Caravan Fridge/Battery Charging from car.

First the problems with charging a caravan battery from the car.
1) Reverse current flow from caravan to car when cranking, this is stopped by the Sterling B2B charger.
2) Volt drop from car to caravan, this is also stopped in the main with the Sterling B2B charger.
3) Discharging the cars battery, not quite sure how the Sterling B2B charger works with this, but likely is will stop over discharge of the cars battery.
4) Ensuring the fridge only uses 12 volt power when there is 12 volt power to spare. Likely the old voltage triggered split charging relay can do this, but first you need to look at caravan wiring.

So caravan wiring so from BS7671:2008 we have
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done as a picture as wanted to keep in order, it also says
BS7671:2008 Annex A said:
A721.55.5 Charging of auxiliary battery and operation of refrigerator
A721.55.5.1 The circuit to charge an auxiliary battery should be separate from a circuit to operate a refrigerator.
A721.55.5.2 The charging circuit for an auxiliary battery should be completed only when the ignition of the towing vehicle is switched on.
A721.55.5.3 The 12 V heating facility of a refrigerator should be completed only when the ignition of the towing vehicle is switched on. This may be performed by a device built into the refrigerator.
In the main this is done by using relays in the caravan, so the core number 10 switches number 9 so giving you a second ignition controlled supply.

However you say no S plug so I will go into history, core 2 has over the years changed what it is used for, originally it was for caravan inertia light, then it was used for reversing light, and also a pressure switch on the vacuum or air tank, I always fused this wire in case the tow vehicle was used to tow a trailer with vacuum brakes, this is what it means by "coding for coupled trailer" (core 12) however it can today also be the anti-snake system.

So I found it took some time to work out have the caravan was wired, there was a fuse box with relays, and it seems these have been updated over the years so finding exactly what it all did was not that easy, seem to remember there was one more fuse to labels on mine, at some time in the past the printed circuit board must have been changed, and a simple blown fuse was my main problem but it could get power from both sides, so it needed me to do a lot of looking up and testing to find how it all worked.

Some where I do have notes, but not on this PC, and until we know what you have rather pointless anyway.

So history before the battery to battery inverter charging the caravan battery while towing was a bit useless, the blocking diode dropped the voltage too low, the split charging relay seemed about the best, but again volt drop was a problem, putting the second battery in the boot of the car did help, but still needed a lot of miles to recharge it.

The battery to battery charger supplies the domestic battery with between 14.4 and 14.8 volt depending on type of lead acid used, often there is a setting for flooded and absorbed glass mat batteries (both lead acid) but I use a smart charger at home, and it is surprising how long it takes to recharge a lead acid battery, up to 80% the charger can bang in the charge, but to charge a lead acid from 20% to 80% takes around 5 hours how ever big the charger is, it's down to how fast the battery can absorb the charge, and from 80% to 100% can take another 5 hours, even on a narrow boat which has the engine running around 8 hours a day it is hard to re-charge the battery, after being discharged over night.

So in the main we look for a mains supply at least twice a month, to top up the battery fully, it is simply how long it takes, no charger can speed that up. So we also look at using as little as possible, the caravan has a huge advantage over the narrow boat, drilling a hole under the fridge to allow flue gases to escape would sink the boat, but in a caravan the fridge can work on gas, when I started all the lights were also gas, and the water pump was a dome in the floor you pressed with your foot, but today with LED lighting it seems all electric.

However solar panels can also help today. There are special flexible ones for the caravan roof. But if your careful then 5 days no hook-up and one day with hook-up seems to work well. Generators are a bit anti-social unless using a Wisper-gen with Sterling engine, but the price for an external combustion engine is silly.

The car alternator comes in three flavours, it was traditionally a simple voltage controlled regulator, there are stage charging regulators mainly used in boats, and also controlled by engine management the latter is why you need a battery to battery charger, which in the main is a stage charger, it will charge slightly less than a simply split charging relay for first 2 minutes, but after that it will charge far more, so a float charge 20% to 100% looking at around 24 hours, a stage charger with a sealed battery is around 10 hours, with a flooded battery where you can top it up, can reduce it to around 8 hours, but it's hard on the battery.

The float charge voltage for a 12 volt lead acid is around 13.4 volt, but on a car we realise it will not be charged for long so normally 13.8 to 14.2 volt it is a compromise, old dynamos were set to 16 volt, with two bobbin regulator, although in practice it never got that high.

If you want advise try a narrow boat forum, they have been trying to live off 12 volt and 24 volt for years, I trained as an auto electrician but moved to mains back in around 1980 so not so well informed about modern methods, just what I have learn with own caravan and sons narrow boat, but three 180 Ah batteries in a caravan gets a tad heavy. I know the anti-snake devices have a problem with stop/start technology, as when the car stops charging the anti-snake stops working with the old voltage activated split charging relays.
 
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Thanks Eric, so there is some advantage of the sterling. I'm always a bit precious about my batteries and try to limit the discharge (I had 15 years out of one second hand battery and only recently destroyed it by leaving one of those tiny "car" type usb phone chargers connected over the winter). I think it's that last 20% I'm after, with 2 batteries and the solar panel topping up I think I could rotate the batteries to the boot every 3/4 days which would be about 5 hours driving.

I think I want to bypass the entire box of tricks in the caravan when charging so I'll just have a seperate set of quick release clamps on the charging cables, with a cable tee'd off to the fridge with a VSR.

Regarding the reversing light, I think I have 2 options to avoid needing an S, either pinch one of the tail light wires from the 12N - a bad idea as we have two tow cars and also tow a trailer, and obviously problems if we need recovering, or "remove" the reverse lights (they're in a shared cluster so it would be bulbs out, something stuck over the holders and a bit of red spray paint on the lenses - in reality I don't think as bodgy as it sounds).

The Sterling has loads of settings for different batteries but I think the instructions tell you to check the actual voltage you're getting and if necessary chose a different setting.

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Thanks Eric, so there is some advantage of the sterling. I'm always a bit precious about my batteries and try to limit the discharge (I had 15 years out of one second hand battery and only recently destroyed it by leaving one of those tiny "car" type usb phone chargers connected over the winter). I think it's that last 20% I'm after, with 2 batteries and the solar panel topping up I think I could rotate the batteries to the boot every 3/4 days which would be about 5 hours driving.

It is letting any battery go flat, which always finally kills them :)

My own caravan's battery must be around 12 years old, 120amp/hour now on it's second caravan and only needed on route and for the mover. My regime is to put it on the 13.8v PSU when I remember, every month or two for a couple of days, a couple of days after we return and a couple of days before we leave - though if I remember I might put it instead on the intelligent charger for those two days. PSU is permanently connected to 240v.

The PSU originally output 14.00v, concerned it was too high - I trimmed it down to 13.8v

The thing is, just how long would you like a battery to last and what is the cost and extra complexity of a B2B compared with a new battery? I priced a replacement 120amp job up at £70 a few weeks ago.

So far as the 12s socket is concerned, why not convert to the later version with more pins?
 
I didn't pay much for the charger, and it's something I've always fancied trying, although probably overkill. As we have 2 towcars, a caravan and a trailer it's a lot of messing changing to the big plugs and sockets, and for some strange reason they're part of the MOT now.
 
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I didn't pay much for the charger, and it's something I've always fancied trying, although probably overkill. As we have 2 towcars, a caravan and a trailer it's a lot of messing changing to the big plugs and sockets, and for some strange reason they're part of the MOT now.

Only the larger single socket, the last time I checked. The 12n/12s was exempt.
 
Is it just the 13 pin sockets that are subject to the test?

Yes, the last time I checked. Not too bothered to check, because my 12n/12s system constantly monitors the socket is working and instantly flags up any issues.
 
Some trailers use an electric anti-snake device, it puts the brakes on I think if it starts to snake, however clearly the system must ensure there is power to the trailer for this to work, and the old relay used for split charging switches when the voltage raises, if the car uses the engine management to switch charging on with over run and off when under load, this could mean the anti-snake is disabled when the car is not charging, so good reason to check wired correctly.

However this system was introduced after we went to 13 pin, so can see why 7 pin exempt.

I back in the 70's charged the third battery in the boot, I had two caravan batteries, in a Lucas type box, and a strap in the car boot to hold the box in place, last thing you want with a crash is a lead battery in the back of your head, it had a three pin plug and socket, three pin so not reversible, I took the three phases out of the alternator into three large diodes from a fork lift, and fed this to the battery in the boot, so impossible to get reverse flow, and also since from the three phase supply no 0.6 volt drop through the diodes, it worked, at that time we did not have inverter chargers, but also only uses the caravan battery for water pump and maybe a light although we had gas lights.

As the caravan moved to all electric lighting a 60 Ah battery went from lasting a month to if careful a week, today maybe 3 days, so we came to the conclusion we needed an electric hook up to use the items in the caravan, microwave, radio, we would not put TV in the caravan, whole idea was to get away, but did have a radio.

My father-in-law got a small generator, but we felt three days on no electric was enough, in real terms it means we had called into a site en-route, main site if no electric we simply would not use the site, in the main as if no electric likely some one else's generator is thumping away, so no electric and site is rejected.

OK I will admit today we have solar panels so we can recharge battery, but in the main a site with electric costs £5 more than one without, so would need to do a lot of caravanning to be worth getting solar panels or generator. Except for the Forest of Dean most sites which are any good that have electric hook up also have wifi. Forest of Dean was an odd one out, no broadband in the area.

The fridge is either gas or 230 volt, yes it can use 12 volt, but never tow for long enough to be required, if cold at start, still OK 8 hours latter, no way am I towing for over 8 hours.

So no split charging with cars today, I would with a motor caravan, but not a caravan.
 
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