No, I’m not starting an Agony Aunt post..!
A couple of days ago one of Caravan Chronicles readers, Vic, posted in the comments section of “Understanding Caravan and Tow Car Electrics” that he recently flattened his car battery by leaving his caravan connected to his car. Here’s what he posted:
Hi Simon, while travelling across Spain I made an overnight stop and left the caravan connected to the car electrics I also hooked up to mains. The next morning the car battery was flat, I’m sure I’ve done this before with no consequence. All I can think is that european site wiring can be suspect.
brilliant article.
OK, that got me thinking… Was it the car-off-van switch faulty? Was there another fault of some sort? I asked Vic for the details of his caravan and he replied it was a 2009 Bailey Senator California. I don’t know the exact details for Bailey electrical schematics, but it should not be too difficult to work out what might be happening. I started to think of various failure scenarios that ‘could’ account for Vic’s problem. At this point I have to say I’m indebted to David Rose for sending me information about the electrical services in Bailey Caravans.
Looking closely at the schematic of the control unit, it all hinged around the correct operation of four relays. Here is the schematic I have drawn out in a simplified form:
The drawing (above) shows the four relays – R1, R2a & R2b, R3 as they are set when the caravan is not connected to the vehicle and the master switch is off. The red lines show what part of the circuits are live. The master switch controls relay R1 and when energised by turning the Master Switch on allows current to flow to the caravans 12 volt electrical system via the 12 volt fuse board.
Above: We have now turned the Master Switch on and can see that relay R1 has energised and is allowing current to flow to the caravans 12 volt services. The caravan is not connected to the tow vehicle.
Above: The caravan is now connected to the tow vehicle and the engine is running. The caravan master switch is off. With the fridge circuit powered from the running engine, it energises R3, R2a and R2b. Relay R2b disconnects the master switch circuit so that it cannot operate R1. Relay R2a connects the leisure battery to the 12 volt permanent feed from the tow vehicle. R3 energises, but only switches the feed for the master switch from the leisure battery to the permanent 12 volt supply from the tow vehicle. As this stage it cannot be used to supply the master switch due to R2b being energised.
Above: The caravan is still connected to the tow vehicle, but the engine is off. Relay R2a and R2b that are controlled by the fridge circuit are now not energised and revert to the default position. However as the caravan is still connected to the tow vehicle, the 12 volt permanent feed keeps relay R3 energised, therefore supplying the Master Switch – not from the leisure battery but from the permanent 12 volt feed from the vehicle.
This now leaves us in the following position:
Above: The caravan is connected to the tow vehicle but the engine isn’t running. The master switch is on. The caravan now receives it’s 12 volt feed from the tow vehicle to power all the 12 volt services. I have greyed out the live circuits from the leisure battery. Even though the caravan is plugged into a EHU bollard, the 12 volt services will be run from the vehicle battery and not the mains charger/ leisure battery circuit. The fridge will continue to work on 240 volts as will any mains powered equipment.
So, it’s not a ‘fault’… it’s a ‘feature’ ! Leaving the caravan plugged into the car and turning the Master Switch on allows you to power the caravan’s 12 volt services from the car. Disconnect the lead from the car and the caravan will be powered from the leisure battery.
I hope this solves Vic’s puzzle.
.
Copyright © 2011 – 2015 Simon P Barlow – All rights reserved
I take it there’s no car/off/’van switch fitted to the caravan then ……??
Hi Andy
No, the caravan post dates the old ‘car-off-van’ switch. I’m not sure when this would have been dropped from the Bailey range of caravans.
S
Hi Simon
thanks for your time and effort in solving the problem.
Regards and many thanks
Vic
Hi Simon, Thank you for a most informative article, explained in plain English. Great website, keep going please.
Cliff
HI Simon,
Very Very interesting, upon following the shown diagrams I totally agree with your conclusions.
In my opinion, it’s an oversight by the manufactures not undertaking their in-house homework upon designing the on-board electrical systen.
The purpose of relay three (R3) ;
Correct me if I’m wrong, but I understand that there was a change sometime ago within the caravan safety regulations regarding the isolation of mains operated appliances within caravans becoming in contact with the drawing vehicles’ circuitry if the mains 240v was connected whilst the towcar was connected.
The function of relay three (R3) is to disconnect the mains battery charger/mains transformed 12v power supply from being connected to the tow-cars’ circuitry while the vehicle is still electarically connected.
In this case, the wiring of R3 is totally incorrect in my opinion as you correctly say Simon, if the tow-car is left connected, the caravan domestic 12v services are running off the tow-cars’ battery.
To add to this, the said relay is NOT isolating the mains charger/transformer from being able to connect to the drawing vehicles’ circuitry after the tow-cars engine has been started. If you look at the relays, as you correctly say; R3, R2a & R2b are energised but the charger/transformer is still allowed to connect to the drawing vehicle as it is connected in parallel with the caravans’ leisure battery even though R3 has supposedly isolated it to conform with the supposed change in regulations.
I say no more, I rest my case.
Snowy
sorry, I’ve made a slight mistake; i.e. One of the aims of R3 is to isolate the charger/ traisformer from the drawing vehicle should the caravan be connected at the same time as the mains, the relay should also be wired to act as an automatic, van-off-car, seletor switch. The said relay cuold be wired to undertake these functions with a few wiring alterations & diodes.
snowy
Hi simon,
In my eyes, you should be well credited for solving this mystery, it has taken a lot of time and effort posting the home-made customised diagrams etc, I don’t know of anyone else who would go to the lengths and efforts to find answers to these sort of mysteries. Behind the scenes, I have a feeling that some caravan manufactures read your posts within your site and pick your brains, well, looking at the above scenario, they certainly appear not to know it all.
Hi Snowy
I have had a few ‘issues’ emailed to me from manufacturers/suppliers asking for my input. I have in the past emailed them back, and received no credit. Now I will only answer technical questions if they are posted in the blog.
It’s the same when manufacturers have contacted me about reviewing a bit of kit… I tend to email them back with all sorts of technical/difficult questions… and suddenly they disappear. That’s why there are very few reviews. I’ll ask awkward questions and tell the truth about a product.
Hi Simon
Can I add my appreciation for the clarity of your explanations on this website?
This particular thread is most useful as we have made the odd overnight stop while touring abroad where disconnection has not been necessary. Seemingly our 12V useage is not as high as Vic’s!
Incidentally we tow a 2014 Bailey Unicorn.
This got me thinking. In the old days, some 35 years ago, when on site in Spain with no EHU I used to connect the caravan to the car battery for limited 12V use overnight and rely on the car battery recharging on journeys out and about during the day. It seems that I can still extend my leisure battery use when off electrics by connecting the car but this would be quite inconvenient – backing up and parking within range of the A-bracket.
Could I make up an extension cable with 13-pin connections using just two of the pins – presumably 9 and 13? There would presumably be some limitation on the length of the cable – I was thinking about 10 feet would allow parallel parking nose to tail.
Hi Keith
I have made myself a lead from 4 core 4mm2 cable (the cable was originally designed for 3 phase mains equipment) with a 13 pin plug on one end and 13 pin socket on the other. It’s about 3 metres long and use this to connect the vehicle to the caravan. As I have the Sterling Power Wildside unit installed (DC to DC Charging) with the vehicle on tick-over I can put around 24 amps back into the caravan battery (measured at the battery terminals). Don’t forget you MUST use BOTH earth leads for the lead – only one earth connection in the lead will be overloaded!
Hi Simon
Don’t think we’ll be off electrics often enough to warrant anything too complicated. I just felt that if I could use the car battery whenever we’re at the van then the leisure battery would last much longer. Not even worried about running the car to charge the battery. So I’d just want to connect the relevant pins with the extension cable so that the van recognised and used the car battery. Would my original cable suggestion work for this purpose – presumably just the one pin 13 earth would be adequate if not charging through the cable.
Hi Keith
Yes, your cable would work 🙂