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PostPosted: Tue Aug 21, 2018 11:41 am 
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There has been a change with the car’s charging system, it is now controlled by the engine management so the voltage is no longer maintained at 13.8 to 14.2 volt, so a relay which switches on at 13.4 volt to connect caravan to car will no longer work.

The basic idea was:-
1) To stop current flowing from caravan to car, it would only flow from car to caravan.
2) To power Fridge or Anti-snake devices only when the ignition is switched on in the car.
Although the old relay will still do 1) it would switch off under engine load so the fridge and anti-snake device would not work. So using a blocking diode and relay triggered by ignition will allow power from car to caravan but not caravan to car, and it would be powered all the time. However the added 0.6 volt drop will mean it will not charge caravan battery.

In the caravan however we could add a DC to DC inverter which would raise the voltage and allow charging, once added the diode in the car is no longer required; however it must be added in case a non converted caravan is towed.

Now in caravan forums I hear how people travel with head lights on to stop the relay in car dropping out, so it would seem some auto electricians are still fitting the voltage switched relay. Or cars are being wired by untrained personal? However I may be wrong, as modern cars have complex lighting systems for rear lights, so today one is left with no option but to buy a ready made harness to power the twin 7 or 13 pin sockets. As to if these include or don’t include the two power supplies to caravan I don’t know?

Again question does only the non ignition supply needs a diode in the supply? My caravan uses the ignition supply to operate a relay which connects non ignition supply to battery; the ignition supply works the fridge. So volt drop on ignition supply due to fridge does not reduce charging current. However although this is what BS7671:2008 recommends, some caravans may use the ignition supply for both.

However what is done needs to follow a standard practice and be failsafe. My caravan has a 10A fuse between caravan battery and car when charging from car, so even if the car does try to draw current from the caravan battery when cranking car engine it will fail safe. However my caravan does not have ATC to apply brakes to stop snaking, clearly with that system loss of car supply could be dangerous.

So question is what is the new standard practice.


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PostPosted: Tue Aug 21, 2018 11:54 am 
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Moved to Vehicles.


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PostPosted: Tue Aug 21, 2018 3:08 pm 
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ECU controlled alternators have been around 20 years, what car and year, sounds like you just need a bypass relay and a split charge module

the 12v will only charge the battery and the charger/invertor in the caravan should have protected charge system so it doesn't back feed


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PostPosted: Tue Aug 21, 2018 7:40 pm 
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Vehicle used at moment does not charge caravan, we are about to change it, there were faults on the caravan which needed correcting, what I wanted to do was emulate the tow vehicle using a 7Ah battery so once changed I will know it will work.

However on doing that I have found some items which I see could cause problems with modern car, I want to be able to lend caravan to my son and daughter without anything I have done causing problems, so looking at a Jag and what ever we buy.

So want what every is fitted on car to work with all caravans, and what ever is fitted on caravan to work with all cars. Without blowing a fuse, or messing up vehicle engine management. I know if you change a vehicle battery you have to go into the engine management and tell it the battery has been changed. So it is possible having a second battery in parallel can mess things up.

However cars have cig lighters and USB sockets so a drain on vehicle battery is unlikely to be a problem, only a feed is likely to mess things up. So blocking diodes come with two outputs so car would need blocking diode to ensure what ever caravan is towed it will not back feed, and for the ignition supply a relay after the diode.

But then we come to caravan, some thing like this Image it seems is needed, it can also be used with solar panels so has duel use, but it also has a safety system, it will switch off if donor battery less than 12.6 volt and back on once it reaches 13.1 volt.

So in real terms if one of these is fitted to caravan there is no need for anything on the tow vehicle. I had wondered if this would fit on caravan or car, but since it works the solar panels clearly it fits on the caravan.

So non ignition feed all it needs is a fuse, on the ignition side the drain is only that taken by the fridge, so again only a fuse, but 30A is a fair amount of current, which likely if no hook up the caravan will need, but existing fuses in caravan are just 15A. I would say likely 15A is enough, but Ring is 30A, and more to point £179, Sterling do a 20A version but that's £250, getting a bit silly. Even a 6A one looking at £70.

It seems best option is pull the charging fuse and pay for sites with hook up. So do people actually pay out £179 or do they use generators and have 230 volt as well.


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PostPosted: Wed Aug 22, 2018 2:52 pm 
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tow wiring is all standardised, if the car has multiplex wiring aka can-bus it need a bypass relay, a intelligent relay will do the power 30 amp module is under £20 or less (

Traditional setup is a 12N 7 pin (lights) with a 12S (power) pin 1 and 2 are swapped pre/post 1997

After 2008 it went to the 13 pin euro (power and lighting combined)

what ever you have there are conversion leads to suit

Towsure have a lot of info on their site

https://www.towsure.com/fit_a_multiplex_bypass_relay

https://www.towsure.com/self-switching- ... -electrics

Edit. The Ring 30A box is a dc to dc convertor, basically a battery regulator for the caravan Battery

Keep it simple car side and caravan side don't confuse them


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PostPosted: Fri Aug 24, 2018 8:41 pm 
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I now have my new tow car, it has the road lights socket, just needs the second socket, it seems odd but can't find 5 or 7 core 2.5 mm sq flex. Well at least in a short length. 30 meters easy enough but 2 meters is another matter. It seems I will have to use 2 x 3 core.


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PostPosted: Sat Aug 25, 2018 7:42 pm 
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7 Core SY armored flex ?

https://www.ebay.co.uk/itm/BRAIDED-SY-C ... qOGLqyKqqw


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PostPosted: Thu Sep 13, 2018 10:17 am 
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When I came to plug caravan in, found 13 pin not 7 pin on car, mad dash to get converter, I have not as yet tested to see if any power to caravan, but all road lights work.

So it seems I have been let off, don't need to wire car, already done.


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PostPosted: Thu Sep 13, 2018 2:53 pm 
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That's ok then eric, the problem sorted its self in the end


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