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I would take a feed from my alternator so that it is only "live" when the engine is running, not sure if a battery in the boot is a good idea though. Someone with more knowledge on this subject will be along soon to advise. 

As Legs down says plus also make sure the battery terminals are insulated. Don't want anything metal touching them.

If you pick up the supply from the 12S socket, it will only charge when the engine is running. You must unplug the battery in the boot when you are towing, otherwise it will discharge very quickly running the fridge. As long as you only use it whilst not towing you should be OK.

I am also doing something along the same lines but I will be using 2 leisure batteries, one in the van and one in the car boot.
I will then be connecting a Battery to Battery charger (the Furnvo UniCharger) and charging the van battery from the boot battery.

My van (see gallery) is very small and changing batteries over is not advisable, plus I need the caravan battery in at all times for security reasons.

The Boot Battery is wired directly to the Car Battery but through a Split Charge Relay which has a 20A fuse on either side of it.

Your main problems are the security of the battery.  As has been said, this means a proper mounting or box, because the loads in an accident are huge.  You don't want your head smashed in and then covered in acid.

Secondly, you do need to make some provision for the escape of the gases produced while charging.

Jim

There may be a small well that the ordinary battery sits in, which you don't have for your second battery.  What you need to ask yourself is whether the battery would stay where it is with the vehicle upside down.  If the answer is yes, then think about covering the top with something to prevent acid running out.  BUT don't forget to leave a route for gases to escape.

Jim

There's no great issue.  A split charge relay is nice, but if you don't have one, just connect the batteries in parallel and they will "find their own level".  Imagine two taks of water, connected by a pipe.  Water will flow from the fuller tank to the emptier one until both are at the same level.  It is the same with electricity.

Of course, the batteries must be the same voltage, usually 12v.  Also, do be aware that if one of the batteries is not in good condition, old or nearly finished, it will drag the other battery down with it.

Best thing is to unplug the extra battery when you have finished your journey,so the two batteries are not connected all the time.  That way, you can be sure of starting your car again.

Scotchlocks aren't the greatest thing in the world. Use them if you have to, but have a look for some place you can make a proper connection first.

Jim

Thanks Jim, but two questions now.

1. Can you take the earth from the body of the car for the Boot Battery?

2. Can you have both batteries at a different Amp/Hour rating?

Agree with points 1 and 2.  Also, a fuse is a good idea, but it needs to be a quite high rating to account for the high amperage the alternator puts out just after startup.

I would prefer not to crank from a leisure battery, but have done so on boats with 3or 4 batteries for many years with no problems.  It would need an electrical expert to tell you why, but I don't believe a 50amp fuse would blow in these circumstances, even though the starting current is a lot more than that.

I think it's because the battery in the car is acting as a kind of buffer, and most of the current comes from that, and the two batteries equalise afterwards.

Simple solution is to put a fuse in and try it.  If the fuse blows, either get a blocking diode or just don't plug the extra battery in (or take the clips off or take the fuse out) until the engine has started.

Going back a few years, when I was doing offroad recovery for the RAC rally, I had a trailer crane (still have it) with two large batteries in it to run two electric winches.  No fuses, no diodes, worked fine.

I don't believe it's that big a problem.

Jim

I recently bought a Split Charge Relay for the purpose of putting a leisure battery in the car boot and that comes with two fuses, each to be put on either side of the Relay.

The Battery to Battery unicharger that I will use to recharge the van battery from the boot battery, was originally used to charge my TT Battery from the Car Battery.

So I have used this set up before, but it is the idea of having the extra battery in the car boot that is different this time.

It is my intension to fit a third socket to the towbar to run the power for this set-up between the car and the van.