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PostPosted: Sun Oct 22, 2017 12:33 pm 
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Fully understand why you need a second caravan ("leisure") battery. You need to keep this charged up. I imagine this is for pitches without EHU.

We had such arrangements in the 1970s - early'80s. The car had no electronic systems. An electromechanical Lucas SPLIT CHARGING RELAY turned on the supply to a charging socket installed in the boot when the system voltage rose high enough to charge both batteries simulataneously (car battery and spare caravan battery). When the alternator was not charging, the charging socket was dead, so there was no risk of high starting current being drawn from the caravan battery plugged in in the boot and frying its connection cables.

Few pitches then had even an EHU, so one had to live off the battery. That became quite demanding with our 1981 caravan, which had "twin electrics" an onboard 240/12 Volt trickle charger (not a PSU, so you couldn't run anything much off just the charger) and plenty of lights, and a powerful 10 l/m Shurflo water pump, and placed pretty heavy demands on the 12 Volt system. So we had to alternate between two batteries, ensuring that we used the towcar enough solo when on site to keep the spare battery charged

I imagine that your needs are similar. For whatever reason you do not want to be obliged to use a pitch with an EHU (though there are surely few without them now?), and a modern caravan will place even higher demands on its 12 Volt system than our 1981 Elite "Britanny" did.

As regards charging a spare battery while en route, things are different nowadays. It's no longer legal to have a lead-acid battery on charge in the car unless (as in the caravan) it is in a compartment sealed from the passenger/living area and with its own vent.The risk is that an explosive air/hydrogen mixture will accumulate inside the car/caravan - especially if the battery is overcharged or faulty. The latter is usually achieved for the car battery by installing it in the engine compartment. However, to keep the battery cooler, and apparently as part of the weight distribution calculation, our current towcar (2003 BMW 530D estate) has the battery in the boot, alongside the offside wheel arch, in a sealed compartment with a removable cover and with one-way venting through the wheel arch inner skin. It's a fact that one does need a big and very heavy battery to start the 3-litre 6-cylinder diesel engine.

We no longer use pitches without EHU, but I have occasionally wondered what we would do if we wanted to, and so had to have two caravan batteries.

Leaving aside carrying a small generator to charge the second battery on site (there might be objections from the site owner), this would be my provisional proposal, if it is any use to you.


BACKGROUND

The BMW towbar electrics control box supplies the usual constant live to the car (pin 9 of the 13-pin socket/pin 4 of a 12S) and a switched supply for the caravan fridge (pin 10/6). In the old days there were separate switched supplies for (1) constant live (2) fridge and (3) en-route charging of the caravan's own battery. These three functions are now performed by the two supplies mentioned above a and an automatic relay in the caravan electrics. This relay is NO, in which mode the constant live (pin 9/4)supplies the accessory fuseways. When pin 10/6 goes live to supply the fridge, it energises this relay, so diverting the constant live from the caravan accessories to the positive pole of the caravan battery. In this mode, the fridge is working on 12 Volts (if so switched,) and the caravan battery is in parallel with the car battery. The car's charging system charges both batteries.

In the old days, 12S pins 2 (caravan battery charging) and 6 (fridge) were dead until the car's electrical system reached at least 12 Volts. Then they were switched on by two voltage-sensitive electromechanical NO Relays in the car'S towbar electrics control box.

The arrangement on our BMW is different, reflecting the changes to the 12S for 1999 which copied the the-new German 13-pin socket. There is an unswitched supply as a constant live (pin 9/4), which becomes the link between the positive poles of car and caravan batteries when there is a sufficiently high voltage across the combined systems. The operation of the relay in the caravan which performs this changeover of function for this supply has been described above.

Apart from containing only one relay controlling a voltage-sensitive-switched supply (pin 10/6), the other difference in our present towcar's towbar electrics control system compared with what we were used to between 1979 and when we gave up caravanning in 1985 (only to make a "come-back" to caravanning in 2007) is that the relay which controls pin 6/10 is electronic and not electromechanical. Therefore, it is an NC relay. This leads to behaviour which puzzled me until I investigated. When you turn on the car's main switch (AKA ignitions switch), pin 10/6 goes live and the fridge 12 Volt supply is live. The "towing relay" in the caravan operates to switch the constant live at pin 9/4 from supplying the caravan's accessories to caravan battery charging. Fortunately, the single NC relay in the car's towbar electrics control box is voltage-sensitive. When the combined system voltage drops to 12 Volts (very quickly if the fridge has been switched t o 12 Volts, as it draws over 9.5 Amperes at 12 Volts) the NC relay opens, pin 10/6 goes dead, the caravan towing relay reverts to NO, the supply to the fridge goes off and the accessories section of the caravan's 12 Volt fuseboard is on again.

PROPOSAL

Connect a spur from the output from the car's towbar electrics control box to pin 10/6 to one side of the coil of a simple NO 12 Volt relay, with a switching capacity of 30 Amps. Connect the other coil terminal to the car's structure (negative). When the control box allows this output to be live the relay contacts will close.

Connect the fixed contact terminal of the relay contacts to the positive pole of the car battery, using cable of suitably high capacity.

Install a suitable two-pin non-reversible socket in the car boot. Connect what is to be the positive terminal of this socket with the same type of high capacity cable to the moving contact terminal of the relay contact. Install a 20 Ampere inline fuse in this cable, as close as possible to the battery.

Connect the negative terminal of the socket to the positive pole of the car battery.

Thanks for the voltage-sensitive NC relay in the car's control box electrics you now have a voltage-sensitive switched supply for a spare 12 Volt battery carried in the boot. The battery will need high capacity leads to connect it to the plug for the socket. The battery should at least be carried in a suitable battery box. When plugged in, this battery will be connected in parallel with the car battery (and with the caravan battery when the outfit is en route) only when the combined electrical system, including both/all three batteries, achieves at least 12 Volts across the alternator terminals.

RESERVATIONS/OBJECTIONS

My comments and proposals are based on what I know about the towbar electrics control system on our BMW. Most modern cars probably use basically the same system, but the single relay might in some instances be NO rather than NC. In my opinion, this would be preferable electrically and functionally, and should not involve any changes to the installation that I propose.

Adding an electrical subsystem to the sophisticated electrical system of a modern car, with multiplex wiring and a a lot of electronic sensing equipment, may be unwise (or worse!), and would almost certainly contrave a new car's manufacturer's warranty. Our BMW is 14 years old, so the latter is not a point of concern, but it has loads of electronics and possibly some multiplex wiring. These do not appear to include the towbar electrics control box.

I would be transporting a lead-acid battery which, most of the time, was under charge within the car's passenger compartment. A sealed battery box with a vent tube sealed into a hole in the existing battery compartment might be possible, but it sounds a time-consuming and tiresome aspect of what looks otherwise quite a straightforward job. If I did not make arrangements along these lines, and if the car caught fire, and the insurer's accident investigator became aware of the presence of the spare battery and its charging system, and could see that this was operative when the fire started, I could well find myself without insurance cover as well as in court for driving without due car and attention (at least!). This is a doomsday scenario. It would almost certainly not happen (with or without dedicated venting for the spare battery on charge). Modern "sealed for life" batteries do not normally emit hydrogen (though they usually have a vent for this, should it occur) provided the charging voltage is regulated to 14.40 Volts maximum. The latter is down to the alternator's voltage control box (built in to the end plate), and this is most unlikely to fail in way which would allow an excessively high charging voltage.

Finally, this is an "armchair" proposal. We have not felt the need for a second battery so far (since 2007) and so no provision for one has been attempted.


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For this message the author Carl Haworth has received gratitude : ericmark
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PostPosted: Sun Oct 22, 2017 5:24 pm 
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My car (not the tow car) is adapted to carry a wheel chair or mobility scooter in both cases there are 24 volt batteries in them and there is no vent they are however VRLA so maybe there is a special allowance to carry them.

Thank you for your very long reply, we have decided to have a charged battery at home so if there is a problem we can swap quickly, so if the motor mover is used to get off the site leaving not enough power to get it up the drive, we will swap batteries at that point.


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PostPosted: Mon Oct 23, 2017 10:32 pm 
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As said before when I was sent the link to this DC to DC inverter is seemed to be what I would require. Image It would need some work mounting and setting up and is still a possibility. However my wife states on hook up no stay so unless we used the motor mover a lot to get off the site it is unlikely we would have a problem. And worst case we could remove car battery but I have an old battery which I think would have enough power to allow us to get the caravan on the drive. So for the time being I am shelving the idea.

I think first time I went in a caravan was 1956, seem to remember dad had locking petrol cap because of suez crisis the caravan was designed to be towed behind a motor cycle and side car and belonged to my Uncle very soon after we had our own a Sprite Aerial we has a whole range until around 1988 when I lived in the caravan for around 3 years after which I never wanted to see one again.

My wife wanted a caravan again and around 2010 we got a caravan again, which I gave away, then a second one inherited which was too heavy for our car, so son has it, then a very cheap small one which I thought ideal, however my wife has gone for another large one. Lighter than the one we can't tow but not by much.

Early days I made my out towing brackets, remember the mini I had supported a ball at the front to launch the boat with that would not be allowed now, and hired trailers which were far too heavy for the car, in theory trailer loaded can't exceed 30 cwt with a ball but I am sure they did, one was made to carry a car and still with a 50 mm ball no way was that under 30 cwt.

So I can see why the law was changed, today the car manufacturer says what we can tow no longer left to us. I have towed with pin and eye and have pulled 20 ton, but that part of my licence is no longer valid. In all those years this is first time with a motor mover, always reversed in using the car, but seeing where one is going is the problem. I have a camera ready to fit on caravan but on my to do list. With the motor mover so much safer. No worries about people walking behind the caravan unseen, and also no worries about it running away.

So in a couple of weeks I will go in it for second time and have a chance to try it out, only work today is to fit an double socket with USB outlets instead of a single.


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