Currently I have a very simple leisure battery system in our van. A leisure battery under the back seat, which takes power to a fuse box full of blade fuses, which then distributes power out to all the 12v stuff around the van that's powered by it (lights, 12v sockets, radio). Then the earth return to the battery through the van's body. This is so basic that I don't even have a split charge set up to charge it on the move - I just rely on charging it up before we go, and have really only ever been on short breaks so this is fine. I also have 240v hookup through a site socket in the engine bay, which runs to a consumer unit under the bed, which powers a number of sockets in the van. (And is earthed to the van's body too, so that I don't fry myself). However, I would like to set up something more sophisticated that I could rely on for longer trips - I'd like to be able to set up something whereby when I'm camped up and plugged into the hookup, the leisure battery is being charged, and the 12v circuit that is normally powered by the leisure battery runs off the hookup power. A while ago I constructed a mains powered 12v power supply that has enough wattage (and more) to cope with the demands of my 12v circuit, but I'm unsure as to what sort of circuit I'd need to construct so that when the hookup is plugged in, this takes over from the battery. Anyone help? Or is there something I can buy that will do all this, and will save me the hassle of all this?
I got a ctek cs800 battery charger (i think that was the model). When hooked up it charges the battery which in turn feeds the 12v lights etc. Always been fine.
As you have already have a 240 to 12 volt power supply big enough to do the job - Use a 12 volt changeover relay with the coil powered by the output of the power supply to change the power supply to be the positive feed to the leisure fusebox instead of the leisure battery. Problem is that this disconnects the 12v for a moment when it switches. Alternatively, if the power supply is regulated , wind up the power supply to 13.8 volts which is about the float charge voltage of the leisure battery and connect it to the battery directly (if it has diodes that stop it draining charge when the mains is off) , or via a suitably rated relay powered off the power supply output as above if the power supply will drain the battery when the mains is off. Of course it will need to have a current limit circuit in it to make sure it doesnt overload/overheat when the leisure battery is flat. A decent leisure battery charger rated higher than the expected current drain of your accessories would be safer and better as it will also shutoff if it or the nearby battery gets too hot - lead acid batteries over 40 degrees C are not happy.
You are 80% of the way there with what you've got. You just need to get something like a ctek unit, plug it into one of the 240v plugs and then run the wires that come out of the other end of the ctek back to the leisure battery and you're sorted. The ctek will then charge the leisure battery on hookup and so run all the 12v electrics at the same time. You could then consider an intellignet split relay to charge the leisure battery while you are driving and you're all covered.
