12 volt power for caravan, charging from car, thoughts?

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OK I am right into keep it simple silly, so jump charging battery pack has a cig lighter socket, it was supplied with a cig lighter to cig lighter lead to charge it.

Caravan has an S 7 pin plug and if power is supplied to plug relay in caravan auto turns caravan lights onto 7 pin plug supply, so a lead cig plug to S Socket and I can put battery pack in front box of caravan with lead to 7 pin S plug so caravan battery not used, so available for motor mover, and which ever car is used, does not need to be tow car, when we go out it will recharge the pack.

Have I missed something, why don't most caravan users do this?
 
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It will work technically, but the batteries in those packs are small and don't like repeated deep discharge so unless your caravan demand is low (in which case your big leisure battery would probably handle it) the battery won't survive long. Also you'll have to drive a fair way to recharge it and if it's deeply discharged (I think the average engine start only depletes a battery by about 3%) you'll need a "smart" charger to recover it, not 12v from your cigar socket.
I have a 500w inverter in my car boot (fed directly from the car battery via heavy duty cables protected by a maxi fuse), I plug in my mains 3 stage smart battery charger and strap the flat (we rotate 2) leisure battery in the boot - it takes a good couple of hours driving to recharge it. My batteries have lasted up to 10 years using this system - up to 3 weeks at a time "off grid".

You can buy a 12v to 12v smart charger that runs off the alternator (made by Sterling) but very expensive and just achieves the same end result as my home made system.

My inverter will just about power my 900w angle grinder - handy when wheel clamping was legal, and my wife's hair straighteners when camping.
 
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Have you thought about a solar panel on the roof? I fitted one to my boat when the marina started metering electricity. Saved me a fortune and on a sunny day means the fridge, laptop etc can all be powered under sail with a bit of a buffer in the service batteries. (3 x 120Ah)
 
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Years ago I modified alternator to have two outputs and also had a second battery in a plastic bread bin strapped down in the boot, also also swapped batteries boot to caravan, however in those days electric hook-up was rare, and the power used in the caravan was very little, lights were gas, fridge was gas with pilot flame, water pump foot operated, no radio built in, and motor movers unheard of.

As long as last day of holiday is with electric hook-up there is no problem, when we arrive home the motor mover will take the caravan up the drive at side of house into back garden where it will be plugged in again. However should the battery not last, there is no room to change it, I could not even get to motor mover to disengage it, so a method of ensuring the battery in the caravan is fully charged on return is a good option.

OK could park on road for day with power lead connected, but not really best option. However not having electric hook-up is rare, so hardly worth massive expense for the odd time. The battery pack has a 300W inverter built in, but the other inverter originally used to run lap top in car is only 120W so tend to take battery pack to run the odd item if required.

When fixing caravan electrics I was using a 7Ah battery to emulate the car, when I realised that the built in relays auto swap to car supply when connected, I thought this could be also used with any other 12 volt supply, only cost is a 7 pin S type socket already have spare cig plugs with lead. So for around £3 I can make up lead to use battery pack.
 
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