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Back in the late 60's I set up the car to charge the caravan battery, I used many methods, blocking diodes, relays, and even second three diodes connected to alternators three phases, latter had some success, but the volt drop on cables, and diodes resulted in only really working when battery in car boot to charge, and swapping with one in caravan.
However at that time very little 12 volt in caravan, gas lights, gas fridge, water pump was rubber done in floor, so battery was for TV so if discharged it did not really matter.
Today however no gas lights, water pump needs 12 volt, and motor mover needed to get caravan in/out of drive, so caravan battery now important. The old split charging was never very good, OK on starting engine 15 amp going into second battery, but after 15 minutes lucky to get 5 amp, and after 1/2 an hour down to one amp.
So avoiding having second battery in boot, and getting a reasonable charge rate, only option is DC to DC inverter, this way likely to get 10 amp charge for an hour or more using stage charging, so 2 to 3 hours towing and 20 to 30 AH into battery, however that is not much for a 110 AH battery, even when charging as home with as much power as battery can take looking at 8 hours to fully charge, and I will never tow for that long.
Split charging needs a method to stop second battery supplying car when cranking engine, but blocking diode drops voltage so reduced charging, and relay will be in/out like yoyo as car switches alternator off when engine under load, and if there is an electric anti snake device don't want 12 volt switching off while ignition is on, even if not charging. So in spite of volt drop, safety of supply means diode, unless no second battery charging or using an inverter.
Now most inverters have a cut off voltage so it will not over discharge the doner battery, so if using inverter better not to have diode in car, but if no inverter then diode needed to stop cranking current being drawn from caravan battery.
Clearly with one car and one caravan you can select inverter and no blocking diode, or no inverter and have blocking diode. But diode fitted on car, and inverter fitted on caravan, so how do make sure car and caravan matches.
However at that time very little 12 volt in caravan, gas lights, gas fridge, water pump was rubber done in floor, so battery was for TV so if discharged it did not really matter.
Today however no gas lights, water pump needs 12 volt, and motor mover needed to get caravan in/out of drive, so caravan battery now important. The old split charging was never very good, OK on starting engine 15 amp going into second battery, but after 15 minutes lucky to get 5 amp, and after 1/2 an hour down to one amp.
So avoiding having second battery in boot, and getting a reasonable charge rate, only option is DC to DC inverter, this way likely to get 10 amp charge for an hour or more using stage charging, so 2 to 3 hours towing and 20 to 30 AH into battery, however that is not much for a 110 AH battery, even when charging as home with as much power as battery can take looking at 8 hours to fully charge, and I will never tow for that long.
Split charging needs a method to stop second battery supplying car when cranking engine, but blocking diode drops voltage so reduced charging, and relay will be in/out like yoyo as car switches alternator off when engine under load, and if there is an electric anti snake device don't want 12 volt switching off while ignition is on, even if not charging. So in spite of volt drop, safety of supply means diode, unless no second battery charging or using an inverter.
Now most inverters have a cut off voltage so it will not over discharge the doner battery, so if using inverter better not to have diode in car, but if no inverter then diode needed to stop cranking current being drawn from caravan battery.
Clearly with one car and one caravan you can select inverter and no blocking diode, or no inverter and have blocking diode. But diode fitted on car, and inverter fitted on caravan, so how do make sure car and caravan matches.