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The Ultimate Guide to Campervan Split Charging Systems

Want to charge your leisure batteries while you drive? Well, you're going to need a split charging system! In this chapter we will learn about the different devices you can use including: a manual switch, split charge relay, voltage sensitive relay, split charge diode, and smart chargers.

I'm Shane, I've been teaching people to convert campervans for years; I'm the author of Roaming Home; The Comprehensive Guide for Converting Your Van Into a Campervan, writer of The Van Conversion Newsletter, instructor of The Van Conversion Course over at Udemy. And full-time vanlifer for 4 years!


So let's jump in and have a look at campervan split charging systems!


Index

 

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


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!

 
Campervan electrics guide

What is a split charging system?

A split charging system charging both the starter batteries and leisure batteries at the same time while you drive.

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. 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 batteries in contrast release energy in a steady flow over a longer period. They are designed to withstand hundreds / thousands of charging cycles.

In this article we will explore many different split charge systems, but if you just want to cut right to the chase...

Smart chargers are the best choice for most people. They have fast charge capabilities and protect against overheating, reverse polarity, and overcharging. They are the only split charge system you should use with lithium batteries, or with vehicles newer than 2015 (smart alternators).

I highly recommend the Renogy smart charger.

How to wire a split charging system

At the most basic level, a split charging system 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 van converter’s humble opinion, a split charger is an absolute necessity.

There are five ways one can split charge a campervan (here they are 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. Smart charger (also known as a B2B or DC-DC charger)

In the Roaming Home 2023 study we found that 55% of van conversions are still using split chargers (SCR / VSR) rather than B2B chargers. Though there is no doubt that with the increasing popularity of lithium batteries, B2B chargers will soon take over. They are a far more efficient charger.

split charger vs smart charger

Note: 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 DC-DC battery charger, you cannot use a SCR or VSR.

How an alternator works: A lead acid battery is fully charged at ~12.8V. 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.

Manual switch

First up we have the 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 (16 mm² 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 charge

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 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.

Note: It is not advisable to use manual switches with smart alternators. You cannot use manual switches with lithium batteries.

Renogy b2b charger

Split Charge Relay (SCR)

What is a split charge relay?

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.

Note: It is not advisable to use split charge relays with smart alternators. You cannot use split charge relays with lithium batteries.

Split charge relay wiring diagram

Note: These days, Voltage sensitive relays (VSRs) or battery chargers are more commonly used than split charge relays.


With that being said, let's look at the split charge relay wiring diagram below.

  • The positives of the starter and leisure batteries are connected through the split charge relay - ensure a fuse (normally 100A, though can vary) is installed on each side.

  • Ensure the two batteries and split charge relay is grounded

  • The split charge relay should have a D+ connection (ie. it is connected to the alternator)

Split charge relay wiring diagram
Split charge relay wiring diagram

The D+ connection is a 'trigger wire' for the split charge relay (which detects when the engine turns on).

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.

Note: It is not advisable to use split charge diodes with smart alternators. You cannot use split charge diodes with lithium batteries.

Voltage Sensitive Relay (VSR)

What is a voltage sensitive relay?

A voltage sensitive relay is the most common split charging device found in campervans. It is relatively simple to wire up, low cost, and an efficient way to charge up your leisure batteries for vehicles made before 2015 (before smart alternators were introduced).

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.

Durite are the most well known manufacturer of voltage sensitive relays. They sell full split charge relay kits that are very easy to install.

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 12V 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 (16 mm²). 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).

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


Voltage sensitive relay wiring diagram

Let's look at the voltage sensitive relay wiring diagram below. As mentioned, Durite is the most popular manufacturer of VSRs.


The wiring for a voltage sensitive relay is very similar (but simpler) to the wiring for a split charge relay.

  • The positives of the starter and leisure batteries are connected through the voltage charge relay - ensure a fuse (normally 100A, though can vary) is installed on each side.

  • Ensure the two batteries and voltage charge relay is grounded

Voltage sensitive relay wiring diagram

In the case of a voltage sensitive relay, a trigger wire to the D+ connect is not needed like with other split chargers. This is a nice benefit!

Note: It is not advisable to use voltage sensitive relays with smart alternators. You cannot use voltage sensitive relays with lithium batteries.

Smart chargers

What is a smart charger?

Battery to battery chargers (B2B / DC-DC / smart charger) are the bomb! I have been completely convinced by them over the past couple years.

B2B chargers are devices that take the power coming from the alternator and then boost or reduce that power in a controlled manner, depending on what is needed. This is known as intelligent charging.

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

One of the most important benefits of a smart charger is that they work with smart alternators. This is important if you own a vehicle that was built after ~2015.

Another big benefit of B2B chargers is that you can use them with lithium batteries.

campervan diagram pack

B2B charger wiring diagram

The B2B charger wiring diagram shown below, is similar to that of a split charge relay.

  • The positives of the starter and leisure batteries are connected through the split charge relay - ensure a fuse (normally 100A, though can vary) is installed on each side.

  • The negatives of the batteries are also connected through the smart charger

  • Ensure the two batteries are grounded

  • The split charge relay should have a D+ connection (ie. it is connected to the alternator)

B2B charger wiring diagram

I'd highly recommend checking out the Renogy smart charger.

It's worth pointing out that all other types of split charge relay kit will never fully charge your leisure batteries. Whereas, DC-DC battery chargers can achieve a 100% charge.

What is intelligent charging?

The key differentiator for B2B 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. This is why they are commonly referred to as “smart chargers”.

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

What size B2B charger should I use?

Alternators can easily be burnt out if overloaded at low revs/low cooling airflow. It is recommended not to exceed 40% loading of an alternator - so if the alternator is 125A rated then the B2B charger should be 50A max. So long as you stay below that 40% threshold, you should get the biggest B2B charger you can.

What size fuse do I need for split charging?

You will need to use a different size fuse depending on the amperage of the battery charger you have. Typically ANL fuses are used.

Here Renogy's recommended fuse sizes for their battery chargers:

20A battery charger

Starter battery to battery charger: 30A

Battery charger to leisure battery: 25A


40A battery charger

Starter battery to battery charger: 60A

Battery charger to leisure battery: 50A


60A battery charger

Starter battery to battery charger: 90A

Battery charger to leisure battery: 75A

Renogy's recommended fuse size for battery chargers
Renogy's recommended fuse size for battery chargers

Split charge relays and voltage sensitive relays

VSRs and SCRs typically come in a kit where you will be provided the appropriate fuses. In general, you will be supplied with two 100A ANL fuses.


Smart alternators

Smart alternators (ECU controlled charging systems) are becoming more and more common these days as vehicle manufacturers seek to meet European emission regulations.

A smart alternator initially runs at a lower voltage compared to a traditional alternator. It then increases 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 very dangerous indeed to send 17V to our leisure batteries as they are not rated for this current.

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


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 B2B 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 very different charging profiles. If we connected them with a split charge relay, the two types of batteries would be 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? Firstly, ensure your starter battery’s chemistry matches your leisure battery’s chemistry. 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 its charging cycle.


What is the best split charging system?

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


Durite split charge relay wiring diagram

Durite are a very well-known manufacturer of voltage sensitive relays (often referred to as split charge relays). Below is the wiring diagram for the Durite split charge relay wiring diagram below. In fact, this is the generic split charge relay wiring diagram, however Durite is the most popular manufacturer.

Durite split charge relay wiring diagram

Note: It is not advisable to use voltage sensitive relays with smart alternators. You cannot use voltage sensitive relays with lithium batteries.


How to test a voltage sensitive relay (or any split charging system)

When the engine of your vehicle is turned on, it will send out between 13.5-14.5V. If you have installed your split charging system correctly, you should see your leisure battery levels match that voltage. You can find the voltage of your leisure batteries using a battery monitor (the Victron SmartShunt is very popular) or with a digital multimeter. If you have a campervan solar system wired up, you can also monitor you battery levels with your MPPT.


Conclusion

I hope you found this article on wiring a split charging systems 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 join).


If you're looking for some guidance with your van conversion, you might be interested in Roaming Home; The Comprehensive Guide for Converting Your Van Into a Campervan. In the 380-page book (or ebook), you'll learn directly from me how to convert a van into your dream home - no prior experience needed!

Roaming Home Book

Until next time,

Shane ✌️

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