Split charge systems and relays

Online resource for 12 volt and 24 volt split charge relays, systems and wiring kits. Your leisure/auxiliary battery bank split charging requirements made simple.


Split Charge Systems

Why do I need a split charge relay?


Not only will the split charge relay/system recharge the leisure battery while the engine is running it will also stop current being drawn from the starter battery when the engine is switched off (ensuring you will be able to start your vehicle) should the leisure/auxiliary bank run out of power.

How do I fit a split charge relay?

We supply voltage sensing relays (VSR) that don't require an alternator connection, you simply wire it from your starter battery to the leisure battery. These are available as complete kits which include the relay and all the necessary cables, fuses. connectors etc to make your split charge relay installation simple. Alternatively, you can buy the volt sensing relay separately and choose your own cables, fuses etc from our extensive range.

If your vehicle is relatively new and has a complicated engine management system we would only recommend using the voltage sensing split charge relay which is easy to fit and requires no connection to the vehicle electrics. Simply wire the relay from your starter battery to your leisure battery.

Always fit suitably rated fuses close the both the starter battery and the leisure battery.

The old, conventional, make/break relays where the split charge relay has to have a trigger connection (usually the alternator warning light) are more difficult to fit and may cause issues with engine management systems and warranty conditions if fitted to modern vehicles.

Split charge relays/systems FAQ’s

What size leisure battery do I need?


The Amp hour rating denotes the amount of energy that can be taken from a battery before the terminal voltage falls below 10.8 volts. This test is usually carried out over a 20 hour period (20 hour rate). Thus a 50 Amp hour battery can be discharged at 2.5 amps for 20 hours before the voltage drops below 10.8 volts (i.e. 20 x 2.5 = 50). With leisure batteries Amp hours is often abbreviated to amps (e.g. 85 amp leisure battery). For further explanation of volts, watts and amps click here.

What’s a battery to battery (DC to DC) split charger?

The DC battery charger uses it's own micro-processor system  to determine the actual level of charge in the leisure battery and tricks the alternator into providing extra charge for long periods in order that the leisure battery reaches full charge.

When using other types of split charge system the leisure battery voltage rises to a certain level the alternator will cut down it's charge rate to minimal amps because it thinks the battery is now charged. However, if the battery is deeply discharged the alternator will cut back too early because it cannot differentiate between what is just surface charge and what is full charge. This is where the DC battery charger comes in... it will keep demanding that power until the leisure battery's actual full charge is reached.

How do I choose the right split charger?

For medium battery power consumption and leisure batteries of up to 120Ah go 70 - 100 amps and for high consumption and larger leisure battery banks go a 100 amps plus. Alternatively, you could choose from one of our range of Durite micro-processor controlled, DC battery to battery chargers which work differently to conventional split charge relays. These units are available in a number of different voltage combinations including 12v starter to 24v leisure and 24v statrer to 12v ;leisure. Go to the page for more info.

If you want to make life really easy for yourself buy one of our complete split charge relay kits which include everything needed for a successful installation to any make of vehicle.

What does voltage sensing/sensitive relay mean?


Voltage sensing split charge relays are is easy to fit and require no connection to the vehicle alternator or other electrics. The relays sense when the starter battery is at full charge and then switch to charge the leisure battery. Simply wire the relay from your starter battery to your leisure battery. This also avoids any vehicle warranty issues.

The relays cut in at 13.7 volts, making sure the starter battery is at full charge first, and cut out at 12.8 volts ensuring that the starter battery cannot be discharged when the leisure battery is out of power and that your vehicle will always start!

Why do I need a split charge relay?

How do I choose the right split charger?

What does voltage sensing/sensitive relay mean?

What’s a battery to battery (DC to DC) split charger?

What size leisure battery do I need?

How do I fit a split charge relay?

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