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Leisure Battery Charging

The reason to install a split charge or battery to battery charging system


The main reason to install a split charge Relay or DC-DC charging  kit in your vehicle, is to charge a second or leisure battery while you are driving. This means you will have enough power or charge in your second battery to use it when you are stopped, camped or working, to use whenever you need it. It will isolate the second battery from the main or starter battery of your vehicle when you are stopped. This will then ensure the main battery doesn't get drained of power when you are stopped, so you will always have enough power in your main battery to start your vehicle. So basicly whenever the vehicle is started the relay will allow power from your vehicles charging circuit usually known as the alternator or generator to charge the second battery or batteries.




A split charge relay explained


A Split charge relay is simply just a heavy duty switch which is controlled by either the vehicles ignition system, a simple on off switch or the more popular voltage sensitive relays which are the Simplest to install and are ativated by a small electric circuit in the relay which detects the rise in voltage when the vehicle is started. It doesn't matter which type you use as they all do the same job. When they are switched on they allow power to pass through the relay to charge the second or leisure battery and when they are switched off they isolate the main battery from the second battery so will always ensure you have enough power to start the vehicle.


(VSR) Voltage sense relays

The voltage sensed relays and kits we supply are the simplest to install to asll vehicles with a standard alternator charging system.

There is no need to supply a switch or ignition feed to control the relay as they work on the rise and fall of the voltage from the alternator when the vehicle is running.

The relay cuts in to allow power to charge the axillary battery at approx. 13.3 volts and cuts out to isolate the axillary battery at approx. 12.8 volts.

All the voltage sense relays and kits are simply installed by connecting the main starter battery to the leisure battery via a suitable fuse as close to each battery to protect the circuit and the relay with no need for any wiring to the vehicles wiring or alternator making it it the best solution for most modern vehicles with standard charging systems.

We offer a range of the very popular 12v 140amp voltage sense relays which range in cost and operating voltages which may suit your vehicle, budget or intention of use better.

All of the relays work in a similar way from the rise voltage from the vehicles charging system when you start start your engine which activates the relay and allows the second or leisure battery to be charged whilst you are driving.

When you are stopped and the engine has been switched off the voltage drops and the relay then isolates both batteries.

All of the 140amp voltage sensitive relays we supply are duel sensing and bi directional which means that if you apply a charge to the leisure battery via a mains battery charger or solar panel it will activate the relay if the voltage rises above the relay activation voltage allowing charge to flow through the split charge system in reverse to the main starter battery. This will need to be isolated using a battery isolator between the leisure battery to the relay if not required.


Common problems and questions with using or installing a Voltage sense relay





Battery to battery chargers


A battery to battery charger or B2B is an alternative to standard split charging converting energy from the main starter battery to a three or four stage battery charger offering the best charging system that you can achieve by charging your leisure battery up to 10 times faster than a split charge system will achieve and to a much higher level. We offer a range of Merlin Equipment, Sterling Power and Ctek chargers up to 60amps and recommend these units for anyone wanting the best charging solution for their vehicles and essential for business vehicles needing a good charge in the leisure battery bank or for all the latest Euro 5 and 6 engines with smart charging systems. The battery to battery charger is also the simplest solution for charging a 12v battery from a 24v vehicle or for charging a 24v battey bank from a 12v battery. Some of our battery to battery chargers are supplied with pre wired leads or can be supplied as kits for simple installs. The chargers are all fully automatic activating via the rise of voltage like a voltage sense relay making them very simple to use but most have the option of ignition controll if required.


Euro 5/6 vehicles that are fitted with smart alternators or SRCS (Smart Regenerative Charging Systems) are not compatible with standard split charging systems and will require a Battery to Battery Charging Systems for the only safe and affective way to charge a leisure battery without causing issues to the vehicles main electrical system or 12v devices connected to a leisure battery receiving very high spikes in voltage


SRCS recovers excess energy from brakes and other rotating parts on the vehicle explained.


The power is delivered in short bursts while braking – often at very high voltage and current. In order for the chassis battery to accept these bursts, the battery is kept at a low level. The alternator is only employed during sustained periods of higher-current charging. So, in some instances, the voltage is too low for conventional split charging, or, the voltage is too high to safely charge specialist auxiliary AGM/Gel Battery Banks.


The DC-DC Charger is the perfect solution.


The unit acts like a battery charger. However, it uses DC power instead of AC power. When the engine is running, the unit will switch on and deliver smooth, regulated power to the auxiliary batteries on board. The unit is smart enough to know if the engine battery is too low for charging and is not affected by stop-start devices (a real problem for charging when vehicles are driven in congested areas).


For all of the latest spec vehicles including Mercedes Blue Efficiency Sprinters, VW T5 and T6 Blue motion, Vauxhall Ecoflex and Fords Econetic Transits it is recommended to use a battery to battery charger form range of battery to battery chargers, smart charging alternator kits



Additional information on a very popular question on if you require a battery to battery charger


It is very difficult to advise 100% and we would always advise if unsure get professional advise.

The simplest way to check if you may be able to use standard relay system or require a battery to battery charger is to do a voltage test on your main battery to see if you have a constant voltage from the alternator or it is controlled via the ECU or some kind of charge control module or battery monitoring system.

Start the vehicle and ensure all vehicle electrics are off ie lights and heating ect.

With a multi meter test the voltage at the starter battery over 15-30 minutes.

The voltage will rise from to approx 12.6v up to 14v to 15v as the alternator is now charging the starter battery.

If the voltage stays constant over the test at 14v to 15v then it should be ok to use any standard relay charging system.

If the voltage drops to 12v to 13.5v then the ECU has cut the power from the alternator as a fuel saving feature and you

will be recommended to use a battery to battery charger.

This test is not 100% as it will be different on some vehicles but will give you the best idea of what type of charging system may be suitable for your vehicle.

It is not recommended to use a relay system in any vehicle with ECU controlled charging or battery monitoring as

this can cause issues with the vehicles electronics.

Any Euro 6 vehicle as this will have some form of fuel management via the charging system

Any Euro 5 vehicles with High efficiency engines or Stop start features

For charging any AGM, Gel or Lithium type battery that all need to be charged at a regulated voltage which a standard alternator charging system may not provide.








Simply 12 Volt is a small family run business offering one of the largest range of split charge kits and systems we think you will find anywhere. We know and understand that one kit does not fit all. If you cant find the type of kit or system you need or require any help or advise on which kit will be best for you then please contact us for help and advise


For a human then please call our sales office

Monday to Friday 9am to 4pm

Saturday, Sunday Closed


01229 718098