A campervan split charging system is the core of any off-grid electrical system, along with a solar system. These systems quickly charge your leisure batteries whilst you drive by taking a small amount of power from the van's running engine. In this chapter, we'll learn about the different devices you can use for split charging, including manual switches, split charge relays, voltage sensitive relays, split charge diodes, and smart battery-to-battery chargers.
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Shane, the founder of The Van Conversion, is a campervan professional dedicated to helping people transform ordinary vans into homes on wheels. He has authored Roaming Home, and teaches The Van Conversion Course, guiding many people through their van builds. Shane also writes The Van Conversion Newsletter, where he shares practical tips and insights. After completing two van builds and living on the road full-time since 2020, he is passionate about sharing his expertise with others.
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What is a Split Charging System?
A split charging system charges both your vehicle's starter battery and your leisure batteries while you drive. A starter (cranking) battery is what kicks your van into action when you turn the ignition. Once the engine starts up, an alternator recharges the starter battery. An alternator is a simple device that detects the charge level of a battery by detecting its voltage. Fully charged lead-acid batteries sit at 12.8V. If the alternator detects a lower voltage, it begins charging the battery. Once the voltage reaches 12.8V, it disconnects.
Normally, the alternator is only wired to the vehicle's starter battery. A split charging system connects to the alternator and uses it to charge your leisure batteries once the starter battery is full.
Our Top Picks
A smart charger, DC-DC charger, or campervan battery-to-battery charger is the best choice for most people. They have fast charge capabilities and smart protection against overheating and other dangerous malfunctions. A battery-to-battery charger will charge your leisure battery more efficiently and more safely than any other split charging system. Further, they're the only split charge system suitable for use with lithium batteries or with vehicles newer than 2015 with smart alternators. Both Renogy and Victron make excellent battery-to-battery chargers with high-quality components and options for bluetooth connectivity.
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 to prevent it being drained by the leisure battery.
In this van converter’s humble opinion, a split charger is an absolute necessity. There are five ways one can split charge a campervan:
Manual switch
Split charge relay (SCR)
Split charge diode (Charge splitter)
Voltage sensitive relay (VSR)
Smart charger (also known as a battery-to-battery charger, B2B charger, or DC-DC charger)
In the Roaming Home 2023 study, we found that 55% of van conversions are still using basic split chargers (SCR/VSR) rather than B2B chargers. We don't doubt that with the increasing popularity of lithium batteries and the increasing prevelance of smart alternators in modern vehicles, B2B chargers will soon take over.
If your van has a smart alternator, which is the case for vans produced after around 2015, or if you are using a lithium leisure battery, you must use a B2B charger.
Manual Switch
First up we have the manual switch. This is a very cheap and simple device that switches between charging the leisure battery and the starter battery using a switch you have to flip yourself.
Manual Switch Wiring Diagram
Simply wire the positive terminals of the starter battery and the leisure battery together. Then, pop the manual switch right in the middle with a fuse on either end. Read more about wiring and fusing in our dedicated articles to learn about wire thickness and fuse size.
In general, we don't recommend manual switches as you must remember to turn the switch off everytime you stop driving the vehicle. If you forget, your electrical system will drain your starter battery, leaving you stranded. Additionally, this form of split charging is inefficient and has very minimal safety protocols. It's possible to overcharge your leisure battery, damaging or even killing it entirely.
A manual switch won't charge your leisure batteries to 100%, as it cannot provide the higher, controlled voltage levels required to charge a battery from 80% to 100%. Finally, manual switches don't work with smart alternators or lithium batteries. Choose a manual switch only if you're on a shoestring budget and don't mind its very basic nature.
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 the engine starting or stopping.
With this useful feature, we fix the main downside of the manual switch. However, SCRs remain inefficient, comparatively unsafe, won't achieve a 100% charge of your leisure batteries, and don't work with smart alternators or lithium batteries.
Split Charge Relay Wiring Diagram
The positives of the starter and leisure batteries are connected through the split charge relay with a fuse on either side.
The split charge relay is grounded to the leisure battery's common negative ground
The split charge relay is connected to the alternator via a D+ connection trigger wire
This trigger wire detects when the engine starts and stops and activates and deactivates the relay.
Split Charge Diode (Charge Splitter/Battery Isolator)
A split charge diode allows current to flow from the alternator to either battery but not between the batteries. This gives them a similar functionality to split charge relays in that they prevent your leisure battery from draining your starter battery.
These devices are not commonly used in van conversions as they have significant drawbacks and are complicated to wire. The biggest drawback is that they incur a large voltage drop. Voltage drop occurs when there is resistance in a system. 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 also tend to generate significant heat which must be managed.
Besides this, they share the drawbacks of SCRs: inefficient charging, minimal safety features, and incompatibility with smart alternators and lithium leisure batteries.
Voltage Sensitive Relay (VSR)
A voltage sensitive relay is the most common split charging device found in campervans. They work similarly to split charge relays, using the alternator to charge your leisure batteries only when the engine is running, and disconnecting the batteries when the engine is turned off.
The difference is that a VSR detects that the engine is running via the charge status of the starter battery. When it detects that the starter battery is full, it switches to charging the leisure battery. This eliminates the need to connect the VSR to the alternator via a D+ trigger wire. This simplifies installation, especially in vehicles where the alternator is hard to reach.
Note that VSRs still charge less efficiently and are less safe than B2Bs, won't achieve a 100% charge of your leisure batteries, and aren't compatible with with smart alternators and lithium leisure batteries.
Durite are the most well known manufacturer of voltage sensitive relays. They sell full split charge relay kits that are very easy to install.
Voltage Sensitive Relay Wiring Diagram
The wiring for a voltage sensitive relay is very similar to the wiring for a split charge relay, but simpler.
The positives of the starter and leisure batteries are connected through the voltage sensitive relay with a fuse at either end
The VSR is grounded to the leisure battery's common negative ground
There is no need to connect the VSR to the alternator via a D+ connection
Smart Chargers
Smart chargers/battery-to-battery chargers/B2B chargers/DC-DC chargers are an excellent bit of kit which have completely convinced me over the past couple of 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 the status of your leisure battery.
This smart charging takes into account your battery's chemistry and charge status, providing the optimum charge profile for fast, efficient charging. They're also unique among split charging systems in that they will charge your leisure batteries to 100%. This is because they are capable of providing the final high, controlled voltage needed to get a battery from 80% to 100% charge.
B2B chargers also incorporate smart safety features that protect against overheating, battery overcharging, and reverse polarity malfunctions. Finally, they work perfectly with modern smart alternators and lithium leisure batteries.
B2B Charger Wiring Diagram
B2B chargers are slightly more complicated to wire than the other devices we've discussed:
The positives of the starter and leisure batteries are connected through the smart charger, with a fuse on either side
The negatives of the batteries are also connected through the smart charger
Most B2Bs don't need to be grounded
The smart charger connects either to the alternator or directly to the ignition circuit, depending on whether the alternator has a D+ connector socket
What size B2B charger Should I Use?
Determining the correct size split charger is a little tricky, as it depends on alternator current output and maximum battery charge current. Our DC-to-DC charger calculator will help you figure out exactly which smart charger you need depending on your alternator and battery bank.
Alternator Current Output
Determining the alternator's current output is crucial for choosing the right DC to DC charger:
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 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.
Our DC-to-DC charger calculator simplifies the process of choosing a B2B charger.
Leisure Battery Maximum Charge Current
The maximum charge current your leisure battery can handle from your split charger 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.
As mentioned above, our DC-DC charger size calculator takes all of these factors into account, simplifying the selection 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 come with built-in thermal protection features which automatically shut down the charger when a certain temperature is reached. However, running a charger at high heat runs it down in the long term and reduces charging efficiency.
To prevent overheating, install your charger in a well-ventilated area away from other heat-generating components like inverters and batteries. Additionally, avoid overheating of your alternator by not exceeding the 40% current load limit discussed above.
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. Some manufacturers provide guidelines for fusing their chargers. If you buy your charger in a kit, it will come with all the wiring and fuses needed to install it. To learn more about fusing your electrical system, check out our detailed guide.
What Size Wire do I Need for Split Charging?
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
Voltage drop is a critical issue in split charging systems. If your cable lengths are too long or the cables are too thin, the resistance in the wires can lead to significant voltage drop due to cumulative electrical resistance, meaning the leisure battery won't charge efficiently or fully.
To minimize voltage drop in split charging systems, you need to use thicker cables and keep cable runs as short as possible. Our guide on wiring covers cable lengths, cable thickness, and voltage drop in detail. Further, our wiring size calculator simplifies the process.
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.
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. Read more about grounding in our detailed guide.
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. Read more about wire sizing here, or check out our wire gauge calculator.
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.
Why do I Need to Use a B2B Charger with a Smart Alternator?
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 dramatically to about 17V when the vehicle moves off. 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. A B2B charger is capable of regulating this variable voltage input, ensuring your batteries always receive the right voltage.
Why do I Need a B2B Charger for Split Charging a Lithium Battery?
Lithium leisure batteries need precise voltage control, current limiting, and specific charging profiles that standard split-charging systems cannot provide. Unlike lead-acid batteries, lithium batteries are highly sensitive to overcharging and rely on a Battery Management System (BMS) for protection. Standard systems, like Voltage Sensitive Relays or Split Charge Relays, cannot limit current, posing a risk to alternators due to the high current draw of lithium batteries. Smart B2B chargers address these issues by delivering optimized charging profiles, integrating with the battery’s BMS, and limiting the battery's current draw. This ensures efficient, safe charging and maximizes the lifespan of lithium batteries.
What is the Best Split Charging System?
As discussed in this article, smart B2B chargers/DC-DC chargers do better than other split charging systems in almost every metric. They charge your batteries more efficiently, more quickly, and more safely. If you have a smart alternator or a lithium leisure battery, they're essential. Both Renogy and Victron make excellent battery-to-battery chargers with high-quality components and options for bluetooth connectivity. Their wide range of capacities means you're almost certain to find one that fits your use-case.
How to Test a Voltage Sensitive Relay or Other Split Charger
When vehicle's engine is turned on, the alternator will produce between 13.5V-14.5V of electricity. 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 a popular option) or with a digital multimeter. If you have a campervan solar system wired up, you can also monitor you battery levels with your smart MPPT solar charge controller.
A battery monitor is an excellent addition to any electrical system, allowing you to monitor your charge inputs, battery levels, and charge outputs from your phone.
Conclusion
We covered a lot, but you should now understand how split charging systems work and feel equipped to choose one for your electrical system. If you're still unsure, consider our Electrical Masterclass, a detailed, step-by-step course on every aspect of campervan electrical systems. If you feel you're ready to choose, check out Renogy's and Victron's smart B2B chargers.
Don't forget to subscribe to The Van Conversion Newsletter for everything you need to get started with your own van conversion (we'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 our founder Shane how to convert a van into your dream home - no prior experience needed.
Until next time.