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There is a lot of confusion over what the differences are and when to use each one. So what are they?  A relay is just a simple switch that allows a low power electrical circuit to turn on (or off) a high-powered electrical circuit. An SCR or Split Charge Relay is a switch that senses voltage and switches over from one circuit to another at a pre set voltage. Finally a VSR – Voltage Sensing Relay is similar to an SCR but turns on and additional circuit at a pre set voltage. So what do you use each one for?

Split Charge Relay – SCR

These are usually installed in 4 x 4’s that have a lot of accessories fitted especially electrical winches. The general idea is the winch is powered from a second battery so that using the winch should never flatten the main vehicle starter battery. As the winch uses a lot of power, it is necessary to be able to recharge this battery as fast as possible using the output from the vehicle’s alternator, so once the vehicle starter battery is fully charged, the output from the alternator is switched directly over to the second battery. Modern SCR’s are usually all solid state and some times have a bypass switch allowing both batteries to be directly connected for either starting the vehicle if the starter battery is flat or to allow the winch to be powered from the starter battery in emergencies.

SCR's are normally solid state, I have shown it as switches to make it clearer.
SCR’s are normally solid state, I have shown it as switches to make it clearer. Note: I haven’t shown any fuses or earth (neutral) cables. Never use the vehicle body as a conductor, always install correctly sized earth (neutral) cables.

How do SCR’s operate?

If you look at the drawing above, you can see the alternator output goes straight to the split charge relay. In normal operation the relay connects the output of the alternator to the starter battery. Once the engine is running and the relay determines the starter battery is fully charged, it switches the output of the alternator to the vehicle accessory battery. If the voltage on the vehicle starter battery drops, the relay switches back. However, these relays are slightly more sophisticated and have the ability to dual feed – i.e. charge both batteries at the same time or the ability to link both batteries for either vehicle starting or to power accessories under extreme load… heavy and prolonged winching for example. Expect to pay serious money for a good quality SCR with a remote bypass facility and battery monitoring.

A true ‘split charge relay’ is not suitable for charging your caravan leisure battery, but could be used for charging leisure batteries in motorhomes.

Voltage Sensing Relay – VSR

Voltage sensing relays were designed to sense the tow vehicles battery voltage and when the battery is sufficiently charged switch on a second circuit, usually the power to the caravan’s fridge. Some of the more sophisticated VSR’s allow the setting of the voltage the relay will operate at. The biggest draw back with these is if the electrical load with the tow vehicle suddenly increases – turning on the headlights, heated windows, air con etc, they can turn off the circuit to the caravan’s fridge until the electrical load is removed. It is for this reason they should be installed with an indicator light on the dash to confirm that they are actually supplying the fridge. With a caravan, if there isn’t a feed to the fridge, the habitation relay will not operate and therefore the caravan’s leisure battery will not be charged.

VSR’s can be a mix of solid state and mechanical switch or all solid state. Again, I have not shown fuses or earth (neutral) cables for clarity.

VSR’s work on the principle that they monitor the vehicles electrical voltage. The VSR’s sensor detects the voltage change when the vehicle battery is fully or near fully charged, it closes the contacts connecting the leisure battery in circuit and allowing it to be charged. However, if the vehicle voltage drops for some reason, the relay will open disconnecting the leisure battery.

Problems with VSR’s

Most of the time they are installed when the rest of the towing electrics are installed and are usually located behind a panel in the rear load area of the vehicle. Because of this, it is rare to see one that has a dashboard indicator light installed to show when it is ON and charging your leisure battery and consequently switching over the habitation relay allowing your fridge to work. The second problem is when they leave the factory the voltage that they are designed to switch on at is set at the correct level if it was installed near the vehicle battery. As it has been installed some distance away, the length of cable between the battery and VSR has its own voltage drop – dependant on length of cable and current through it. So one fault that is often reported is that the relay ‘vibrates’ or ‘chatters’. This is caused by the voltage drop on the connecting cable increasing below the “trigger voltage” when the leisure battery is connected and the relay is simply turning on and off repeatedly. Like most things you get what you pay for and the more expensive VCR’s have a timer circuit built-in so that they only switch over after a pre-determined time.

But wait…. there’s more!

Modern vehicles are sophisticated bits of engineering, electronics and software. In order to reduce emissions a lot of manufacturers removed things like hydraulic power steering pumps and air conditioning pumps and replaced them with electrical motors. This removed mechanical load from the engine and reduced the weight. The trade-off was the vehicle needed more electrical power, so larger alternators were fitted. As power steering and air conditioning wasn’t needed all the time and in order to prolong the life of the vehicle battery, the alternator is now controlled by the software in the vehicles ECU. So when the ECU detects the vehicle battery is fully charged it reduced the output of the alternator by reducing the voltage and therefore reducing the load on the engine and hence reducing engine emissions. If you have a VSR fitted, this drop in the vehicle voltage will stop the VSR working as it was intended to do. So even if the VCR has a timer circuit, it can still fail to work as intended.

Theres is an interesting letter posted in the September 2013 issue of Land Rover Owner magazine:

Article appeared in Land Rover Owner INternational September 2013. (c) LRO /  Bauermedia
Article appeared in Land Rover Owner International September 2013. (c) LRO / Bauermedia

How can we reliably charge the leisure battery in modern vehicles? Well the clue is in the article above. If we can fool the tow vehicle to ‘see’ the leisure battery as part of the vehicles own electrical system the software in the ECU won’t reduce the voltage until both the leisure battery and vehicle battery are suitably charged.

Ordinary Relay

The most straight forward way of providing the leisure battery with a suitable charging circuit is also the cheapest. A simple 30 Amp 12 volt relay can be bought for a few pounds and it will be a reliable way of charging your caravan’s leisure battery.

Ordinary 30 Amp relay
Ordinary 30 Amp relay. Again, I have not shown fuses or earth (neutral) cables for clarity.

The circuit above shows an ordinary 30 Amp automotive relay being used to switch the leisure battery charing circuit. The relay is controlled by the ignition circuit, so when the engine is running the relay is energised and the leisure battery is being charged. As the leisure battery is in circuit all the time the engine is running, the vehicles software in the ECU ‘sees’ just one big battery and will keep the alternator voltage high enough to charge both batteries fully before reducing the output in energy-saving /emission reducing mode.

Some vehicles including a number of Land Rover’s actually have a connection on the main fuse board specifically to control this relay.

I hope you have found this useful.