how does a split charge kit work - guide to split charge kits

Image of a split charge kit

A split charge kit is an electrical arrangement that safely charges the secondary leisure battery of a vehicle.  It protects the starter battery by isolating it from the leisure battery when the engine is off, and resumes charging only when the engine is running.


A split charge kit is an electrical set-up that allows the safe charging of a vehicle's primary starter battery and secondary leisure battery at the same time.  It also protects the two batteries by isolating them when the engine is not running.  This prevents the leisure battery from draining the starter battery when a power source is unavailable.  Typically, a split charge kit draws power from the alternator and charges both batteries when the engine is running.  The leisure battery is connected to the primary via the split charge kit.  When the engine stops the split charge kit isolates the two batteries so that the leisure battery does not draw a charge from the starter battery.  This protects the charge in the starter battery and prevents it running flat avoiding difficulties getting the engine started.  Isolating the batteries when no charging source is present also protects the leisure battery during engine start-up.  When the engine is started it draws a high current from the starter battery over a very short period of time.  This is something a leisure battery is not designed to do and could result in damage.  Isolating it from the starter battery prevents damage during ignition.


A split charge kit charges a secondary leisure battery while a vehicle is driving.  Once the engine stops and the power supply is interrupted, the kit isolates the two batteries and prevents the leisure battery from draining the starter battery.  Most modern split charge kits use a relay switch device positioned between the primary and secondary battery to control this mechanism.  A relay is an electrically controlled switch that can make or break a circuit.  In this scenario the switch will open when no charge is present, to isolate the secondary battery, and it will close again when it receives a current which will allow the secondary battery draw a charge.  This way the leisure battery only charges while the engine is running and a power source is available.  This ensures it never drains the starter battery resulting in a flat battery and problems getting the engine started.

View our range of Split Charge Kit Products - Here


  • Standard Relay Split Charge Kits
  • Voltage Sensitive Relay Split Charge Kits
  • Diode Split Charge Kits
  • Manual Switch Split Charge Kits
  • Battery to Battery Split Charge Kits 

A split charge kit system can be set up in a number of ways but the most common methods involve the use of a relay device.  A relay is an electronically operated switch.  They function using an electromagnet device to mechanically maneuver a switch to make or break an electrical circuit.  This relay switch set up is the most popular method used in split charge kits and is used to control the connection between the starter battery and leisure battery.  Other options include: manual switches, battery to battery chargers and, split charge diode units.  Lets first take a look at the split charge relay kit set up and two different ways it can be done.


A split charge relay kit uses a heavy-duty make-or-break relay switch to connect the leisure battery to the starter battery.  The switch is activated when it receives a current greater than 12V whilst the engine is running.  In this scenario, the switch closes and makes the circuit connecting the leisure battery to the starter battery.  The leisure battery begins to charge as it receives current from the alternator via the primary battery.  When the engine stops, the current drops and the relay breaks the circuit.  The leisure battery is then isolated from the starter battery and cannot draw a charge from it.  This protects the starter battery from going flat and protects the leisure battery from a high current draw during ignition.  The installation of a heavy-duty relay involves making a connection to the vehicle's electrical system, usually the alternator, which means adjusting the factory wiring system.  This could lead to warranty issues down the road.  A heavy-duty relay is required in a split charge kit due to the large currents coming from the alternator during charging. 


  • Cheap & Easy Installation
  • Replacing Parts is Easy
  • Switching Occurs Automatically


  • Tapping Into Vehicle Electrical System May Void Warranties
  • Parts May Need Replacing Over Time
  • Leisure Battery Cannot be Used for Start-Up in Emergency Situation


Installing a voltage sensitive relay split charge kit is the best solution for cars with traditional charging systems.  This set up does not require wiring to the vehicle's alternator or electrical wiring system making it a simpler installation.  The voltage sensitive relay, (VSR), is positioned between the starter battery and the leisure battery.  It functions by sensing the voltage of the starter battery and therefore does not need a connection to the alternator.  When the engine is running the voltage of the starter battery will rise above 13V.  The VSR will detect this rise and close the contacts, thereby connecting the leisure battery.  The leisure battery will then begin to charge.  When the engine stops running the voltage of the starter battery will fall below 12.8V.  The VSR is set to open at this preset value, thereby isolating the leisure battery.  Another feature of the VSR split charge set up is that some VSR models allow for the leisure battery to be used to start the engine in an emergency situation.   


  • Automatic Operation
  • Cheap to Install
  • Easy to Install
  • No Re-Wiring of Factory Wiring System - No Warranty Issues
  • Leisure Battery Can Be Used for Engine Start in Emergency


  • Moving Parts Can Wear Over Time - May Need Replacing


This method involves split charging a leisure battery using a diode unit.  Diodes allow charge to flow in one direction only.  This arrangement sees charge flow from the power source, the alternator, to the two batteries but not from one battery to the other.  Charge cannot flow in the reverse direction between the two batteries thereby protecting the starter battery from being drained by the leisure battery.  One problem with using diodes is that they result in quite a significant drop in voltage across the diode.  This can be as large as 1V, which is a pretty big value in a 12V system.  This voltage drop means the leisure battery never gets optimally charged as it cannot receive full power from the alternator.  This can be corrected by adding in an additional component to up the voltage before it reaches the leisure battery.  An alternator controller can be fitted to boost the voltage output and help charge the secondary battery.  


  • Automatic Operation
  • Easy to Install
  • No Moving Parts to Replace Over Time
  • Unidirectional Current Flow Protects Leisure Battery During Start-Up


  • Expensive
  • Diodes Incur Voltage Drop
  • Leisure Battery Does Not Get Fully Charged
  • Leisure Battery Cannot be Used For Start-Up in Emergency Situation
  • Additional Parts May Need to Be Installed to Compensate For Voltage Drop


    The simplest way of installing a split charge kit is by fitting a manual isolator switch between the two batteries.  Although crude, it will work, and it is the cheapest solution.  This method involves connecting the leisure battery to the starter battery via a basic high current on/off switch.  Although cheap and easy to install it comes with a whole range of headaches so it is not the recommended option.  The switch must be turned on manually every time the engine is started, and again switched off every time the engine is stopped.  Also, if the driver forgets to isolate the leisure battery when the engine is stopped, the starter battery will go flat.  It's a simple option in terms of setup but a real nuisance when it comes to operation.  


    • Cheap
    • Simple Installation


    • Operation is Manual and is a Real Headache!
    • Risk of Starter Battery Going Flat


    The battery-to-battery split charge setup is used in modern Euro 5 and 6 engines that utilise smart charging systems.  It is not suitable for the majority of vehicles.  A battery-to-battery split charge system draws current from the starter battery to charge a three or four-stage leisure battery setup.  This arrangement can charge the leisure batteries much faster than a conventional split charge kit.  Battery-to-battery setup is also recommended for charging a 12v leisure battery from a 24v vehicle, or vice versa, charging a 24v leisure battery from a 12v vehicle.  Battery-to-battery chargers are voltage sensitive, similar to a voltage sensitive relay, which makes them fully automatic.  Charging takes place only when they sense a rise in voltage and thereby removing the risk of draining the starter battery when the engine is not running.   


    Leisure batteries are commonly found in motorhomes, caravans, and boats to run small electrical domestic appliances.  They are sometimes referred to as "auxiliary" or "deep-cycling batteries" and are used to provide a steady flow of power, typically 12v, over a long period of time.  A starter battery, on the other hand, is designed to give an occasional short burst of high power to fire up an engine.  Leisure batteries provide a low steady flow to run small appliances such as lights, fridges, and water pumps.  In an emergency situation a leisure battery can be used to help start an engine, but repeated use in this manner is not recommended as it will damage the battery.  Leisure batteries come in different shapes and sizes and range from 110ah to 225ah capacities depending on what they are required for.  Generally speaking the higher the ah rating the longer the battery will last and the less often it will have to be recharged.  Depending on the size of the battery you choose you will need to make sure you have the correct components for your split charge kit.  In particular, you need to ensure you have an appropriately sized and amp rated battery cable to connect your leisure battery.  Below is an overview of battery sizes and corresponding battery cable sizes.

    • 110ah Batteries - 6mm² 42amp Cable
    • 125ah Batteries - 10mm² 70amp Cable
    • 225ah Batteries - 16mm² 110amp Cable 

    For more information you can read our guide to Leisure Batteries and Battery Cable Sizes.



    BatteriesLeisure batteriesSplit charge kitsSplit charge relay kits

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