charging' in general as a 'split charge relay'. A split charge relay is simply one type of split charger.

A starter (cranking) battery is what kicks your van into action when you turn the ignition. Starter batteries are designed to provide a big burst of energy (which the starter motor needs), the energy is supplied fast and can use as much as 20% of their total capacity per start. Once the engine starts up, the alternator recharges the starter battery so it’s ready to go for the next time you start up.

Leisure/Deep-cycle batteries in contrast release energy in a steady flow over a longer period. They are designed to withstand hundreds/thousand of charging cycles.

When we have wired a split charge relay, we can charge up our leisure batteries while we drive!

At the most basic level, a split charger is simply a switch.

  • Switched on, it allows power to pass through the relay to charge the leisure batteries.

  • Switched off, it isolates the starter battery from the leisure batteries so you won't drain it with your campervan appliances.

In this man's humble opinion, wiring a split charge relay is a necessity in a van conversion.


If your van has a smart alternator (If your van is newer than ~2015), or if you are using a lithium leisure battery, you must use a battery to battery charger. More on this later...

A quick note on how the alternator charges the starter battery.

A battery is fully charged at ~12.7v. The battery will only charge if the incoming voltage is higher than the current voltage of the battery.

voltage requirement to charge leisure battery

It is for this reason that alternators deliver between 13.8 - 14.4V of power.

Five types of split charging

There are five ways one can split charge a campervan (from most primitive to most advanced):

  1. Manual switch

  2. Split charge relay (SCR)

  3. Split charge diode (Charge splitter)

  4. Voltage sensitive relay (VSR)

  5. Battery to battery (B2B) charger

Let's look at each of these five methods! We will also look at split charge relay wiring diagrams for each.

Manual switch

Super primitive. Super cheap. And just a little bit sketchy...

Simply wire the positive terminals of the starter battery and leisure battery together with 6AWG cable. Then pop an isolator switch right in the middle. You will also want to add a 100a fuse on either side of the switch (right beside each battery).

Manual switch split charging van conversion

So why is this sketchy?

Because you MUST remember to turn the switch off everytime you stop driving the vehicle. If you forget to turn the switch you will slowly but surely drain your starter batteries.

It's also worth pointing out that this form of split charging is not very efficient and has no fast charge option.

It is VERY basic.

It is not advisable to use manual switches with smart alternators (If your van is newer than ~2015).

You cannot use manual switches with lithium batteries.

Split Charge Relay (SCR)

A split charge relay (also known as a 12v relay) is very similar to a manual switch, but with one very useful little addition: the switch is automatically triggered when it senses electrical input (ie. the van starting).

With this useful feature, we fix the main downside of the manual switch. However, it is still a very inefficient method of split charging, with no fast charge capability.

Wiring a split charge relay is not advisable with smart alternators (If your van is newer than ~2015).

You cannot use a split charge relay with lithium batteries.

Split charge relay wiring diagram van conversion

It is necessary for us to wire up a 'trigger wire' for the split charge relay (which senses when the engine turns on), and also to ground the device (connect it to the van chassis).

Split charge diode (charge splitter / battery isolator)

In a split charge diode, current can flow from the alternator to either battery but cannot flow between batteries. ie. Either the starter battery or the leisure battery is charged at a given time.

These devices are not commonly used in van conversions; they can be more difficult to wire up (must connect directly to alternator) and there are some significant drawbacks to split charge diodes.

The biggest drawback is that they incur a large voltage drop (around 1V). Voltage drop occurs when there is resistance in a system (can be caused by wires, fuses, connectors, or in this case the split charge diode itself). This voltage drop means that the leisure battery can never be fully charged; it will usually be charged up to 80% or so).

Split charge diodes tend to generate significant amounts of heat which one must be careful about. They also do not have a fast charging option.

It is not advisable to use split charge diodes with smart alternators (If your van is newer than ~2015).

You cannot use split charge diodes with lithium batteries.

Split charge diode (charge splitter) van conversion

Voltage Sensitive Relay (VSR)

A voltage sensitive relay is the most common split charging device found in campervans today. It is relatively simple to wire up, low cost, and an efficient way to charge up your leisure batteries!

So what's the difference between a voltage sensitive relay (VSR) and wiring a split charge relay (SCR)?

As discussed, a SCR will switch on when it senses even the smallest amount of electrical input. In contrast, a VSR will only switch on when it senses a certain level of voltage.

In fact, the VSR monitors the voltage levels of both the starter battery and the leisure battery (dual sensing). The VSR will turn on when it senses a voltage level of around 13.8v. A 12-volt battery produces about 12.8v of electricity when fully charged. Whereas an alternator will typically produce between 13.8 to 14.6 volts of electricity when active.

The dual sensing capability of the VSR is very handy as it means we can charge our starter batteries if our leisure batteries are full (if we are receiving solar or shore power).

Installing a voltage sensitive relay is quite straightforward.

Simply wire the positive terminals of the starter battery and leisure battery together with 6AWG cable. Then pop the VSR right in the middle. You will also want to add a 100a fuse on either side of the switch (right beside each battery). You will also need to ground the VSR, which means running a wire to the ground point in the vehicle chassis.

Luckily, voltage sensitive relays usually come as kits like this one, meaning you don't need to buy the individual parts! 🎈

split charge relay wiring diagram

Unfortunately, it is not advisable to use voltage sensitive relays with smart alternators (If your van is newer than ~2015).

You cannot use voltage sensitive relays with lithium batteries.

Battery to battery charging

Battery to battery chargers (B2B / DC to DC) are the bomb! They have a lot of really cool benefits and are top of their class.

Battery chargers are devices that takes the power coming from the alternator and then boost or reduce that power in a controlled manner, depending on what is needed where (intelligent charging).

They have fast charge capabilities and protect against overheating, reverse polarity, and overcharging.

One of the most important benefits of a DC to DC battery charger is that they work with smart alternators (which we will discuss in the next section) - this is important if you vehicle was built after ~2015.

Another awesome benefit of battery to battery chargers is that you can use them with lithium batteries!!

It's worth pointing out that all other types of split charge relay kit will never fully charge your leisure batteries (due to charging profiles, which is discussed below). Whereas, battery chargers can achieve a 100% charge.

Though, inevitably these benefits do come with added some cost!

DC to DC Battery charger wiring diagram renogy van conversion

I'd highly recommend checking out Renogy's DC to DC battery charger.

What is intelligent charging?

The key differentiator of DC to DC battery chargers is that they have intelligent charging functionality. They transform the voltage and current from the alternator to produce the correct charging profile for the leisure battery being charged.

Charging profiles follow three stages of charging: bulk, absorption and float. The charging limits differ between the different battery types

  • Bulk: a fast, constant current charge up to ~80% State of Charge (SoC)

  • Absorption: much slower, constant voltage charge to reach 100% SoC

  • Float: a constant voltage charge which maintains 100% SoC by counteracting self-discharge

Image from mowgli-adventures

Battery to battery chargers can be programmed to cater to the different battery types.

An important note on smart alternators (N.B)

Smart alternators (ECU controlled charging systems) are becoming more and more common these days as vehicle manufacturers seek to meet European emission regulations. These are commonplace in T6s, Transits , Renault Masters and Mercedes Sprinters, amongst most other new vehicles.

A smart alternator initially run at a lower voltage compared to a traditional alternators. They then increase the voltage (~17v) dramatically when the vehicle lifts off, this high voltage very quickly charges the battery. It is for this reason that we cannot use manual switches, split charge relays (SCR), voltage sensitive relays (VSR), or split charge diodes with smart alternators. It would be verdangerousus indeed to sent 17v to our leisure batteries as they are not rated for this current. You can read more about the different types of leisure batteries here.

So if your van has a smart alternator (lucky you), you will need to grab yourself a DC to DC battery charger.

Why do I need a battery to battery charger for split charging a lithium battery?

Lithium batteries and alternators don’t get along. Compared to lead acid, lithium batteries draw a much higher current from the alternator. This higher current is bad for both the lithium battery and the alternator. The amps pulled by the battery can exceed the alternator’s maximum rated amperage. No bueno.

To add even more fuel to the flame, lithium batteries and lead acid batteries don't get along! The two battery types have slightly different charging profiles. When the split charger is closed, the batteries are directly connected which is not good. In fact, it's never recommended to connect batteries of different chemistries. Their lifespan will be shortened and they become a lot less efficient.

So what should you do? You need to use a DC to DC battery charger to split charge lithium batteries. The battery charger will send the correct current to the lithium leisure battery according to the stage it is at in it's charging cycle.

What is the best split charge relay kit?

For all the reasons mentioned above (better and faster charging + ability to work with lithium batteries), I would highly recommend Renogy's DC to DC battery charger. Ranging from 20a to 60a it is one of the very best split charge relay kits out there.


I hope you found this article on wiring a split charge relay in your campervan useful! Don't forget to subscribe to The Van Conversion Newsletter for everything you need to get started with your own van conversion (I'll send you a free wiring diagram when you sign up).

Be sure to check out the rest of the Electrics Guides. Related articles include: Campervan inverters, grounding electrics, leisure batteries.

If you're converting a van but unsure of how to do it, you could also check out the Van Conversion Course on Udemy. In the course, you'll learn directly from me how to convert a van into your dream home - no prior experience needed!

Until next time,

Shane ✌️

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