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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
done as a picture as wanted to keep in order, it also says
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.
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
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.BS7671:2008 Annex A said:
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.
