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Help & Advise

Please see below a basic guide to all of our split charge systems with help and advise to hopefully help you choose the right kit, first time.


We are sorry that the images and some of the links have been removed whilst this page is being modified 




The reason to install a split charge system


The main reason to install a split charge Relay 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.





Split Charging Cable Sizes Available


We offer a large range of cable sizes in our kits to suit most needs and budgets and rate them as Light Duty, Medium Duty and heavy duty. The cables are a very important part of any split charge system and we would always recommend using the highest spec cable you can as they will be allowing the power through the system via which ever type of relay is used for charging the leisure battery.

The size of cable chosen will depend on the sizes of the leisure batteries used as the higher the capacity of battery or batteries the higher the transfer rate of current will be when you start the vehicle and both starter and leisure battery balance their capacity plus also if you will be intending on using a amp, winch or inverter with the engine running as the current from these devices can also be pulled through the split charge system.


Light duty kits are all supplied with 6mm² 42amp rated cable recommended for charging up to 110ah leisure batteries on a budget for light duty split charging for general camping and leisure use.


Medium duty kits are all supplied with 10mm² 70amp rated cables recommended for charging up to 125ah leisure batteries for a more professional charging system for general camping and leisure use or for use with light duty business vehicles running light duty equipment.


Heavy duty kits are all supplied wit 16mm² 110amp rated cable recommended for charging up to 225ah batteries and is our recommended cable size for anyone wanting a top qualty professional heavy duty split charge system suitable most leisure and business uses including powering amps and inverters up to 1000 watts with the engine running.


We also have a range of Industrial duty kits with 25mm 170amp rated cable and 40mm² 300amp rated cable suitable for charging large banks of leisure batteries or for running high powered winches or inverters with the engine running.




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 and euro 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


Please see the links for our recommended options




Melin Equipent 30amp or 50amp charger with added solar MPPT control for up to 500 watt panels for our top quality system for anyone wanting the best charger in our range 


This system is suitable for charging a 12v leisure battery bank from 12v or 24v vehicles so is especially good for horse boxes and race vans with 24v vehicle electrics




Merlin Equipment DATA sheet







For smaller demands our Ctek D250SA is a very popular option with a 20amp charging output and solar mppt 



Ctek DATA sheet



Another very popular option from Sterling Power is the BB1230 12v 30amp charger which is also available as a 12v 60amp charger or 12v to 24v or 24v to 12v charger



Sterling Power Data sheet





If unsure please click on this video link for more information on how to find out if you have a smart alternoator or regenerative charging





Please click HERE for a very simple plug in volt tester if required




Please see a PDF file to explain more about why you can onle use this type of charging system for your vehicle

Brake Regeneration Information PDF



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.

All we can do is offer our recommended advise to this very popular question.

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 45-60 minutes. 
The voltage will rise from to approx 12.6v up to  13.5v to 15v as the alternator is now charging the starter battery.
If the voltage stays constant over the test at 13.5v to 15v then it should be ok to use any standard relay charging system.
If the voltage drops to 12v to 12.8v 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

Also 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.









(VSR) Voltage sense relays


              Basic wiring diagram for connected a standard voltage sense relay system





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

For the latest Euro 6 ECU controlled or brake renerative charging  vehicles please see our range of battery to battery DC/DC chargers.



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.


This activation voltage and isolation voltage is different in all of our voltage sensitive relays so please read the information below for advise on which will be best for you.


All of the 140amp voltage sensitive relays we supply are duel sensing 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.


Please see the popular range of voltage sense relays and kits can supply



M-Power Voltage Sense Relays & Systems


Actuation voltage 13.3v and isolation voltage 12.8v

The M-Power relay is a high quality popular relay more well known in the marine world and for a good isolation voltage at 12.8v when the engine is switched off is our recommended product for most installations.

As the isolation voltage is higher this means that the relay will disconnect sooner at 12.8v leaving a much higher charge in your main starter battery for when you need to start the vehicle.


           M-Power 12v 140amp Relay        



          M-Power 12v 140amp relay Kits





Durite Voltage Sense Relay


Actuation voltage 13.3v and isolation voltage 12.65v +/- 0.1v

The Durite relay is still the most commonly known and popular voltage sensitive relay.

Please note that Durite lowered the isolation voltage of this relay to 12.65v which means the relay can stay connected for longer periods after the engine has been switched off until either the main starter battery or leisure battery drops below the isolation voltage.



   Durite 12v 140amp Relay                     




  Durite 140amp Relay Kits





HC Cargo Voltage Sense Relays & Systems



Actuation voltage 13.3v and isolation voltage 12.8v

The Cargo relay is from our new lower priced range of kits and relays but offers a fully e-approved professional quality relay and split charge system with good isolation voltage at a great price.

If you are looking for a good quality split charge system at a great price then we also recommend this product.


       HC Cargo 140amp Relay     




 HC Cargo 12v 140amp VSR KIts      







HC Cargo 12v 70amp Voltage sense relay                  HC Cargo 12v 70amp VSR Kits




Actuation voltage 13.2v and isolation voltage 12.6v

We also have a range of HC Cargo 12v 70amp relays and kits that are great value for installing a light duty split charge system for general camping use

Please note this relay is not duel sensing like our range of 12v 140amp kits



HC Cargo 12v 180amp Voltage Sence Relay                      HC Cargo 12v 180amp VSR Kits




Actuation voltage 13.2v and isolation voltage 12.6v

We also have a range of HC Cargo 12v 180amp voltage sense relays and kits which can be for a higher rated system and our kits are supplied with 25mm² 170amp cable or leads for a much heavier duty system.












Heavy Duty ignition controlled relay



The ignition or switch controlled split charge kits are a bit more complicated to install but not too difficult. The relay is simply controlled by a 12v feed which can be taken from the alternator warning light wire, any positive ignition feed when the vehicles ignition is on or even just a simple on/off switch but if using the switch method its important to remember to switch it off when you stop to ensure the batteries are isolated so the starter battery isn’t drained.










All of the above Split charge kits or parts we supply are all top quality, UK supplied and fully tested E approved parts.



Simply Split Charge are 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 at




For a human then please call our sales office

Monday to Friday 9am to 4pm or Saturday 9am to 1pm ... Sunday Closed


01229 718098

07818 603245