Campervan Split Charging: The Complete Guide

When you’re ready to start on your campervan conversion, you’ll no doubt want to have electrically powered devices inside your camper van that you’ll want to use when you’re parked up with the engine turned off.

These devices can be low consumption items such as your LED lights, 12V power sockets through to the more power demanding things such as a 12V coolbox/fridge, laptop, inverter or water pump.

If you try to run these devices from your engine battery without it running in the background, you can soon expect to get a flat battery.

So the standard solution is to fit one of more leisure batteries that are used to power all your devices that are not directly related to starting the engine.

But with installing a leisure battery, you will now need a way of keeping all the batteries charged with that all-important power.

This is where campervan split charging comes in to play.

How does a Split Charge System work in a Campervan?

split charging wiring system in a campervan
A typical split charge system in a campervan.

In simple terms; a campervan split charging system is a way of charging both the starter battery and leisure battery from the same power source.

On your campervan, the power source will almost always be the vehicle alternator. The vehicle alternator is powered by the engine when running.

How does the leisure battery charge?

split charge wiring diagram engine off on
A super simple wiring diagram on how a split charge device directs power towards the leisure battery one it has been engaged.

A typical campervan split charging system has a device that connects the battery used to start the engine to the leisure battery when the engine is running.

Once the engine is running, the power from the alternator can now flow to both your starter battery and leisure battery, charging both batteries simultaneously.

What are other ways to charge your leisure battery in a Campervan?

other ways to power your leisure battery can be solar, shore power, generator
Solar Panels, Shore Power and a Generator can all be used to power your campervan.

There are many other ways you can charge up your leisure battery that does not involve your alternator.

These can be;

All these methods described above can be used to charge your leisure battery.

Why do I need to install a split charge system in my campervan?

By installing a campervan split charging system, you can avoid flattening your starter battery.

One reason why you would want to install a split charge system in your campervan is to isolate both the starter and leisure battery.

It will ensure that the use of one battery does not draw current from the other while in a stationary position and not receiving any external power (alternator, mains charger, solar).

However, it is essential that the engine starter battery is isolated to prevent using all the power in both batteries.

If this happens, you will eventually end up with a flat starter battery, and this can leave you stranded.

By installing a campervan split charge system, you can prevent a flat starter battery.

This will also guarantee that the starter battery can always start your engine.

Another reason why you would want to install a split charger system in your campervan is you want to avoid your leisure battery to be accidentally used when starting the engine.

Some dedicated leisure batteries are not designed for the initial high current draw used to turn the vehicle engine over.

Campervan Split Charger Methods/Technologies

Below, we have listed the most common split charge methods used in a campervan or motorhome.

1: Manual Switch

12v campervan manual switch split charge wiring diagram connected to the starter battery and the leisure battery.
A typical 12V manual switch split charge campervan wiring diagram.


  • Cheap and straightforward to install.
  • Robust.
  • Quick and easy to combine both batteries to start the campervan in case of emergency (e.g. starter battery is flat).


  • Inconvenient.
  • Installing a switch relies on the driver to always remember to flick the switch ON and OFF.
  • Forgetting to turn the switch back to off and leaving your batteries connected could result in a flat battery.

The simplest way to connect your leisure battery to your power source (alternator) is by installing a cable from your starter battery to the leisure battery.

You would then install a high current switch located in a convenient location (usually on your dashboard) that you manually turn on and off to connect the starter battery and leisure battery together, once the engine is running.

By switching the switch to the on position, this will allow an electrical charge to flow to the leisure battery. Once you turn the engine off, simply flick the switch back to the off position to now isolate all batteries.

As a result, this will prevent you from flattening the main starter battery when using power from the leisure battery.

What We Think

Unfortunately, this is not the ‘smartest’ solution out there. One of the issues with installing a switch is that you can easily forget to turn it back to the off position. This can then leave you vulnerable to flattening the starter battery. And we are sure you will agree that getting stranded with a flat battery can be a right pain!

The only real benefit to this method is that its a cheap and relatively easy to install.

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Products We Recommend

We have listed below what we believe are the best value for money manual switches you can install for you split charge system.

2: Split Charge Relay

12v campervan split charge relay wiring diagram connected to the leisure battery.
A typical 12v campervan split charge wiring diagram that connects the starter battery to the leisure battery.


  • Simple automatic operation.
  • Inexpensive compared to alternate methods.
  • Simple electro-mechanical construction.
  • Easy to replace.


  • Internal moving contacts can wear out. This can lead to increased resistance / voltage drop.
  • There is no ‘intelligence’ built into this relay.

Inside a split charge relay, there is a solenoid coil that energises once it receives a +12V signal. This signal is usually from a charge signal wire that is connected to the alternator and will only turn the charge splitter on once the engine is running. When energised, the solenoid inside will activate and connect the two internal contacts.

As the internal contacts are now connected, power will begin to flow down a separate cable that is connected from your split charge unit to your leisure battery positive terminal. Once you turn the engine off, the split charge relay will detect the drop of current and disconnect the internal connections. It will now isolate both batteries.

As a split charge relay may have to handle a high current coming from the alternator, you may have to install a more heavy-duty device.

What We Think

Before the availability of the more intelligent and the now popular Voltage Sensing Relay came to market, this device was the most common type of split charge relay used.

Although this type of relay is still available today, many people are wary of tapping into their vehicles electrical system in case of damage and potentially affecting the warranty of the vehicle.

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Products We Recommend:

3: Voltage-Sensitive Relay (V.S.R.) / Intelligent Split Charge Relay

12v campervan intelligent voltage sensitive split charge relay wiring diagram
A typical 12v Voltage Sensitive Relay (VSR) campervan wiring diagram.


  • Intelligent.
  • Automatic operation.
  • Relatively low-cost.
  • Simple to install (you don’t need to modify the factory-fitted electrical system – so no warranty issues).
  • V.S.R.’s are usually dual-sensing. It means that when the leisure batteries are charging by solar panels or an AC battery charger, and it is above 13.2V, the V.S.R will automatically engage, which will then charge the crank battery.
  • Some models have a smart manual override switch. This is used to engage and connect the crank and leisure batteries manually. What’s brilliant about this feature is that it can use it when the crank battery is low on charge, and the leisure batteries are full.


  • Over time, internal moving contacts can wear out which can lead to increased resistance and voltage drop

A voltage-sensitive split charge relay, or V.S.R., intelligently detects the incoming voltage coming from the engine at a preset level and automatically connects the contacts inside.

How does a VSR work to charge my campervan leisure batteries?

When you start the van engine, the alternator will now begin to produce an electrical current. This is is usually around 14 volts (assuming the battery is charged to its maximum).

The voltage sensing relay will detect that the voltage has increased passed a certain level, and will now automatically connect its internal contacts. As the internal contacts are now connected, power will begin to flow through the unit. The power will leave the relay down a connected cable and will travel to your leisure battery positive terminal. The leisure battery will now begin to charge.

Once the engine is shut off, the V.S.R. will detect the drop in voltage and the relay will now disconnect the internal contacts.

What voltage does a voltage sensing relay activate and de-activate?

A voltage sensing relay typically activates around 13.3 volts and deactivates at 12.8 volts. (make sure to check over the manufacturer’s specs as certain models vary)

What We Think

Voltage-Sensitive Relays are probably the most commonly used relay for many self-build campervan split-charge methods.

They’re simple to fit, reliable, and are relatively cheap compared to other technologies available.

Some of the more intelligent voltage-sensitive relays can monitor the voltage of the vehicle over a long period.

Once the VSR has enough data of the electric system, it is then used to open and close the relay at a more accurate voltage level.

As vehicles can suffer from voltage fluctuations, this feature should help eliminate unwanted opening and closing of the relay.

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Products we recommend:

After extensive research, we have listed what we believe are the best voltage seeing relays on the market right now.

4: Battery Isolator / Charge Splitter (Diode)

12v campervan charge splitter battery isolator split charge diode relay wiring diagram
A typical 12v Charge Splitter / Battery Isolator (Split Charge Diode) campervan wiring diagram.


  • Automatic operation.
  • No internal moving parts so contacts won’t wear out.
  • Easy to install.
  • Battery splitters often have multiple outputs for numerous batteries.


  • They can suffer from voltage drop and may not charge batteries to 100%.
  • You might have to install an after-market alternator regulator to increase the voltage.
  • Can become hot under operation.
  • You will not be able to combine your batteries for emergencies unless you install a bypass switch.
  • Higher-costs.

A simple yet reliable way of splitting the charge in your campervan is with the use of a charge splitter/battery isolator.

Inside a charge splitter, you will usually find two separate diodes (a diode allows an electric current to pass in one direction while stopping it going in the opposite direction. Or in even simpler terms, a one way valve.

Once the engine is running, the power will make its way from the alternator to the split charge unit by a connected cable via a common stud.

campervan charge splitter
A charge splitter splitting the power.

From here, the power is then equally split internally and diverted down each of the internal diodes (hence the name charge ‘splitter’).

Once the power has finally passed through each of the internal diodes, it will eventually come to the end of the unit where you will find two separate ring terminals.

Each of these terminals will now have an independent positive power supply.

You connect a cable to one of the ring terminals that you then run back to your starter battery positive (+) terminal.

And on the other ring terminal, you connect another cable that will run along the inside of your vehicle all the way to your leisure battery positive (+) terminal.

Once all connected; When you turn the engine on, and the power begins to flow from the alternator – the split charge device will now efficiently supply both the starter battery and leisure battery with an equal amount of power. Once you turn the engine off, as both the batteries are connected through individual diodes, the power cannot flow back on its self preventing you using any power from the starter battery.

The only issue with using a charge splitter in a campervan is they can have a significant voltage drop. In some charge splitters, this can be as much as 1V.  When fitted in a 12V electrical system, this can be quite a significant drop.

It means that the leisure battery will never get the total voltage coming from the charge source (alternator).

Therefore this can prevent the leisure battery from becoming charged to its maximum.

What We Think

Charge splitters are popular in the marine sector and slowly growing in strength in the self-build campervan market.

Many people choose to install a charge splitter for their simplicity and durability despite having a voltage drop.

Some manufacturers are now starting to use more advanced technologies to reduce voltage drop issues by the use of field-effect transistor-based charge splitting and low-voltage drop hot-carrier diodes.

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Products we recommend

What is the best Split Charge System for my Campervan?

common split charge technologies to use in your campervan.
A list of the common split charge technologies to use in your campervan.
Please Note: Every self-build campervan installation will differ, so please use the points listed below as a guide only.

Simple Split Charge Relay System

Answer: Voltage-Sensitive Split Charge Relay (VSR)

If your electrical system consists of a single leisure battery with a few electrical devices such as some LED lights, fridge, water pump and a few 12V power sockets, then a VSR is your best option.

Complex Split Charge Relay System

Answer: Battery Isolator 

If your electrical system consists of a multi-battery setup with several electrical sub-systems and many electric devices, then one or more battery isolators is the best option, although a VSR is still very practical.

EURO 5 + 6 Compliant Systems

smart alternator battery-to-battery charger

Important: Many new vehicles come with a ‘smart’ alternator. A smart alternator allows the vehicle to control the output voltage from the alternator intelligently.

Answer: Battery to Battery Charger

Generally, you will find smart alternators in vehicles fitted with Euro 5 or 6 compliant engines.

If you do have a vehicle fitted with a smart alternator, the tips outlined above will not be suitable for these types of alternators. You will have to install a Battery to Battery charger instead.

Sizing your Connecting Cables

When a battery becomes deeply discharged, it will now have low resistance. When attached to a charging source, the battery will draw significant current.

For this reason, it is imperative to install the correct cables size when installing your split charging system. All cables should be rated to handle the maximum current the alternator is capable of supplying.

Failure to fit correctly rated cable sizes can result in permanent damage to your batteries, split charge unit and other electrical devices.

In the worst-case scenario, it can cause an electrical fire which can have disastrous consequences.

What size cable should I use for my split charge relay?

When installing the cable in your split charge relay system, use a cable of at least 16mm2 cross-section (110A) and upwards. It should be sufficient for most vehicle alternators.

Please Note: This is dependant on many factors including the alternator output, the application and length of cable.

The same practise should be used for the split charging device itself. Cables should be rated to handle high current draw from the alternator at under regular load as well as if the batteries are discharged to a low level.

You should also note that the starter battery is also a charging source. It will try to equalise the voltage when connected to the leisure battery.

If your leisure battery becomes severely drained, the combined current from both the starter battery and power source (alternator) can exceed what the alternator produces on its own.

It is why you should always try to prevent a leisure battery becoming severely drained (dropping below 50% of its total capacity).

To prevent this from happening, you can either install a battery warning alarm or connect a battery guard.

Sizing your Split Charge Relay Unit

When selecting your split charge unit, it is essential to purchase one that can handle the maximum output of the alternator of your vehicle.

Due to the widespread rise of self-build campervan projects, there are now many campervan relay charge kits available on popular online retail sites like eBay and Amazon.

We advise that you should research your vehicles technical specifications as many of these kits are not rated high enough for the maximum output for the alternator of your vehicle.

If you do end up buying a kit that is not the correct rating, providing that your electrical system is adequately fused, this should not be a problem as you will blow fuses.

Always carry out the correct research for your vehicle.

Multiple-battery split charge system

If you want your campervan electric system to be able to run multiple electronic devices, or you are building a campervan that will be going ‘off-grid’ for long periods, then you may require more than one leisure battery setup in a parallel circuit.

It will allow you to increase the total capacity of energy available in the campervan. This is a battery bank.

If you choose this option, it is essential that all batteries have an equal charge.

Practical tips when installing a multiple-battery split charge system:

    • When running multiple cables from the split charging device to each of the batteries, try and use a cable of the same gauge and length and try to keep the cables as short as you can.
    • Furthermore, the negative (ground) cables running from each battery should be of the same gauge and length and try to keep the cables as short as you can.
    • When installing a battery bank, the cables that go from battery to battery should also be as short as possible.
    • Try and use batteries that are the same brand.
    • All batteries must have the same amp-hour rating.
    • Likewise, the batteries should be of the same age as batteries lose charge holding ability with age.
    • Batteries should be discharged evenly so that they ‘age’ at the same pace.

We know that these tips here may not be practical in real-world scenarios. If you use the tips above when installing your leisure batteries, this should extend the entire life of the batteries.

For more advanced battery health solution

Installing an intelligent battery monitor that can optimise the charging for each battery can make the above guide less critical.

Can you combine the leisure battery to the engine battery in an emergency?

You jump in the driver’s seat after a nights camping, go to start the engine then…click, click, click! – Ooops…you have a flat battery!

Looking around, you notice you forgot to turn the exterior lights off and unfortunately left the lights on overnight while camping, resulting in the flat battery.

What do you do now?

Thankfully, you have a leisure battery on board and depending on how much power is available in the battery from the night before; you should be able to transfer the power successfully to start your vehicle engine…

Method Via: Manual Switch

Switch the manual switch to the on position to let the power flow back from the leisure battery to the starter battery.

Give it around 10 minutes to build up enough charge in your starter battery and you can now safely close the switch.

Turn the key, and hopefully, the engine should turn over and should now successfully start-up.

Method Via: Split Charge Device (Split Charge Relay / VSR / Charge Splitter)

Some split charge devices come with a feature that allows you to close the internal contacts momentarily.

To engage this feature on the device, this is usually with a button installed on the device itself. Some models feature an external remote switch that is wired directly into the cab department for ease of use.

The split charge device will now allow power to flow back from the leisure battery to the starter battery for a specified period. (Check out manufactures specifications on the amount of time)

The split charge device should have allowed the leisure battery to have charged your starter battery up to the point where you can now start your engine successfully.

What if I have no type of emergency switch?

If your split charge device lacks such a device, you might want to consider adding a manual switch to the circuit to by-pass the split charge device.

Important: Always take care when using this feature. Currents used by the starter motor to start the engine can be extremely high. It is vital the switch that you install has a sufficient rating. Failure to do so can result in blown fuses, your switch welding together making it useless or in worst-case scenarios, cause an electrical fire.
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