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.
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
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.
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
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
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!