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If you own a motorhome, campervan or boat then you’ll almost certainly have electrically powered equipment installed that you’ll want to run when your engine is off. These can be simple, low consumption items such as 12V sockets and LED lights through to more power-hungry items such as a water pump, coolbox/fridge, navigation equipment, fan, laptop, TV or sound system.

If you try to power these from your engine starter battery without the engine running then before long you’ll have flattened it, leaving you stranded. So the common solution is to fit one or more auxiliary batteries (often referred to as leisure batteries in vehicles or house batteries on boats) that are used purely to power those items not related to the starting of the engine. But once you have a second battery installed, how do you keep it charged up in the same way that the starter battery is? This is what split charging is all about and this article will give you an overview of the various technologies commonly used, as well as some pointers to consider when specifying your own system. 

What is split charging?

Split charging is the term used to describe the simultaneous charging of the vehicle starter battery and the leisure battery (or batteries) from a common charging source. On vehicles and boats, this charging source is almost always the vehicle alternator which is in turn powered by the engine, but there can also be other charging sources not related to the engine such as a mains powered charger, solar or wind-powered re-charging systems and portable generators.

Modern split charge systems typically have a device that connects the starter and leisure batteries only when a charging source is operating, and electrically isolates them otherwise, ensuring that the use of one battery does not draw current from the other. It is important to ensure that the starter battery is isolated so the engine can always be started, but you also don’t want the leisure battery to be accidentally drawn upon when starting the engine because dedicated leisure batteries are not designed for high current output over short periods of time and can be damaged.

A few words on battery charging

Before we look at connecting batteries for split charging, it’s worth taking some time to understand how a battery obtains charge from the charging source.

12V batteries typically provide a voltage output in the region of 12-13V, depending on their level of charge, and when a charging source is attempting to deliver current to a battery to recharge it, it must actually provide a voltage higher than that output by the battery in order for the charge to flow into and be accumulated within the battery. It might be helpful to think of the voltages as acting in opposing directions with the voltage from the battery acting one way and the voltage from the charging source acting the other. Since a difference in voltage drives current flow (e.g. between +12V and 0V/ground), then with them each delivering identical voltages in opposing directions there is zero current flow. Only when the voltage of the charging source exceeds that of the battery will charge start to flow into the battery. 

For this reason, alternators will typically deliver current to a battery at between 13.8 – 14.4V to enable them to start charging. A voltage regulator is fitted to the alternator to control the rate of charge and ensure that the upper voltage is limited and does not cause damage to the electrical systems.
It is also important to note that whilst an engine’s alternator is supplying a voltage higher than that of the battery, all of the electrical systems are powered by the alternator, not by the battery. With the battery at a lower voltage than the charging source, no current can flow from the battery to the rest of the electrical system because current will be flowing into the battery. A typical charging system arrangement used in vehicles is shown in the diagram below:  

Split Charging methods/technologies

There are various split charge devices that can be fitted to allow charge to be distributed to both the starter and leisure batteries. The following is an overview of the various options along with the advantages and disadvantages of each (please note that if you have a vehicle with a ‘smart’ alternator then these methods will not be suitable and you will need to use a Battery-To-Battery charger instead): 

1 – Manual Switching 

The most simple way to connect and disconnect the starter and leisure batteries is by using battery isolation or master ON/OFF switch. This just uses a high current switch to manually connect the batteries in parallel once the engine is running and allow charge through to the leisure battery, and then disconnect them once the engine is turned off to isolate them and ensure the starter battery can’t become flattened.

Advantages • Very inexpensive
• Simple
• Robust
• Batteries can be combined to start the vehicle in an emergency (e.g. if the starter is flat)

Disadvantages • Inconvenient
• Relies on the user to manually connect and disconnect batteries
• Risks leaving batteries connected and flattening starter 

This is not a very ‘smart’ solution because it can be easily left on, flattening the starter battery. However, on boats it is still quite common to have a multi-position rotary switch that can select either the starter battery, the leisure battery, or both and also dis-connect both of them from the charging system (sometimes referred to as a 1 / 2 / 1+2 / OFF switch). In addition to remembering to switch to house-batteries only when the engine is off (so as not to flatten the starter battery), care must be taken when using these types of switch as disconnecting all batteries from the alternator with the engine running can damage the alternator rectifying diodes (for the same reason you should never disconnect your car battery when the engine is running). 

2 – Split Charge Relay 

This system uses a heavy-duty version of a simple ‘make and break’ relay, the coil of which is energised by a signal that is only +12V when the engine is running (usually the charge signal wire from the alternator). When energised, this relay connects the two batteries together and disconnects them when the engine is turned off (it is important that the relay closes only once the engine is running so that the leisure battery is not connected to the starter during engine cranking, so a switched ignition signal should not be used). A heavy-duty version is required so that it can handle the potentially large current that could be passed through it by the charging source. 

Advantages • Automatic operation
• Inexpensive
• Simple electro-mechanical construction
• Easily replaceable 

Disadvantages • Requires connection to alternator or other parts of vehicle electrical system which means tapping into the factory wiring

• Moving contacts can wear out over time, leading to increased resistance and voltage drop
• Batteries cannot be combined for emergency engine start unless a bypass switch is fitted
• No ‘intelligence’ built into the relay

This was the most common split charging solution before voltage sensitive relays became available, and is still sometimes used, however, some are wary of tapping into the vehicle electrical system in case of damage or problems with warranty issues and some more modern alternators are not compatible with these relays.

3 – Voltage Sensitive/Sensing Relay (VSR) 

Voltage-sensitive relays, or VSRs, sense the voltage of the starter battery and close the contacts inside the relay at a pre-set threshold, normally around 13.7V (check manufacture’s specs). The contacts then open again at a pre-set lower threshold (normally around 12.8V). So when the alternator starts turning the voltage rises to around 14V at the starter battery terminals (assuming the battery is already well charged) and the relay will close to allow current through to charge the leisure battery. When the engine shuts off and the alternator stops turning, the voltage will gradually drop away at the starter battery terminals and the relay will open again, isolating the batteries once more.
For more information on how to make a split charge kit with a voltage-sensitive relay, check out our YouTube Video 

Advantages • Automatic operation
• Relatively inexpensive
• Simple to install (no need to modify factory-fitted electrical system – no warranty issues)
• Are generally dual-sensing, meaning that they operate when sensing a voltage rise on either side of the relay. This means that the starter battery will also charge when the leisure battery is being charged by a mains battery charger by solar power or another charging source.
• Some models have an emergency engine start feature to combine the batteries 

Disadvantages • Moving contacts can wear out over time, leading to increased resistance and voltage drop
These types of split charge devices are probably the most popular on the market for smaller applications with a single leisure battery (typically up to around 150Ah) such as found in campervan conversions, 4x4s, smaller motorhomes, etc. They are relatively inexpensive, simple to fit, and reliable. Some of the more intelligent VSRs have voltage trending software built-in to monitor voltage over time and determine if and when to open/close the relay-based upon the trend, rather than using fixed lower and upper voltage thresholds. This is useful to eliminate unwanted opening and closing of the relay due to momentary voltage fluctuations.

4 – Battery Isolator / Charge Splitter

This split charging method utilises the property of diodes to allow current to flow only in one direction. So current can flow from the alternator (or another charging source, e.g. battery charger) to both batteries but cannot flow between batteries, thereby isolating the starter battery from the leisure battery and preventing it from being flattened. The biggest drawback with the use of diodes is that they incur a relatively large voltage drop across them, typically around 1V, which is a 12V system that is quite significant. This voltage drop means that the leisure battery never sees the maximum voltage from the alternator and doesn’t fully charge, so split charge diodes are sometimes used in conjunction with an after-market alternator controller to increase the regulated output voltage and compensate for the voltage drop. 

Advantages • Automatic operation
• No contacts to wear out
• Simple to install
• Units often have multiple outputs for several batteries
• No risk of contacts arcing and so protects against ignition (important on boats where gas or petrol fume build-up is a concern) 

Disadvantages • Suffer voltage drop so don’t charge batteries fully
• May need to be used with after-market alternator regulator to boost voltage
• Generate significant amounts of heat
• Batteries cannot be combined for emergency engine start unless a bypass switch is fitted
• Relatively expensive

Split charge diodes are still widely used in the marine sector due to their simplicity and durability which offsets their slightly lower charging performance compared with other systems. Manufacturers have also started to employ new technologies to minimise the voltage drop issues such as the use of low-voltage drop Schottky diodes and FET-based charge splitting.

Design considerations for a split charging system

The following are a few points to consider when designing your split charge system. Please note that every installation will differ slightly so this is intended as a guide only.
The complexity of your leisure/auxiliary electrical system
If you have a relatively simple electrical system consisting of a single battery with a few consumers, e.g. lights, fridge/coolbox, water pump, power sockets, etc. then a VSR is probably the best option. Example installations would be campervans, small motorhomes, and horseboxes, small commercial vehicles small day/fishing boats.
If you have a complex electrical system with multiple batteries, several electrical sub-systems, and many consumers then one or more battery isolators are probably the best option. Example installations would be large motorhomes and horseboxes, larger sailing/motorboats or commercial vessels, and heavy commercial vehicles.

IMPORTANT: If you have a vehicle with a ‘smart’ alternator (as a rule of thumb this applies to vehicles with Euro 5 or 6 compliant engines onwards) the split charging methods outlined above won’t be suitable and you’ll need to use a Battery-To-Battery charger instead. Find out more info by reading our Knowledge Centre article and discover the benefits of using a battery-to-battery charger over split charge systems. 

Sizing your split charge unit and connecting cables when a battery is deeply discharged it has a very low resistance and so will draw a large current from any charging source attached. For this reason, the cables used to connect the batteries and the split charge unit itself should be rated to handle the maximum current that the charging source is capable of supplying to ensure that they are not damaged. For any reasonably sized alternator, this means using battery cable of at least 16mm² cross-section (110A) and upwards, depending on the application and cable run length. The same applies to the split charging device itself; it should be rated high enough so that is is not damaged if the leisure battery (or batteries) is deeply discharged and draws a very high current from the alternator.

It’s also worth considering that in a split charging system the connected starter battery is also effectively a charging source that will try to equalise the voltage in the leisure battery when they are connected, so if your leisure battery is drained to a very low level you could see a combined current from the alternator and the starter battery for a short period of time which might be larger than the alternator could supply on its own. Allowing a battery to become too deeply discharged is damaging and you can prevent this using a battery guard.
There are many commercially available split charge kits featuring relays and cable that are not rated high enough for the maximum alternator output of the vehicle in which they’ll be installed. Whilst this will not be a problem if the system is fused properly, it does mean that you could easily keep blowing fuses if your battery is deeply discharged and a large current is drawn from the charging source. An annoyance at best and extremely inconvenient at worst. We would advise against using any system that is not capable of handling the maximum output of your charging source.

For more information on choosing the correct cable please take a look at our Cable Sizing & Selection guide.

Balancing your split charging system

If you intend to charge multiple connected leisure batteries at the same time (i.e. a bank of connected batteries used to increase total capacity) then to ensure that all batteries are charged equally and that one does not get prioritised over the other it is good practice to design your system as follows:
• If there are multiple cables running from the split charging device to each of the batteries then they should be of equal size and length and as short as possible.
• Similarly, the earth cables running from each battery should be of equal size and length and as short as possible.
• Battery-battery connecting cables in a battery bank should be as short as possible
• The batteries should be of the same type, size, and age (batteries lose charge holding ability with age)
• The batteries should be discharged equally so that they ‘age’ at the same rate
These are not always practical in a real-World installation but will give your batteries the best chance of lasting for the same amount of time. Some of the more advanced charging systems can overcome differences in battery health and monitor usage to optimise charging for each battery, so the above becomes less critical.

Protecting your split charge circuit

Whatever split charge system you opt for, the most important aspect is to ensure that all connecting cables are protected by appropriately rated fuses to prevent an electrical fire. Each battery that connects into the split charge device should have an appropriately rated fuse installed in-line with the cable, as close as possible to its +ve terminal. This minimises the unprotected length of cable (the section from the terminal to the fuse) and the associated risk of an accidental short circuit due to, for example, mechanical abrasion, vibration damage, or termination failure. It is also a good idea to install manual battery isolation switches to totally disconnect the batteries from the charging sources in the event of an emergency, or simply for convenience when carrying our work on or around the batteries. For more information on fusing please take a look at our Fusing Guide.

Combining batteries in an emergency

Although split charge systems are designed to keep the starter and leisure batteries separated when the engine is not running, there might be emergency circumstances where you want to intentionally join them, for example, if the starter battery fails and you need to start the engine from the leisure battery or if your leisure battery fails and you need to run some emergency lighting or safety-critical equipment (e.g. VHF) from your starter battery. If your split charge device does not already have this facility then you might want to consider adding a manual battery switch to the circuit to by-pass the split charge device.

Note: Care should be taken if using this feature for engine starting as starter motors can draw very high currents, so the switch should be rated sufficiently. Although some VSRs have this emergency override facility built-in, the high currents drawn by a start motor could weld the contacts together and render it useless, so it might be prudent to use a manual by-pass switch anyway and avoid the risk of damage.

Battery-to-Battery (DC-DC) Chargers

Battery-To-Battery chargers (also commonly know as DC-DC chargers) allow you to charge an auxiliary/leisure battery from your alternator whilst driving. They are installed in-line between the positive terminals of two batteries (one of which is connected to an alternator, typically a starter battery) and offer several benefits when compared with traditional split charge systems that utilise a Voltage Sensitive Relay (VSR) or heavy-duty, alternator-triggered relay.

Direct alternator charging used in a traditional split charge system with a VSR will only achieve around an 80% charge state. VSRs simply allow charge to pass directly from the alternator to the auxiliary battery whereas Battery-To-Battery chargers take the alternator output and boost or reduce it to provide a stable voltage output according to a multi-stage charging profile. This provides a 100% re-charge for your leisure battery in a controlled way and typically achieves it faster than with direct alternator charging.

Current flow is limited by the charger’s current rating, eliminating potentially damaging high in-rush currents and often allowing the use of smaller connecting cable than with VSR-based split charge systems.

Using a battery-to-battery charger is the only reliable solution if you intend to re-charge a second battery in a modern vehicle that has an ECU-controlled ‘smart’ alternator (generally Euro 5/6 compliant engines onward). These have a highly variable voltage output that is often too low to provide a charge or (in the case of regenerative braking systems) so high that it can damage some battery types. Due to this highly variable voltage, VSRs are not suitable for use with ‘smart’ alternators.

Battery-to-battery chargers are simple to install and will engage/disengage automatically on engine start/stop, ensuring the batteries are isolated and cannot discharge one another.

Auxiliary Battery Charging In Vehicles With 'Smart' Alternators

Vehicle manufacturers are having to comply with increasingly stringent environmental regulations so are continually researching and employing clever technologies to lower fuel consumption and reduce emissions. One of the technologies that has been around for several years, but which is now becoming standard on new vehicles, is the ‘smart’ or ‘intelligent’ ECU-controlled alternator.

For those of us wanting to charge a second (auxiliary or leisure) battery whilst driving this presents a problem because traditional split charge systems cannot be used. This article explains why and looks at how Battery-To-Battery chargers overcome these issues and the other benefits they have when compared with split charging.

Note: this issue is not generally applicable to boats as the emissions regulations that apply to vehicles don’t apply to leisure craft and so ‘smart’ alternators on boats are relatively rare.

What are ‘smart’ alternators and how do they help with emissions?

Smart alternators are essentially those that have their output voltage controlled externally via the Engine Control Unit (ECU) rather than by an internal voltage regulator as found on traditional alternators.

When an alternator is running and producing a charging voltage the electromagnetic field generated inside it produces a mechanical load on the engine via the alternator drive belt. This load increases as the output voltage rises, meaning that the engine has to work harder to turn the alternator which consumes more fuel in the process. Traditional alternators maintain their voltage between 13.8-14.4V depending on the state of charge of the battery and various other factors. This means that there is always a significant load on the engine from the alternator which requires fuel to be consumed.

As smart alternators are ECU-controlled it enables manufacturers to vary the voltage output more than can be achieved with an internal regulator. This provides the opportunity to bring the voltage below 13.8V during periods when no further charging is required (e.g. when the battery is nearly full) meaning that engine loads are reduced, fuel consumption is reduced and emissions are lowered, all helping manufacturers to comply with industry environmental regulations.

What is regenerative braking?

Regenerative braking is an energy recovery technology that takes the kinetic energy of the vehicle that is normally converted into wasted heat in the brake pads and discs during braking and instead converts it into electrical energy to re-charge the starter battery. This is made possible because of the use of smart alternators that can be controlled by the ECU when deceleration is detected. During deceleration (for example when taking your foot off the accelerator) the ECU boosts the alternator voltage output as high as 15V+ to create a burst of charge into the battery. This high voltage puts an increased mechanical load on the engine, resulting in increased engine braking, meaning less of the kinetic energy is converted to wasted heat in the pads and discs. So the deceleration of the vehicle is putting charge back into the battery, saving fuel that would otherwise be required to re-charge it.

This charge is then used to power the vehicle’s electrical systems when the vehicle is accelerating or travelling at constant speed. During this time the alternator output voltage is reduced (to as low as around 12.5V), which reduces the load on the engine with a consequent reduction in emissions.
Regenerative braking is only effective if the starter battery has some spare storage capacity to absorb the charge created by the alternator during deceleration. If the starter battery was fully charged the electrical energy created would be wasted and so the ECU aims to maintain the battery at around 80% state of charge (low enough to have spare storage capacity but high enough to guarantee engine starting if required).

Why are these technologies a problem for auxiliary battery charging?

Whilst these new technologies are great news for the environment they cause problems when it comes to charging a second battery in an after-market application for the following reasons:

An auxiliary battery will not be charging for significant parts of a journey
During the periods when the smart alternator output is below the charging voltage threshold there will be no charging taking place. These periods can be significant when combined over the full length of a journey meaning that any connected auxiliary battery will not receive as much charge as it would if connected to a system with a traditional alternator.

Gel and AGM batteries can be damaged. Gel and AGM batteries (which are now popular for use in auxiliary battery applications) are sensitive to over-charging and voltages above 14.4V can cause gas bubbles to form in the electrolyte gel which can permanently damage the battery.

Voltage Sensitive Relays cannot be used

Voltage sensitive relays rely on voltage thresholds that trigger them to engage and disengage. Typically they engage to connect the starter and auxiliary battery together at around 13.7V and disengage to separate them at around 12.8V, which in vehicles with traditional alternators coincides with engine start-up and shut-down. This is ideal as it makes sure that the auxiliary battery is charging whenever the engine is running. However, with a smart alternator, when the output voltage drops to below 12.8V a VSR would disengage, meaning that the auxiliary battery would have significant periods where it wouldn’t be charging, even though the engine would be running (some models of VSR may not engage at all, dependent on the on-board software). VSRs can also engage and disengage frequently with the large and rapid voltage changes from a smart alternator and so the high current contacts may fail prematurely.

How can I tell if my vehicle has these technologies?

Almost all new vehicles will now have smart alternators fitted as standard, and if your vehicle is branded as an eco-efficient model then it is very likely also to have regenerative braking technology. Another indicator is to check the European Emissions Standard to which your vehicles engine complies. If the engine is compliant with the Euro 5 or Euro 6 emissions standards onward then it will almost certainly have a smart alternator. If in doubt you should contact the manufacturer and they should be able to advise.

So what is a Battery-To-Battery charger and how does it help?

If you have a smart alternator then you will need to use a Battery-To-Battery charger (also known as a DC-DC charger) to ensure that you can charge your auxiliary battery effectively. Battery-To-Battery (or DC to DC) chargers take the highly variable voltage output from a smart alternator and boost or reduce it to maintain a stable voltage output according to a multi-stage charging profile (in the same way as a mains charger does) providing a safe, controlled and fast charge for your auxiliary battery.
They are easy to install and, just like a VSR, are simply connected between the positive terminals of your starter and auxiliary batteries, together with a negative connection to the vehicle chassis or starter battery-ve terminal.

Benefits of using a Battery-To-Battery charger for auxiliary battery charging

  • Battery-To-Battery chargers are becoming increasingly common in leisure, commercial and marine applications, not only as a way of over-coming the issues with smart alternator technology, but also because they provide several benefits over split charging systems when used with traditional alternators
  • Batteries charge much faster when using a Battery-To-Battery charger when compared with direct alternator charging (as found in split charge systems using VSRs, HD relays or diode isolators). This can typically be around 5x faster.
  • A much deeper state of charge can be achieved because the Battery-To-Battery charger uses a multi-stage profile to maximise the depth of charge, something not possible in standard split-charge systems which will typically charge a battery to around 80% of its capacity.
  • Charging will always take place when the engine is running, regardless of the alternator output voltage, because low voltages are boosted to meet the required charging profile.
  • As in a split charge system, the starter and auxiliary batteries are electrically isolated when the engine is not running to ensure that one cannot discharge the other.
  • The charger protects auxiliary batteries from high voltage spikes produced as a result of regenerative braking systems, so preventing damage to sensitive Gel and AGM batteries.
  • The current reaching the auxiliary battery is determined by the charger rating (rather than by the maximum the alternator can produce), meaning that potentially damaging current in-rush is eliminated. This current in-rush can occur in split charge systems if the auxiliary battery is dead-flat or very low and can overload cables and blow fuses if they are not large enough to cope with the alternator output.

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