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Rather than fit a third socket, you might explore the second socket, the grey one.  It's my understanding, and I'm happy to be corrected, that pin number 5 is not normally used for anything. Perhaps you could run a wire to this?  Just check first, but I think I'm right.

Jim

The grey socket on my van is part of the security circuit and I would be loathed to touch it once we were "on site".

Also I doubt, but I may be wrong, if the wiring would be heavy enough to use for charging from one battery to another. Certainly the wiring that is supplied with the unicharger is heavier than what is used in 7 core cable.

Wire it to the 12s cables in the boot. Positive to pin 6, negative to pin 7. This will be after the split charge relay you already have so there will be no cranking etc involved.

Fit a fuse in the positive lead. Do not charge the boot battery whilst towing or it will try to run the fridge when you stop the engine. It will soon go flat. Make sure it is well secured. Make sure the terminals are well insulated as any metal touching them will cause a big spark.

The space should be ventilated as hydrogen is given off when a battery charges. Hydrogen is explosive. Do not let the battery discharge too much before you recharge it. It will accept the charge quicker.

>>> The space should be ventilated as hydrogen is given off when a battery charges. Hydrogen is explosive.  <<<

What is the difference between a battery being charged in a car boot and a battery being charged in a caravan?

I have yet to come across a "ventilated" battery box in a caravan.

Quote: Originally posted by LobeyDosser on 17/2/2010

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>>> The space should be ventilated as hydrogen is given off when a battery charges. Hydrogen is explosive.  <<<

What is the difference between a battery being charged in a car boot and a battery being charged in a caravan?

I have yet to come across a "ventilated" battery box in a caravan.

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I would agree with all of that.  The small indent where the cable sits represents plent of ventilation.

Just going back to the fuses issue, and not trying to be contentious, but I've just remembered something that's been niggling at me.

In the dim and distant past, I used to race F1 Stock Cars.  These are purpose built and have no alternator - or at least they didn't in those days.  Obviously you would charge the batteries before each meeting, but if you were away for the weekend, everybody used to just run a twin core wire (nothing heavy) from the truck battery to the car on the back.

This never caused any problems, and everybody did it.  There was no fuse, but the wire never got warm, even when staring the truck. Maybe if you were cranking the starter for two or three minutes, there would be a problem, but I think for a normal start, where the starter is only running for a few seconds, it seems to work all right.

Jim

Quote: Originally posted by navver on 17/2/2010

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The stock car battery would be a car battery not a leisure battery. Leisure batteries cannot stand putting out the very high currents involved when starting an engine. They are made to provide low currents for a long time, something a car battery is not so good at.

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No, they were leisure batteries on the stock cars.  We always found them better because the equivalent size car ones were so expensive. We had 2 x 110 amp hour batteries fitted. Certainly in the marine industry, it's quite common just to use leisure batteries.

I know it's not perfect, but it does work.  The distinction between car and leisure batteries is not as great as you might think.  Also, of course, the trailer crane I mentioned before had 2 leisure batteries, because they were better at producing winching current for long periods.  We tried car batteries, which provided the power without problems, but they soon went flat.

Jim

Bailey estimate the charging current of the van battery when towing will be 0.5A.

The purpose of the fuse for the boot battery is to protect against a short circuit in the car or van, by stopping current from the battery feeding into a fault. 

Other websites on this subject suggest 27.5A wire and 25A fuses if you are charging a Boot Battery directly from the Car Battery and 17.5A wire and 15A fuses if you are charging from the cabling to the 12S socket.

I think I would be more inclined to go with this advice than any "0.5A" advice from Bailey's.

I would have presumed that the Amperage of the wiring would be subject to what power is available and the state of the charge in both the Car Battery and the Boot Battery.

I'm not saying that 0.5A is the current rating of the cable, however taking into account the current drain of the fridge and the battery at the same time, Bailey estimate that only around 0.5A will flow into the battery in the caravan. Cable size and fuse rating have to be selected for maximum currents that could possibly flow, under normal conditions and fault conditions, rather than the current that is likely to flow most of the time.

The boot battery will charge at a higher rate when not towing with van battery & fridge taking current.

Hi Navver, I would imagine that anyone, especially those who are thinking about needing a Boot Battery for topping up, would start their journey with a fully charged Van Battery and therefor any charging that is required to the van battery en route, would be only what the fridge is taking out, en route.

Ofcourse I am slightly different in that my fridge is a Waeco compression fridge and is a 12v only so I need extra 12v on site. Hence the combination of a solar panel and a boot battery.