Split Charge Circuits



This page is only to give a brief overview of charging circuits for vehicle etc. 

Whilst the drawings can be used as working drawings to carry out an installation, they are also intended to show the simplicity of some of the Sterling Power products.



Split charge relay

Charging an additional battery can be quite simple, however some people are concerned about flattening their engine starter battery (vehicle battery) as well as their auxiliary battery. Its quite simple to safely keep batteries isolated from one another when the engine is not running, thus preventing accidental draining of the vehicle battery. This is done by using a relay that is activated from the alternator. This simple circuit is commonly known as a 'split charge relay circuit'

I have tried to make the following drawing as simple as possible.

I have coloured the main cable red which is most likely the same as your vehicle, however the other vehicle cables will be a variety of colours. The terminal numbers above are the standard numbers found on automotive relays. Terminals 85 & 86 are the coil, which activates the relay. Terminals 30 & 87 are the switch contact terminals.

Basically, all you need to do is identify the cable which runs from the alternator to the warning light on the dash board. Rather than pull your dash to bits, it may be easier to have a look at the cables on the back of the alternator. You'll find one or two large cables which will be the cables which run back to the battery. You will also find one smaller cable which is the charge indicator cable (no doubt someone will come up with some exceptions)

Now you've identified the charge light cable, its a simple mater of connecting it to the one side of the coil on a 12 volt relay (assuming your vehicle is 12 volts), the other side of the coil is then connected to an earth point (which eventually goes back to the negative terminal on the battery).

I have drawn the above with 30 amp fuses in line with the battery link cable (do make sure the cable between the vehicle and auxiliary battery is capable of handling 30 amps, otherwise you can quite easily create a fire hazard). This size is normally sufficient, however if you have a really dead flat second battery they may blow. Don't increase the fuse size unless your cable and relay are rated for higher currents.

The above is reasonably reliable, however some alternators don't like additional loads being applied to its charge indicator circuit, in which case the alternator not only fails to charge, but also fails to activate the split charge relay. In this situation a voltage sensing relay could be used.

We presently don't stock these relays as split charge diodes are so much better, however we are looking at suppliers

Voltage Sensing Relay

There is another relay available which is more expensive than the above plain relay, however it consists of some electronics to activate a relay to connect the two batteries together.

The relay input is basically connected to a live cable on the vehicle, preferably the main battery feed. The relay output cables are then connected up to your auxiliary battery (no need to run a cable to the alternator charge indicator wire).

The relay operates by sensing the vehicle voltage, as the voltage to the vehicle battery will rise when the alternator is producing power, when the relay senses an increase in voltage, it activates its contact to allow power to the auxiliary battery.

We presently don't stock these relays as split charge diodes are so much better, however we are looking at suppliers

Split Charge Diode Block

There is now another way to split the charging of your batteries. Instead of using relays, its now possible to use a split charge diode block. The first units produced  did  use actual diodes, whilst the units worked, they were not efficient due to the voltage drop of around 0.8 - 1.2volts, this doesn't sound a lot, but in terms of charging batteries it is  quite a drop. To overcome this voltage drop, the modern units as supplied by Sterling use clever circuitry with mosfets, the result is that the voltage drop is now lowered to around 0.04volts

Split charge diode blocks any problems associated with the above relays and provide a good positive supply to both battery banks, whilst maintaining total isolation between them.

The split charge diode blocks are incredibly easy to install, as shown in the following drawing.

The best way to install this unit, and to avoid any changes to the vehicles wiring, is to disconnect the main feed cable off the back of the alternator, thoroughly insulate it and tie it out of harms way with something like cable ties. You can now run a fresh cable from the alternator to the diode block, and two more to the two batteries. Basically that's it.

By using the above method, its very easy to remove the diode block an re-instate the original wiring.

Go to our split diodes

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Advanced Alternator Regulator

Having now fitted the above split charge block, its a good idea  to fit an advanced regulator to the charging circuit. This extra (Smart Regulator) will then give the optimum charge rates for your batteries and will also take care of any voltage/power loss in your charging system. 

This system does required the removal and stripping the alternator to add additional cables. Yes its quite technical to strip the alternator, but the end result is worth the effort.

It is possible to fit the advanced regulator and use a split charge relay, but could cause problems with the voltage regulation, so not recommended.

The combined wiring of the split charge block and advanced regulator is shown below

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Alternator To Battery Charger (A2B Charger)

Obviously not everyone wants to get involved with stripping alternators, well there is a far easier route which combines the benefits of a split charge diode and advanced voltage regulation, by using Sterling's Alternator to Battery Charger.

The A2B unit combines all the above circuits shown above devices into one tidy package, and the good thing is, it only requires the installation of four cables (Three main cables, plus an earth). If you were paying someone to install a system for you, go for this combined unit as the extra cost would be set against the time to install the unit. However if your doing the work yourself, the previous systems would work out cheaper.

This combined unit can easily be configured (using switches) to take account of the types of batteries your using. Its capable of charging 5+ times faster and put an extra 50% extra power in to your batteries. Think about this for a moment, why buy extra batteries, which will take a while to recharge, when you can fit this to reap long term benefits!
The easiest way to install the A2B unit is the same as described above to install the split charge diode block.

Go to our Alternator To Battery Charger

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Battery to Battery Charger with Hook-up Charging
Using Sterling's Battery to Battery charger is probably as simple as it gets. There's no need for anything additional, as the B2B unit does it all. The unit detects when there is spare power (when the engine/alternator is running), and uses that spare power to charge the house/second battery at the correct voltage and rate. What could be simpler? 

It's worth pointing out that the B2B unit will not drain the starter battery, as it effectively relies on the surplus power of the alternator to charge the second battery, so you'll always have power to start your engine.

Having installed a B2B unit, then installing one of Sterling's Pro Budget battery chargers to the main starter battery, will give you dual battery charging capability. The mains battery charger will charge the starter batter, the the B2B unit will detect spare charging capacity, and use the spare capacity to charge the house/second battery. This set-up will also allow you to use 12 volt appliances whilst on hook-up.

The B2B is quite and advanced unit and although described in its simplist form above, its quite advanced. For a more in depth description of the B2B unit click here

Go to our Battery To Battery Charger

Want a more in-depth explanation


The manufacturer (Sterling) have done a lot of research into their products and their use with various batteries. Their recommendations for batteries is to stick to standard lead acid batteries, rather than gel, leisure, carbon fibre etc. as they have found no benefit for the extra expense. The down side to them is that they don't charge as quickly as standard batteries. They recommend standard lorry batteries, we have found tractor batteries to be as good. In fact we have found they are dramatically cheaper than using ordinary vehicle batteries.

More information here


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