Charging a leisure battery through the car

[Surfer]

We have a Jeep Grand Cherokee that we use for towing. Obviously it has the towing electrics fitted. It also has a 12v socket in the rear. How would we go about charging a leisure battery placed in the rear while driving around. The 12v socket would be inadequate as we would probably burn out the wiring.
Is it possible to tap into the 12v line feeding the caravan and have that charge the 12v 85 amp battery assuming the wiring is of the correct standard. Again I would probably need an auto electrician to do it as my hands are now past their "sell by date".

 

[RogerL]

The 12v socket isn't inadequate, it'll deliver 8 or 10 amps which is as much as you can expect - it will need a 7.5 or 10 amp fuse though! The biggest problem on many cars is that the boot socket is live even with engine off so you'd equalise the car and boot batteries when engine is off.
If you want to be more complex, it needs to be wired by someone with good auto electrics knowledge, more to design the circuit than install it.
When towing you'll have three batteries being charged, that in itself needs to be controlled - you'll also need the batteries isolated when the engine isn't running.

What I would do is use a 100/150w Inverter plugged into the 12v socket in the boot and a smart charger like the Aldi/Lidl/CTEK - it'll only charge at 4A but it will fully charge.

 

[Surfer]

The 12v socket isn't inadequate, it'll deliver 8 or 10 amps which is as much as you can expect - it will need a 7.5 or 10 amp fuse though! The biggest problem on many cars is that the boot socket is live even with engine off so you'd equalise the car and boot batteries when engine is off.
If you want to be more complex, it needs to be wired by someone with good auto electrics knowledge, more to design the circuit than install it.
When towing you'll have three batteries being charged, that in itself needs to be controlled - you'll also need the batteries isolated when the engine isn't running.

What I would do is use a 100/150w Inverter plugged into the 12v socket in the boot and a smart charger like the Aldi/Lidl/CTEK - it'll only charge at 4A but it will fully charge.

Thanks Roger. We do have a 150w invertor lying in the caravan that we have never used although we have had it for several years. We also have a CTEK charger. Never thought you could do that!

 

[ProfJohnL]

A cheaper solution is to purchase a caravan type plug and connect to the caravan/fridge pins, bring the wire through the soft seal in the cars boot/hatch lid and connect to the battery in the boot. The both wires (yes both) need to be fused to protect the circuits.

The battery must be secured so it cant fly about in the event of an accident, and also it should be in an externally vented box to prevent charging fumes from reaching the cars occupants.

 

[JTQ]

What I would do is use a 100/150w Inverter plugged into the 12v socket in the boot and a smart charger like the Aldi/Lidl/CTEK - it'll only charge at 4A but it will fully charge.

This is what I have done in the past; it works well and requires no "wiring up", just plugging in and clipping on.
If you watch it does not unduly deplete the vehicles battery and are lucky enough to have always live 12V outlets you dont even need to drive around.
You dont even need to man handle the battery. Just plug in the inverter and bridge car to caravan/battery with a mains lead. Placing the charger by the battery.
If you dont have always live 12 v outlets like I now suffer from, then make up a "grey plug" [or 13 pin unit as applies] with a 12v socket, and utilise the vehicles always live feed/earth.
Or if you are very lucky get somebody to sell you a old Welsh "Unicharger" which did exactly this car to van power shifting function and had a low level cut out to ensure the car would still start.