Understanding the Leisure Battery Charging Circuit (2023)

There is a lot of confusion and a lot of myths as to what actually happens when you plug your caravan into the tow vehicle and start the engine. To really understand whats going on, we have to understand how the tow vehicle’s battery and alternator work first. It is then easy to see how connecting your caravan allows the engine to charge the caravan leisure battery.

I’ve written this to try to remove some of themystery(or should that be misery?) out of what happens when you plug your caravan into your tow car. A better understanding of what’s happening will hopefully help you fault-find if there are problems.

A bit of history…

Oh no… it’s like school again“…. don’t panic! We do need to look back a few years though. Not too long ago, the only device to charge a car battery was a dynamo. It had fixed heavy magnets wrapped round a rotating coil that was driven by the engine usually via the fan belt. It wasn’t very efficient though. It was heavy, it has a commutator that allowed the rotating coil to pass the output current via carbon brushes which required some regularmaintenance. The output of the dynamo went to a regulator that controlled the DC (Direct Current) voltage…. that eventually went to the battery.

Dynamo’s typically had an output of around 25 to 30 Amps – whichcomparedto modern alternators of around 120 to 160 amps was tiny. Dynamo’s were usually geared so that the turned twice as fast as the car engine, which was great when you average Ford engine only managed to do about 4500 RPM…. with car engines now revving up to 7000 RPM our poor old dynamo would be spinning at 14,000 RPM and lasting about a week as thecentripetalforce acting on those heavy copper coils rotating at that speed would pull them apart pretty quickly.

Dynamo’s had few other issues – not only were they heavy but the carbon brushes needed changing on a regular basis, the commutator needed cleaning and if it became pitted through arcing would usually need re-finishing in a lathe. The regulator was inefficient too and often caused overcharging and boiling of the battery and at slow speed, they just plain didn’t work. So this is where one of the myths comes from – “you need to rev your engine to get your alternator to charge your battery!

Err… no you don’t. The old dynamo’s did need to have the engine going at a fast – 1800 to 2000 RPM idle speed to generate enough voltage to charge your battery but modern alternators will produce sufficient output to charge the vehicle battery at your engine normal tick over or idle speed. Why’s this? I hear you ask. Well, the old dynamo’s were rotating too slow at tick over or idle speed, the voltage was generated was around 9 or 10 volts… notenoughto charge the battery, however at the higher end of the vehicles rev range, the output could be as high as 20 volts, so they had a box or regulator to control this excess voltage by switching the dynamo in or out of circuit.

Car designers have to take into account that in winter you will start your car, turn on the heated windows and mirrors, heater fan,stereo, and then maybe leave the car idling to warm up. They have to ensure that even attick-over, the output of the alternator is sufficient to put some charge back into the battery, power the engines electrical systems and power all the other things. If they didn’t do this, just leaving your car idling in winter could flatten your battery!

Ok, lets skip a bit… vehicles are now fitted with alternators. These lightweight alternators are more efficient. For a start, they generate AC (AlternatingCurrent) and don’t have heavy magnets. Instead of spinning a coil inside a magnet to produce electricity, they spin a magnet inside a coil, however, this magnet is actually a small ‘electro magnet’ (sometimes called the ‘rotor’) which is powered (or ‘excited’) from your battery. The current for these spinning coils is still fed through a commutator and carbon brushes, but as the current is so small, they generally don’t wear out. The coils wrapped on the outside of the rotating coils are called the stator – are the ones that actually produce the AC voltage. Obviously, we need DC to charge out battery and run the vehicles systems, so there are that is rectified using a bridge rectifier, which is made up of 6 diodes. There is a diode trio that powers the rotor windings…. but its starts to get a bit involved so we’ll leave it there. There are also brushless alternators… but that’s a different story.

The output of the alternator is controlled by a regulatorwhich may be mounted inside or on the back of the alternator (internally regulated), or sometimes on the firewall of the vehicle (externally regulated). This is more common on larger 4 x 4’s especially if they have twin battery systems. These regulators are nothing like the old dynamo regulators, they are solid state and they are a lot more sophisticated in operation. The regulator works by ‘simply’ increasing or decreasing the field current supplied to the rotor to control the output.

If you have an alternator that can produce 120 amps (max) and the total current demand from the electrical accessories (including the battery) is only 20 amps, the alternator will only produce the necessary current (20 amps) to maintain the target voltage. This is determined by the voltage regulator and the resistance of the electrical load on the alternator. If the voltage starts to fall below the target voltage, the current increases allowing the voltage to remain the same. The full output of the alternator however is not usually available at your engines idle speed. The output power increases with RPM and maximum output is around 1800 to 2000 RPM.

On newer vehicles now however, there is a moveawayfrom the alternator having its own or aseparateregulator and the engines own ECU controls or regulates charging output so it can balance electrical load and engine efficiency. In some modern ‘eco’ cars, the ECU shuts down the alternator when the engine is at idle say at traffic lights. If the ECU detects a high load on the alternator… like on a winters morning you start the engine, turn the lights on, heated windows and car radio, then the ECU knows exactly how to alter the fuel going into the engine to make it as fuel-efficient aspossible. Previously, the alternator would have just put a dynamic load onto the engine and the idle speed would have dropped, so the early enginecontrolswould have just added more fuel to bring the idle speed back… prior to that, you might have even just pulled the choke out a bit to stop the engine running roughly and stop it from stalling.

On some vehicles now, they have electric CAT pre heaters to get thecatalyticconverter up to its workingtemperature…. and while this is warming up, some carsdon’tallow you to turn the heater blower fan on full or turn the heated windows or seats on. This is often a source of complains by owners that have bought these cars and are unaware of the system.

So how does all that affect my caravan?

Well…. remember the Dynamo? Way back then, it would have taken a week of running about to charge your caravan’s leisure battery, that’s why people took them home and sat them in their garage on a charger plugged in to the wall. When some of the first alternators came out, they still had a crude regulator and the output was a bit ‘iffy’ but it worked ok. The next leap forward was to be able to charge your caravan leisure battery while towing you caravan along the road. In the UK we looked at our 12N and thought…. “we need another plug” so we came up with the 12S…12 for 12 volts and S for ‘supplemental. (Yep you can get a 24N and 24S as used on commercial vehicles) However, our continentalcousinslooked at things and decided to adapt an existing 13 pin military plug and socket.

This was great, we could now charge our leisure battery while driving at some ungodly hour in the morning just to beat the neighbours to the best pitch.However, the old dynamodinosaurswarned “you will not charge your vehicle battery up unless you have a voltagecontrolledrelay” Well, yes and no.

Some of the larger 4 x 4’s just have a lead that goes from the battery via a 15 or 20 amp fuse straight to pin 9 (13 pin socket) or pin 4 (on the 12S) and have no other control. So when you plug your van in, there is a voltage from the tow vehicle present in the caravan all the time…. but we are getting ahead of ourselves here.

Modern alternators and their regulators are quite sophisticated.The actual output voltage produced by the alternator will vary depending on temperature and load, but will typically be about 1-1/2 to 2 volts higher than battery voltage. At idle, most charging systems will produce 13.8 to 14.2 volts with no lights or accessories on. You can measure this by connecting the positive (+) and negative (-) test leads of a voltmeter to the battery terminals while the engine is running.

Understanding the Leisure Battery Charging Circuit (1)When you first start your engine, the voltage should rise quickly to about two volts above the battery voltage, then remain steady. The exact charging voltage will vary according to the battery’s state of charge, the load on the vehicle’s electrical system, and temperature. The lower the temperature the higher the charging voltage, andconversely, a higher temperature requires a lower charging voltage. The “normal” charging voltage range is 13.9 to 15.1 volts at 25 deg C, but below 0 deg C the charging voltage might be 14.9 to 15.8 volts.

Modern alternators are capable of 120 to 160 Amps output, which is more than enough for them to charge the vehicle battery, caravan leisure battery and run the fridge all at the same time and still have enough in reserve to power all the lights and car accessories.

What happens when you plug your caravan into the towing vehicle?

Well in Europe we have different regulations to the USA and Australia. We (in Europe) have to comply with a rule that basically says that only the road lights can be powered when we are travelling. This was done to make sure that nothing in the caravan could affect any of the safety systems in the tow vehicle, a side effect of this means we cannot use electric brakes on trailers which have been in use in the USA andAustralia successfully for a number of years which allows agreater weightto be towed. It also casts some doubt on the status of some of the aftermarket brake assist and stability systems currently on the market…. these operate in a bit of a grey area currently.

To stop anything being powered up in the caravan when the engine is running on the tow vehicle, there is a “habitation relay” which when it detects the engine running, it switches over and disconnects the caravan from the leisure battery and connects the car charging circuit to the leisure battery. This ensures anything in the caravan is disconnected.

Remember when I said “Some of the larger 4 x 4’s just have a lead that goes from the battery via a 15 or 20 amp fuse straight to pin 9 (13 pin socket) or pin 4 (on the 12S) and have no other control. So when you plug your van in, there is a voltage from the tow vehicle present in the caravan all the time” well this circuit is the one that charges the leisure battery. Well some vehicles have aseparateoutput from the ECU for this circuit, others have a second relay controlled by the ECU for this output too. In the future, it’s going to get very involved installing towing electrics into vehicles as they become evermore complex. Now we need to add something else into the mix…..

Understanding the Leisure Battery Charging Circuit (2)

Typical tow vehicle towing electrics – the relay can becontrolledby the ignition switch, ECU or the vehicles ECO power management system. It’s NOT a split charge relay although often mistakenly called that.

The fridge is also allowed to be connected to the electrical system, but only when the car engine is running other wise it wouldquicklyflatten the tow vehicle battery. The fridge can never be run off the caravan leisure battery (although there is a connection so it can operate the gas safety valve and auto gas igniter). So how do we do this? Well, there is another lead that comes via the relay in the towvehicle. Remember I said that there was a relycontrolledby the engines ECU and only turned it on when the engine was running… well there is a circuit that runs from this relay to the caravan via pin 10 (13 pin socket) or pin 6 (on a 12S). When the caravan sees a voltage on this pin, it switches the caravan habitation relay over, disconnecting the caravan leisure battery from the caravan and connecting it to the charging circuit (pin 9 -13 pin socket, pin 4 on the 12S). This ensures two things… first – the caravan leisure battery can never supply the car with power – so if you have a flat car battery, it will not use the caravan battery totry to turn the engine over allowing the high starting current todestroythe caravan wiring and second – it disconnects the caravan’s internal 12 volts systems from the battery so nothing can operate and potentially affect the safe operation of the tow vehicle.

Understanding the Leisure Battery Charging Circuit (3)

Simple drawing showing the difference between the relays when the engine is OFF and the engine is RUNNING. It shows it is impossible for the tow vehicle to use the caravan battery for engine starting.

It’s not a split charge relay….!

Now this is not a ‘split charge relay’. It is a simple switching relaycontrolledby the ignition light circuit on the car or by the car’s ECU to switch on theheavycurrent feed to the fridge circuit.A lot of people call this the split charge relay andit’s not, you could call it the “fridge relay” if you wanted!.

Understanding the Leisure Battery Charging Circuit (4)

Simple SCR installation

A “split charge relay” and “voltage sensing relay” were originally designed for vehicles with dynamo’s or early limited output alternators and detected when the vehicle starting battery was charged by the voltage output of the dynamo or alternator. When the starting battery was charged, the voltage rose to 13.6 or 13.8 volts and the relay switched the output of the alternator over to the second battery so it could be charged. If the voltage of the startingbatteryfell below a pre determined level, the split charge relay would switch back and charge the starting battery again. On some of the more expensive voltage sensing relays you can adjust the voltage.

If you have an aftermarket electrical towing loom fitted it might well be supplied with a voltage sensitive relay. On installation, the location of the relay is important. Some fitters install this in the rear of the car rather than the engine bay. This is incorrect, as the relay now has to ‘sense’ the voltage at the end of a cable that has run the length of the vehicle and itself is subject to a voltage drop. This will require the relay to be adjusted to accommodate this voltage drop. It also means that when you turn on additional accessories – headlights, heater fans, screen heaters, the voltage drop delta will change and the relay could then start to ‘cycle’ on and off. Realistically it should be installed in the engine compartment to be as accurate as possible.

UPDATE:August 2013: This article appeared in LRO regarding the smart alternators in Land Rover Discovery 4 vehicles. It might also be relevant to other 4 x 4 and tow vehicles.

Understanding the Leisure Battery Charging Circuit (5)

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

SCR’s as opposed to VSR’s are now more than just simple relays. Originally they had to ensure that the were ‘make before break‘ switches, which means they connected the second battery before disconnecting the starter battery, otherwise allowing thealternatorto go ‘open circuit’ was a sure way ofprematurelyending its life.

SCR’s now (and I don’t mean the trade name acronym used by General Electric to describe Thyristors!) use MOSFET and other technology to monitor batteries and dynamically switch and proportion charging… in fact many now incorporate some form of smart charging technology, facilities for connecting more than just two batteries and some even have shore power charging ability and AC inverters built-in. There are a number of SCR’s on the market that now also handle the alternator regulator function and require modifications to your alternator to improve charging capabilities. These however are usually more suited to marine use. One of the best companies I know off for this sort oftechnologyisSterling Power.

You often find ‘split charge relay’s’ in 4 x 4 vehicles that have a second battery for a 12 volt recovery winch or in boats, where they have a normal engine start battery but may have a bank of several 12 volt batteries to provide power while under sail or moored without shore power. Some top end 4 x 4’s have alternators with a dual output for handling a twin battery system. For more information about the difference between Relay’s, Voltage Sensing Relay’s and Split Charge Relay’s read my post Relay, VSR, SCR… What’s the difference?

Understanding the Leisure Battery Charging Circuit (6)

Typical 13 pin connected caravan wiring showing habitation relay

As a fall back safety system, the caravan manufacturer will install a fuse between the caravan’s leisure battery and the 12 volt systems on board the caravan. This does two things… it protects the caravan’s 12 volt electrical system if there is an overload or fault, thesecondthing is does is protect the caravan wiring from damage if the habitation relay should fail and you start the tow vehicle’s engine with the caravan connected. The fuse will blow before any damage can be done by the car attempting to use the leisure battery as a source of power. This fuse is typically rated at 15 or 20 amps.


Some people will tell you that you can check your alternator’s output by starting your engine, then disconnecting your battery to take measurements… some will tell you if you have a flat caravan battery to start your car, disconnect the good battery and connect the flat battery.

Well, if you disconnect your battery with the engine running…. your off shopping later for a new alternator. Disconnecting your battery when your engine is running is a really bad idea. Youwillcause voltage spikes on the tow vehicles electrical system that might just see off your ECU and radio…. it willdefinitelysee of the diode trio in the alternator.

“OK give me all that in English…”

1 – When you plug the caravan into the car with the engine-turned off, pin 9 (13 pin) or 4 (12S) will supply a voltage to the caravan this will energise your Al-Ko ATC system if you have one fitted and on some caravans will allow you to manually switch from the caravan’s leisure battery to the tow vehicle battery. All the road lights will work. The caravan battery is NOT connected to the charging circuit (so you can’t use the caravan leisure battery to start your car if the carbatteryis flat!)

2 – Start the engine. This now makes pin 10 (13 pin) pin 6 (12S) live and provides power to the fridge in the caravan. It also energises the habitation relay, disconnecting the caravan leisure battery from the caravan and reconnecting it to the tow vehicle charging circuit only.

Actually is quite simple really…. even I got it then!

Understanding the Leisure Battery Charging Circuit (7)

13 Pin or ‘Continental’ socket fitted to my Freelander

Some things to look out for….

The cables on the towing electrics should be of sufficient size to carry the currentrequiredby various components in the caravan. Typically the road lights only need a 1mm or 1.5mm square sized cable. However, the charging circuit requires a minimum of a 2.5mm square cable as does the fridge circuit. If you look at the wiring diagrams above, you will also see that there are a number of neutral or earth cables. These are keptseparatein the caravan and should never be linked within the caravan. The only time these neutral or earth cables are connected is when they come together to a central earth point in the tow vehicle. These cablesshouldalso be a minimum of 2.5mm square.

If you want to install twobatteries inparallelin your tow vehicle, or even in your caravan – especially if you have a heavy twin axle with a motor mover fitted, there is a right and a wrong way to do it. I’ve written a “How To: Connect TwoBatteriesinParallel” guide.

For fault finding on theleisurebattery charging circuit, the first step is to make sure all is well with the tow socket on the vehicle. To help with out dismantling it I’ve done a drawing showing the pin connection from the socket side….

Understanding the Leisure Battery Charging Circuit (8)

The connections for a 13 pin towing socket seen from the socket (plug) side.

You can click on the link to down load a PDF copy: 13 Pin Towing Socket 01

Further Reading:

Understanding Watts, Amps, Volts and Ohms–A very basic introduction to some simple maths that allow you to work out power, current and resistance.

Relay, VSR, SCR… What’s the difference?– goes through the different types of relay that might be used to charge your leisure batteries.

Understanding Caravan and Tow Car Electrics

13 Pin Socket – Basic Fault Finding

ISO 11446 – 13 Pin Trailer Connections

Caravan Road Lights – Basic Fault Finding

How To:ConnectTwoBatteriesin Parallel


Copyright © 2011 – 2020 Simon P Barlow – All rights reserved

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