Durite 0-727-33 split charge relay
Durite 0-727-33 split charge relay

Split Charge System FAQ's

Our guide to split charge relays, DC battery charger systems, fitting a split relay charge set-up, what size leisure battery you require and which system will suit your battery charging requirements best.

For the latest vans with Smart Alternators and Regenerative Braking Systems where conventional relays will not work efficiently see DC Battery Chargers.

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Why do I need a split charge relay?

To charge your leisure / auxiliary battery bank from your alternator while the vehicle engine is running.

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 choose the right split charger?

Which split charge system option to choose.

If you're fitting a split charge relay system from scratch, your vehicle is relatively new and has a complicated engine management system we would recommend using a volt sensing relay which are easy to fit and requires no connection to the vehicle electrics. Simply wire the relay from your starter battery to your leisure battery. This also avoids any vehicle warranty issues.

Voltage sensing split charge relays and fitting kits are easy to fit as they only need wiring from the starter battery to the leisure battery/batteries. They are suitable for older and also modern vehicles with INTELLIGENT ALTERNATORS, CANBUS and MULTIPLEX wiring systems (not smart alternator systems with regenerative braking - see DC Battery Chargers for these applications). With our range of volt sensing/sensitive split chargers there's no need to cut wires or take feeds from other sources (i.e. the vehicle alternator). Beware of other systems on the market that may cause electrical malfunction or invalidate the warranty on modern vehicles.
The relays usually cut in at around 13.7 volts, making sure the starter battery is at full charge first, and cut out at around 12.8 volts (figures may differ slightly on some relays) ensuring that the starter battery cannot be discharged when the leisure battery is out of power and that your vehicle will always start! Our range of volt sensitive split chargers are suitable for all sizes of leisure battery bank from 40AH to 200AH.

If you regularly discharge your leisure / auxiliary battery to quite a low state of charge, or are very reliant on battery power then a DC battery split charge system might be a better option. DC Battery Split Chargers are also suitable for the latest vehicles with regenerative braking systems.

Durite DC battery to battery charger
Durite DC battery to battery charger

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

DC Battery Split Chargers are suitable for the latest vehicles with smart alternators and regenerative braking systems.

DC battery chargers are easy to fit and will ensure that the vehicle's starter battery system is at full charge before switching to fast charge the leisure/auxiliary battery system while the engine is running. Ideal for those who use a lot of battery power.

The difference between DC battery chargers and other split charge systems is that whereas a split charge relay is ruled by how the vehicle alternator senses the leisure battery charge 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.

Confused? Let us explain further...

As stated a split charge relay is governed by what the vehicle alternator does. When 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; in most cases (i.e. light caravan and camping uses) where the leisure battery is not deeply discharged this would be acceptable. However, if the battery is deeply discharged the alternator will cut back too early because it cannot differentiate between what is just surface charge (i.e. the voltage has reached a level, but the battery actually needs much more charging) and what is full charge. This is where the DC battery charger comes in... Basically it reacts the same as a top quality mains battery charger, but instead of using mains electricity is uses power from the alternator instead and it will keep demanding that power until the leisure battery's actual full charge is reached.

So, to summarise, if you use a lot of leisure battery power and deep discharge the battery/batteries regularly then the DC battery charger should be your preferred option.

The DC charger will also isolate then starter battery from the leisure battery when the vehicle is at rest to prevent any risk of discharging the starter battery so that you will always be able to start the vehicle.

The DC split charge systems are available in 12v to 12v, 24v to 24v, 12v to 24v and 24v to 12v starter and leisure battery bank options and combinations.

DC Battery Split Chargers are also suitable for split charging in the latest vehicles with smart alternators and regenerative braking systems where other split charge relays will not supply sufficient charge.

Volt sensing relay wiring diagram

How do I fit a split charge relay?

If your vehicle is relatively new and has a complicated engine management system we would recommend using modern style voltage sensing split charge relay which are simple to fit and require no connection to the vehicle alternator or other electrics. Simply wire the relay from your starter battery to your leisure battery.

Voltage sensing split charge relays  and DC to DC split charge systems are easy to fit as they only need wiring from the starter battery to the leisure battery/batteries. Click to enlarge the volt sensing relay fitting diagram to the right and you will see that the installation process is relatively simple. They are suitable for older and also modern vehicles with INTELLIGENT ALTERNATORS, CANBUS and MULTIPLEX wiring systems. With our range of volt sensing/sensitive split chargers and DC battery to battery chargers there's no need to cut wires or take feeds from other sources (i.e. the vehicle alternator). Beware of other systems on the market that may cause electrical malfunction or invalidate the warranty on modern vehicles.
Older style split charge relays will need to be connected to a "Trigger" wire (usually the alternator warning light) and might be a more daunting prospect for those not familiar with auto-electrics.
Always fit suitably rated fuses close the both the starter battery and the leisure battery.

12 volt 110 leisure battery

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.

The 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).
Need to work out your 12 volt battery power consumption? Confused? Click here and we hope to unravel the mysteries of volts, watts, amps and ampere hours.

More about leisure batteries

What's the difference between a leisure battery and a starter battery?

A starter battery is designed to give a high level of current over a short period of time. This is achieved by each battery cell being made up of the largest amount of thin battery plates possible, thus creating more surface area in order to achieve the maximum amount of cranking current amps (CCA) being delivered to the starter motor so the engine will start more efficiently in all kinds of weather.

A leisure battery is designed to deliver current at a lesser rate over a longer period of time. This is achieved by each battery cell being made up of less (than a starter battery) but thicker battery plates, thus creating a more sturdy environment for the repeated discharge and recharge cycles that are necessary to run the typical appliances used for camping, in caravans, camper vans, motor homes, boats etc.

 

In truth, these days most leisure batteries are dual purpose, i.e. they can be used for both engine starting and running leisure appliances (deep cycle) by reaching a happy medium between the 2 different types of battery plates.

There are 3 main types of leisure battery:

  • Flooded, or Wet cell, leisure batteries - the battery is filled with liquid sulphuric acid (electrolyte) in the conventional way and periodic maintenance (topping up with de-ionised water) may be required. Wet cell leisure batteries are the most commonly used and suitable for most leisure/auxiliary battery applications.
  • AGM (Absorbed Glass Mat) leisure batteries - the battery is filled with acid, but the liquid is totally absorbed making AGM batteries non-spill and so that they can be fitted in any orientation, i.e. on their side or at an angle.
  • GEL leisure batteries - the battery is filled with an electrolyte gel thus making them non-spill and GEL batteries can also be fitted in any orientation.
  • GEL and AGM batteries may provide more cycles (battery discharging and recharging) than conventional wet cell batteries and, it is claimed, have deeper cycle parameters, but these advantages must be weighed up against the initial cost.

In most cases we would suggest using flooded, wet cell leisure batteries unless the non-spill advantages of AGM and GEL are a crucial factor in your leisure battery system.