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Caravan, Caravan Electrics, Caravanning, Caravans, Charging, Leisure Battery, Maintenance, Split Charge Relay, starter battery, Technical, Towing, vehicle battery, Vehicle Wiring
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.
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 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:
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.
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.
Caravan Chronicles 
Hi Simon
Just a quick update,I ran a new earth and live to the relay and we now have a working fridge when engine running.
Regards john
Great news…. cold beer/wine now on arrival 🙂
Hi Simon
Just read this and it is very good. Had some trouble with the fridge power from tow car. I traced it back to the vsr 12v in 12v to permanent live no 12v on fridge pin when engine running,changed vsr all working fine down to 13 pin socket plug caravan in fridge still not working even after engine running for quite a while. Fridge works fine on the test box at caravan service centre. This has now got me scratching my head.
Hi John
You need to check that the voltage on the leisure battery charging pin and the fridge pin in the caravan under load. It could be that there is a bad connection somewhere and all seems OK just checking the voltage, but when under load the voltage will drop too low.
Hi Simon
Thanks for your reply,I’m going to run a new power and earth cable of 2.5 mm direct from battery to the relay and see how that goes. Will 2.5mm be ok for the mod as it is about 5 metres from battery to relay? It’s a navara pick up so is quite long.
Regards John
My 2004 autotrail 696gse has a simple relay fitted by autotrail on production. This works fine but the terminals on the relay and fuses get warm and over constant use they melt the fuse holder connections.
Hi Eddy
Sounds like the crimp terminals on the cable ends and/or the terminals on the relay are showing signs of corrosion and causing resistance in the circuit so when a current is drawn, its warming things up.
My first step would be to remove the relay and clean the terminals with some ‘scotch bright’ (the green fibre pads) or if really bad, some very fine emery paper. Next would be to try and check the crimp connections to see if the copper cable inside the crimp is bright or a dull brown or green. If it is, they will need re-terminating. The same goes for the fuses, pull them out and see if the contacts are bright or dull and corroded, replace if they are.
Sometimes electrical connections inside caravans and motorhomes, even though not directly exposed to the outside elements do corrode.
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Sorry, minute not “MINUATE”
PS, forgot to mention that the aim of the 3 minuate delay is to allow the tow-cars engine to partially warm up and replace the bulk of the cranking power taken before connecting the caravans’ auxiliary loadings.
Snowy
Hi simon,
My latest invention which is in fact down to you, in other words, you triggered my thinking cap when you mentioned within your previous post (above) about the 3-4 minuate delay some modern tow-cars which control the output to the caravans’ ignition controlled supply by means of direct switching (via relay) by the tow-cars ECU.
Most of us know that upon many vehicles the tow-cars ignition controlled power supply (12s/pin 6 or euro/pin 10) is controlled by a Voltage Sensing Relay (VSR) which do not provide optimun efficiency even if they are installed and adjusted correctly i.e. they even need periodic adjustment to compensate for battery deterioration.
I have come up with the following invention and it’s tryed and tested, once installed, you can forget about it as it functions with optimum efficiency;
Using a 555 Silicon Integrated-chip, I have made a small electronic circuit to control the tow-cars’ ignition controlled supply for vehicles which are unable to control said supply directly or indirectly via their ECU’s, it functions as follows;
Ignition off, no output supply.
Ignition on, no output supply.
Ignition on, after 3 minutes, live supply.
Ignition off, supply instantly terminated.
This cycle, recycles when the engine is stopped and then restarted even if the engine is fully warm, it only terminates momentarily if the engine operates on auto stop/start, if fitted.
There is a safety feature built into this system, if the vehicle has auto stop/start or the vehicles’ ignition was switched on and the engine was not started untill after 3 minuates the vehicle would try to draw chanking power from the caravans’ leisure battery, this cannot happen as the said unit can detect when the vehicles’ starter is operated.
Connections of the unit/circuit;
1/ Input, constant live supply 2.5mm²
2/ Output, switched live supply 2.5mm²
3/ Input, vehicle ignition switched supply 0.2mm²
4/ Ground, vehicle chassis earth 0.2mm²
5/ Positive, vehicle starter solenoid connection 0.2mm²
I don’t know what readers will make of this, all I can say, it works and never misses a beet.
Snowy
Well, I’m stunned. All I ever needed to know about connecting 2nd. batteries, SCR, VSR, connecting batteries in parallel/series etc. All explained clearly and without unnecessary verbiage. Outstanding! Many thanks to Simon Barlow, Steve Jones et al. I’m happier now about attempting electrical add-ons on my motorhome. Mac1937.
Many thanks for you feed-back Simon, very interesting reading
Snowy
Just something that I have started to pick up on is the fact that some vehicles take 3 to 4 minutes to turn ‘on’ the relay feeding the caravan supply… this is causing some headaches with caravan service engineers as customers are reporting the fridge isn’t working when they start their engine and the service engineers are testing out the caravan… and everything checks out OK. They connect to a test set and everything is OK. The customer hooks up and the caravan fridge isn’t working. They are then referred back to the installer that fitted the tow bracket and everything checks OK. My advice is to leave the tow vehicle ticking over for five minutes and check the fridge.
You would be surprised how many service engineers have gone “DOH” when I’ve told them!
S
HI Simon,
Reading the above artical written by Mr Steve Jones with regard to Smart Alternators, I agree with everything he has stated bar the connection of the relay to the auxiliary connection on the ignition switch. On quite a few modern vehicles, this connection is not always cut when cranking is in progress like it was in past time. Secondly, some modern vehicles do not have an ignition switch as we know it, they have as you know, a key-fob with no actual key on it and it only has to be within the vehicle’s cab, not inserted in anyway, said vehicle usually has a push button starter switch. Thirdly, some modern vehicles have auto-stop-start, if the relay could be connected as said, it maybe bypassed and not deactivate when anto-stop-start operated. I am not going to criticise this point without coming up with the goods myself. If the relay coil was connected a follows; Terminal 85 to a standard ignition switched supply and terminal 86 to the small terminal on the starter motor solenoid in which the solenoid is feed from the starter switch/ignition starter position, this would allow the relay to earthout through both of the solenoid’s coils, ie the moving coil and the holding coil. Upon the starter switch feeding the starter soleniod the relay coil would at this point be receiving a positive supply at both ends, therefore deactivating instantly. Connecting said relay in this manner would allow the relay to function on any vehicle no matter what wiring differences they may have.
Snowy
Hi Snowy
On cars that have keyless starting and eco start there is already a set of terminals that power down when the engine is cranking under start condition. One set goes to 0 volts for all cranking operations and another set remains at 12 volts on eco cranking (engine turns off automatically and restarts automatically) to ‘keep alive’ certain functions.. like the radio for instance, when in eco stop mode. Having looked at several wiring diagrams for a cross section of vehicles, provision for switching caravan supplies has been catered for. Unfortunately there is still a perception that a voltage sensing relay is required on modern vehicles… and I get a lot of emails from people with problems with them. Once they swop to a standard switching relay in all but one case the problems vanish.
The article that Steve Jone wrote was specifically for Land Rovers. Landrover OEM towing electrics don’t have split charge relays only standard switching relays. There are non OEM suppliers that were/are supplying a voltage sensing relay for Land Rover towing electrics to cut down on installation wiring, time and therefore cost.
S