Help in getting head around relays in caravan.

Aug 11, 2018
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Going from bottom up, relay for water pump OK takes load off micro switch. A master relay again OK one switch and everything off great. Next connects to non ignition circuits of car, so when car is connected all lights work off car battery, again no problem with that.

So now the top two relays, tests show activated with ignition supply, however although I could test relay and confirm they actually change over, the question is why. The fridge was switched to mains and since test battery just 7 Ah don't really want fridge load on such a small battery.

But why would you have a relay to work fridge?
 

Damian

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Mar 14, 2005
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Probably because when the fridge is operating on 12v when connected to the car with the engine running, it draws around 10amps.
 
Oct 8, 2006
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ericmark said:
Going from bottom up, relay for water pump OK takes load off micro switch. A master relay again OK one switch and everything off great. Next connects to non ignition circuits of car, so when car is connected all lights work off car battery, again no problem with that.

So now the top two relays, tests show activated with ignition supply, however although I could test relay and confirm they actually change over, the question is why. The fridge was switched to mains and since test battery just 7 Ah don't really want fridge load on such a small battery.

But why would you have a relay to work fridge?

This is the so-called habitation relay. When you connect up and the car engine is running, the relay operates and removes ALL power from caravan 12V circuits so that incoming power from the car feeds only the fridge 12V and provides current to charge the caravan battery. It matters not that the fridge is switched to mains operation. the relays will still operate.
 
Aug 11, 2018
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Slowly getting head around the problem. Power to 3 and 4 on S plug and all power seems to switch to car, power 6 and 7 and fridge works. Also it connects caravan battery to pins 3 and 4 and they become live.

Found if I remove fuse 5 then car and caravan batteries are not connected so then car does everything.

Not sure about connecting car and caravan battery together, will depend on tow car, but to my mind DC to DC inverters yes as no way can the caravan battery discharge to the car, other methods depends on the car.
 
Oct 8, 2006
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ericmark said:
Slowly getting head around the problem. Power to 3 and 4 on S plug and all power seems to switch to car, power 6 and 7 and fridge works. Also it connects caravan battery to pins 3 and 4 and they become live.

Found if I remove fuse 5 then car and caravan batteries are not connected so then car does everything.

Not sure about connecting car and caravan battery together, will depend on tow car, but to my mind DC to DC inverters yes as no way can the caravan battery discharge to the car, other methods depends on the car.

No, you still have it slightly wrong.
When you plug the car in (12S) the first relay will activate and disconnect the caravan battery so all 12V operation in the caravan is from the car battery.
When you start the car the habitation relay activates and disconnects the whole 12V system in the caravan except the fridge and the caravan battery - by definition if the car engine is running then it can and is supplying charge current to the battery as well as powering the fridge. In a modern 13-pin connection the fridge supply actually provides the charging supply as well, but under 12S the two supplies are independent.
Note also to regularly check the earth pins (4 and 7?) as they have a habit of getting hot and melting their mounting (most often pin 7) so that you fridge (and possibly charging also) may not work.
 
Aug 11, 2018
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Woodentop said:
No, you still have it slightly wrong.
When you plug the car in (12S) the first relay will activate and disconnect the caravan battery so all 12V operation in the caravan is from the car battery.
That bit at least works centre relay does that.
Woodentop said:
When you start the car the habitation relay activates and disconnects the whole 12V system in the caravan except the fridge and the caravan battery
That it not happening. The lights continue to work.
Woodentop said:
- by definition if the car engine is running then it can and is supplying charge current to the battery as well as powering the fridge. In a modern 13-pin connection the fridge supply actually provides the charging supply as well, but under 12S the two supplies are independent.
Note also to regularly check the earth pins (4 and 7?) as they have a habit of getting hot and melting their mounting (most often pin 7) so that you fridge (and possibly charging also) may not work.
Table A721 - Functional allocation and cross-sectional areas of cores for caravan connectors in BS7671:2008 sets out connections and both BS EN ISO 11446 (13 pin) and BS AU 177a (7 Pin S type) both have two positive and two negative supplies, in my caravan the latter terminates in a 6 pin multi-coupler with just 5 wires, the 5th being Reversing light whatever Coding for coupled trailer is I don't have it.

I don't have the BS AU 177a wired up on our tow car, we are about to change it, and idea was to get caravan ready, I knew the caravan had problems, the master switch did nothing. I had two in-line fuses which gave an alternative supply to lights, and by-passed the master switch. So I removed fuses and have been attempting to return it to standard. Lighting was easy, just a blown fuse, the master switch it seemed easy to start with, but then realised all 12 volt was only from charger, the battery was simply not connected. Top fuse was marked "Van Batt+" so I connected battery to that fuse.

All now seemed to work, however seemed prudent to test it all. Found 14 fuses and 12 labels, but removing one at a time, it seemed the bottom 9 seemed to follow labels. And clearly the top one as I put it there. Which left one clearly unused and three left to work out two labels left are Fridge and Car. I tested with fuse removed and Fridge still works on 12 volt battery, used a small 7 Ah battery to emulate the car. And so I have two fuses not a clue what they do. However one seems to have not power at any time. And one fuse which only has power when an external battery is connected. Left side becomes live when connecting power to pins 6 and 7, and right when power is connected to 3 and 4, however not the power from 3 and 4 but the power from the caravans battery.

So if I power pin 3 and 4 then the caravan battery becomes connected to pins 6 and 7. So assuming it was connected to a car with all 4 pins connected then on turning on ignition the caravan battery would be connected to car battery. As long as the car has some device to stop current going from caravan to car that's OK.

However if the car does not have a one way device, be it relay or diode, then when the starter cranks or when the car is under load and engine management switches off alternator then car will be likely using caravan battery. Which is the reverse to what is wanted.

Before engine management controlled alternator output a voltage dependent relay could be used so car only connected when alternator is charging, however that no longer works, it would result in the power to caravan being switched on/off all the time, OK for charging, but not for fridge operation.

I have not read how modern cars are wired but I would guess a relay not voltage dependent but just connected to an ignition circuit and a diode to prevent draw from caravan? Since there is a 0.6 volt drop through diode in the caravan it will require a DC to DC inverter to lift the voltage up again.

However removing fuse 5 so caravan battery is not connected to car would remove need for diode in car, not sure however I would want to take the chance that a trailer with battery connected is ever plugged in, safer to fit diode anyway, clearly to both supply's but the standard blocking diode has one input and two outputs anyway so not really a problem, the relay contacts clearly after the diode.
 
Oct 8, 2006
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With 12N/S working the usual method was to connect the caravan battery through a power diode to prevent the caravan battery back-feeding the car battery.
 
Mar 14, 2005
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Woodentop said:
With 12N/S working the usual method was to connect the caravan battery through a power diode to prevent the caravan battery back-feeding the car battery.

Or powering the fridge!
 
Aug 11, 2018
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Well now swapped tow car, and new one does not have the alternator controlled by engine managment, so I can forget about that problem. Already has N 7 pin socket, I need to add a S type. Still not sure if non return system should be fitted to car or caravan, but likely latter as then it will also do solar panels.

I remember the old blocking diode system, it was designed for use with a CAV alternator using a 440 regulator which had three terminals hi, med, low and one had to move the wire to hi setting, it would work with some battery sensed regulators, but many alternators were machine sensed and so the charge voltage was reduced.
 
Aug 11, 2018
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I got a Kia Sorento 2007 much cheaper than originally intended to pay, but it will do what we want, it is only used to tow with, we have other cars for day to day use. It already has tow bracket fitted, and single 7 pin socket, I intend to add second 7 pin socket to run fridge and reversing light.

I did consider split charging, main reason for split charging is to ensure battery has enough power when we get home to get it up the drive, that can be done with a second battery left at home on charge, which is cheaper, and more reliable.

I still have two fuses I don't know what they do, one is always powered, the other never seems to have power. Not found anything which does not work when they are removed. There is one multi connector with nothing connected to it, clearly for some optional extra, so I assume the unknown fuses are the same.
 
Nov 16, 2015
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Eric with so many problem, and your mind blowing , thoughts about your van, why not take the car to a tow bar fitter who will check out your car,
That way you will start from a basic. If you are deciding that caravaning rather than basic electronics for is for yourself, then change to a 13 pin system and just get an adapter.

By the way, have you had a nice time away in the caravan yet, I hope you :) have.
 
Aug 11, 2018
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Found was 13 pin on car, mad dash to get adaptor, now caravan also 13 pin, not tested charging, likely will be left to next year now, extra power has made a huge difference to towing, returned to Forest of Dean, so good comparison. However seems three routes to Monmouth, A4137 and A40 we took early in year bit winding and narrow but OK, this time took the A466 that had some steep hills second gear high ratio would love to know how steep, then return A40 and A49 a much better road.

As to getting some one else to wire for me, not reached that point yet, but I am finding jobs which were easy are now more taxing, and only 67, but I will test myself, however that will be a nice little job next time I go on holiday. Must remember to put meters in the caravan ready.
 

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