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, a van conversion professional dedicated to helping people transform ordinary vans into homes on wheels. I've authored Roaming Home, and teach The Van Conversion Course, guiding many people through their van builds. I also write The Van Conversion Newsletter, where I share practical tips and insights. After completing two van builds and living on the road full-time since 2020, I’m excited to share my expertise with you.
So let's jump in and have a look at campervan split charging systems!
Index
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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):
Manual switch
Split charge relay (SCR)
Split charge diode (Charge splitter)
Voltage sensitive relay (VSR)
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.
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.
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).
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.
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)
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
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.
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)
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?
Determining the correct size DC-to-DC charger is a little tricky, to help we built a DC-to-DC charger calculator which will help you figure out exactly which smart charger you need depending on your alternator and battery bank.
To determine the correct smart charger size you will need to know the current output of your engine's alternator.
Finding Alternator Current Output
Determining the alternator's current output can be crucial for choosing the right DC to DC charger. Here’s how you can find it:
Check the label on your alternator, which usually lists its current rating in amps (A).
If the label is not visible or legible, search for the alternator’s part number online or consult your vehicle's manual.
For convenience, you can also use online parts databases such as Euro Car Parts (for UK and Europe) or Parts Geek (for the US) by entering your vehicle's registration number.
Alternator Output by Vehicle Type
Different vehicles come with different alternator sizes, which dictate how much current (measured in amps) can be generated to power your van's electrical system. Here’s a quick breakdown of typical alternator outputs for various vehicle types:
Vehicle Type | Example Models | Typical Alternator Output (Amps) |
Small Vans | Citroën Berlingo, Ford Transit Connect | 70A - 90A |
Medium Vans | Volkswagen Transporter, Mercedes Vito | 90A - 130A |
Large Vans | Mercedes Sprinter, Ford Transit | 130A - 180A |
Heavy-Duty Vans | Fiat Ducato, Renault Master | 180A - 250A |
Alternator Output by Specific Van Models
Certain van models are equipped with alternators that have a specific range of output, depending on engine size. For instance:
Van Model | Engine Size | Alternator Size (Amps) |
Ford Transit | 2.0L EcoBoost | 150A |
3.5L V6 Gasoline | 150A - 200A | |
Mercedes Sprinter | 2.1L Diesel (I4) | 180A |
3.0L V6 Diesel | 200A - 220A | |
RAM ProMaster | 3.6L V6 Gasoline | 180A - 220A |
Ford E-Series (E-350) | 5.4L V8 Gasoline | 130A - 160A |
Chevy Express/GMC Savana | 4.3L V6 or 6.0L V8 | 145A - 160A |
Nissan NV3500 | 5.6L V8 Gasoline | 130A - 170A |
Matching Alternator Output to Your Split Charging Needs
To size your alternator correctly, you must also consider the battery type and size in your campervan. Lithium batteries, for example, can draw a much higher current, and this can place a heavy burden on the alternator.
As a general rule of thumb:
For AGM or Gel batteries, you can typically get by with a smaller alternator if you have a small battery bank (under 200Ah).
For lithium batteries, it’s critical to have a larger alternator, especially if your battery bank exceeds 200Ah or if you are running a high-power system (e.g., a 2000W inverter).
You’ll also need to ensure your alternator is not overloaded. As a safety measure, avoid exceeding 40% of your alternator's output with your split charging system. For example, a 150A alternator should not support more than a 60A DC-DC charger.
Again, I recommend you use our DC-to-DC charger calculator to simplify the process.
Thermal Protection and Heat Management in Charging Systems
Thermal protection and heat management are critical for maintaining the longevity and efficiency of split charging systems. Chargers, alternators, and relays generate heat during operation, especially under high loads. Modern DC-DC chargers often come with built-in thermal protection features like automatic shutdown and temperature sensors to prevent overheating. If the device reaches a specific threshold, typically around 85-95°C, it will automatically shut down or reduce output to prevent damage.. Additionally,you should install devices in well-ventilated areas help dissipate heat effectively.
To avoid overheating, ensure the alternator load stays below 40-50% of its maximum output.
Leisure Battery Maximum Charge Current
The maximum charge current your leisure battery can handle depends on the type of battery you’re using. Exceeding these values can reduce battery life or cause damage. Here’s a general guide:
Battery Type | Max. Charge Current (% of Battery Capacity) | Example (100Ah Battery) |
AGM | 20% | 20A |
GEL | 30% | 30A |
Lithium-ion | 50% | 50A |
These values are based on manufacturer recommendations and should be checked against your battery’s specification sheet for accuracy.
Alternators can easily be burnt out if overloaded at low revs/low cooling airflow. So as a general rule of thumb 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.
As I mentioned before, sizing a DC-to-DC charger is a little tricky so I recommend using our calculator.
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
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.
What size wire do I need?
Choosing the correct wire size is essential to ensure safety and efficiency. Use the table below as a guide for selecting the appropriate wire size based on the charger's amperage and the distance from the alternator to the leisure battery:
Charger Amperage | Distance (Up to 3 meters) | Distance (Up to 6 meters) | Distance (Up to 9 meters) |
20A | 6 mm² | 10 mm² | 16 mm² |
40A | 10 mm² | 16 mm² | 25 mm² |
60A | 16 mm² | 25 mm² | 35 mm² |
Voltage drop in split charging systems
When it comes to split charging systems, voltage drop is a critical issue. Split charging relies on transferring power from your van's alternator to the leisure battery while driving. If your cable runs are too long or the cables are too thin, the resistance in the wires can lead to a significant voltage drop, meaning the leisure battery might not charge efficiently or fully.
To minimize voltage drop in split charging systems, you need to use thicker cables (low-gauge wire) and keep cable runs as short as possible. A general rule is to size your cables based on the charger’s amperage and the distance between the alternator and the leisure battery. For instance, a 40A split charger may need at least 10 mm² cables for short runs but could require 16 mm² or larger cables for longer runs. Inadequate wiring can result in poor charging performance and even damage your charging equipment over time.
The wire sizing above is purely informational, please do your own wire sizing.
Voltage drop can be surprisingly sinister, check out the graph below to see how cable length and wire diameter affect voltage drop.
Regulatory Guidelines for Split Charging Systems (BS 7671 & NEC)
When installing a split charging system in your campervan, it's essential to follow guidelines from both BS 7671 and NEC to ensure safety and compliance. Below are the key points from each regulatory standard.
BS 7671 - UK Wiring Regulations
Article 721.521 – Cable Management: Cables must be installed in a way that prevents mechanical damage and exposure to heat. Split charging cables should be adequately supported and protected in conduit or sheathing, especially where they pass through metal chassis or compartments.
Article 721.411 – Protective Earthing and Grounding: All metal parts of the system, including the leisure battery, alternator, and bus bars, must be connected to the vehicle chassis to provide a low-resistance path for fault currents.
Article 721.43 – Isolation and Switching: Ensure that all circuits, including split charging systems, can be easily isolated. You must install an isolator switch between the starter battery and leisure battery to prevent discharge when the system is not in use.
Article 433.2.2 – Overcurrent Protection: This article specifies that all electrical circuits must have appropriate overcurrent protection. Fuses or circuit breakers should be installed close to the batteries (both starter and leisure) to protect the system from short circuits or overload.
National Electrical Code (NEC) - United States
Article 551.54 – Overcurrent Protection: All circuits, including those for split charging, must have overcurrent protection. Fuses should be placed as close as possible to the source (batteries). For example, install fuses on the positive lines between the starter and leisure battery in a split charging system.
Article 551.56 – Grounding Requirements: This article outlines that all electrical systems in RVs must have proper grounding. The split charging system should ground the leisure battery and alternator to the vehicle chassis to avoid electrical faults.
Article 551.20 – Voltage Drop: This article emphasizes minimizing voltage drop in long cable runs to ensure efficient charging. It recommends using low-gauge wires, especially for high-current systems like B2B chargers, to maintain proper voltage levels during operation.
General Guidelines for Split Charging Systems
Fusing:
Both BS 7671 and NEC mandate the use of appropriately sized fuses in your split charging system. For a B2B charger, install fuses on both the starter and leisure battery sides. The fuse size depends on the charger's amperage.
For example, a 20A charger typically requires a 30A fuse, while a 40A charger needs a 60A fuse.
Grounding:
Proper grounding is vital to prevent electrical faults. Ensure that all components in the split charging system, including the battery charger, are grounded to the vehicle chassis according to the regulations.
Cable Size and Voltage Drop:
To avoid significant voltage drop, use thicker cables (low-gauge wire) based on the current rating and the length of the cable run. For instance, a 40A charger may need 10 mm² cables for shorter runs, but up to 16 mm² for longer distances.
Smart Alternators:
If you have a smart alternator, you must use a DC-DC charger (B2B charger), as smart alternators have variable output that can damage a traditional relay-based split charging system.
Lithium Batteries:
Lithium batteries have different charging profiles from lead-acid batteries, and traditional split charging systems (SCRs, VSRs) are not suitable. You must use a DC-DC charger to regulate the voltage and current appropriately for lithium batteries.
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 (Euro 8 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.
Here is a brief overview of smart alternators:
Energy Efficiency: Unlike traditional alternators, which continuously charge the battery at a constant rate, smart alternators dynamically adjust their output based on the vehicle’s current energy needs. This reduces the engine load, thereby improving fuel efficiency and lowering emissions.
Regenerative Braking: Smart alternators are often integrated with regenerative braking systems. During braking, they maximize energy recovery by increasing the alternator's output to charge the battery. This energy can then be used to power electrical systems or assist in starting the engine, further reducing the need for fuel.
Stop-Start Systems: Many vehicles with smart alternators also feature stop-start technology, where the engine shuts off when the vehicle is stationary and restarts when needed. The smart alternator ensures that the battery remains sufficiently charged to support repeated engine starts without additional strain on the system.
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.
To figure out the correct size smart charger for your van I recommend using our smart charger calculator.
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.
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 our book Roaming Home, or in our online course The Van Conversion Mastery Course. You'll learn directly from me how to convert a van into your dream home - no prior experience needed!
Until next time,
Shane ✌️