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[05omegav6]

Have a genuine pair of towbar wiring kits, one for trailer lights and one for caravanist things...

If I were to fit a battery box in the boot for several obscure reasons, could the caravan wiring be used to charge it? 
I don't suppose it would be any different from charging a battery bank of a caravan if plugged in whilst engine running

Want to fit trailer electrics anyways...

Also, how much ventilation, if any would the battery need? Would a vent hole straight into the boot suffice?

Thinking 18mm board for the box

Would I be better off replacing current battery with summat like this...

http://www.halfords.com/webapp/wcs/stores/servlet/ProductMobileDisplay?zygonManPartNum=HSB019&zygonPrice=0&zygonId=12774105&zygonArticleNum=161078&zygonGenericCode=000000&catalogId=10151&storeId=10001&productId=734844&categoryId=165762&langId=-1 

Reason for asking is that current battery goes flat if the headlights are left on for ten minutes, but otherwise behaves as expected. Contents of the boot are unlikely to be used without engine running, but if the battery is weak then I might be taking a chance, even if left idling via a runlock

Car is currently fitted with 140amp Plod alternator and a Bosch Silver battery which is 3.5 years old, HSB 075 iirc,although I do have a brand new HSB 096 Yuasa battery provided by halfrods in exchange for another 3.5 year old Bosch equivalent Always keep the receipts kids

[zirk]

12v Auto Batteries as a rule dont like being connected in Parallel for long periods if they are they really need to both the same type and rating and in similar (good) condition, otherwise the poorer of the two will start to discharge the good one.

Having said that Ive done something similar with a Mig TD Estate where I have to work out of the back on it, using Flood Lights and UPS Inverter running Tools and Lappy, soon become apparent that the only way this was going to work was to fit a second Battery and also fit a HD Isolation switch betwwen the two batteries, that way I always had the Main Battery for normal car use and start the thing should the second battery run low.

I found using this method worked well, and before setting off with the Engine running, I would flick the Isolation switch on and then charge the second battery whilst on the move.

Wiring wise I used semi heavy duty Positive red and Neutral black cable directly to each battery (about the same cable thickness as a set of cheap jump leads) via the Isolation switch on the Positive side as well as Earthing the second Battery locally.

On the Battery vent side I just sorced a BMW Boot Battery, actually the same size and Rating as the 065 Mig ones but has a small vent tube running down through the floor pan via a rubber grommet. 

[Kevin Wood]

I would be concerned about the resistance between front and rear if using the car's existing wiring. It doesn't take much voltage drop before your aux. battery isn't charging at all!

For that reason, I'd run your own wire back from the alternator B+ terminal and rate it as generously as you can. Fuse it at the front end with an appropriate value of fuse, of course.

Use a caravan charge relay or an ordinary relay triggered by the fuel pump feed, oil pressure switch, etc. to isolate the battery except when it's charging.

You might want to look into using an AGM battery to avoid spillage. Failing that, many wet lead acid batteries have a vent connection to which you can connect a hose, which can then lead outside the vehicle. If it's going to be boxed in at all, it needs to vent outside the vehicle, and using a hose avoids the corrosive fumes from rotting the boot.

Also, bear in mind that you might be better off with a battery designed for cyclic use if it's going to be  used a great deal with the engine off - i.e. a leisure battery rather than a starting battery.

[Bigron]

I certainly agree with KW's comments regarding venting, or maybe use a gel-electrolyte type of battery?
Regarding charging, there are automatic devices to allow both batteries to be charged, but not to discharge both at the same time - i.e. just use the boot battery for auxiliaries, as caravanners do - using deep-cycle leisure batteries, as suggested.
Caravan shops will supply a unit which is basically two fat diodes in a box, and charge caravan prices!
Relays are an alternative, and you might find this link useful?

www.ybw.com/forums/showthread.php?307638-Split-charge-diodes-versus-split-charge-relays

If I've misunderstood your intended use, please come back to me and I'll try to help you more......

Ron.

[05omegav6]

Thanks chaps

The engine will be running when the boot contents are in use, so I don't know if I would actually need a second battery or not

The towbar wiring kits I have are:

93160931, which is a fused relay controlled trailer charging supply/separate power supply plus reversing lights (12S),

93165101, which is a trailer lighting kit (12N).

The instructions for the first thing are pretty clear with a couple of very ambiguous connections, namely Pin 86 on the relay (shown as connected to +15 next to a picture of a fuse box), and a spade connector on a red fly led in the boot area. Pin 85 goes to the starter motor (+50 is all that is shown next to a picture of the starter motor... surely that should be the alternator ), Pin 30 to the battery and Pin 87 to the towbar socket. So would presume that the black wire from Pin 86 goes to the switched ignition circuit...

The thinking being that the power board would be connected directly to its own local battery, with the yellow feed from the relay connecting to the positive terminal. There by keeping the engine battery nicely isolated from the extra one, whilst charging the second one when the engine is running

The other slight issue is the fact that the trailer lighting wiring kit was supplied with the exact same instructions as the charging one

Used to have a diode pack on the boat, which had a starter battery along with three domestic ones, so aware of the concept, but the system was fitted by the boatyard, so I never paid much attention to it beyond topping up the batteries

[Kevin Wood]

Actually.. What am I thinking? You need to fuse it at both ends, of course!

The problem with involving diodes is that they cause another 0.7V of voltage drop each, and that really limits the charging capability. You go from 14V at the alternator to 13.3 which isn't really enough.

I'd be happy to look over the bits you have and figure out what you can make out of them, if you like?

I guess this might be a good time to mention Curry, too?

[05omegav6]

Ah yes, the C word... posting elsewhere 

Thanks for the offer, should have most of the gubbins fitted by then, and therefore a more accurate idea of power demands

[steve6367]

I have exactly this setup in mine - a 85AH Gel battery (to remove spill risk) secured in a battery box in the boot. I used the existing wiring to feed through a voltage sensitive relay so that the boot battery is connected when the engine is running but can't flatten the main battery.

Voltage drop is a little high with the in car wiring, but it does work - I get 13.8V with engine running, which is actually ideal for my Gel battery, but would be too low to fully charge a conventional open battery.

Steve

[05omegav6]

Is your second battery wired independently of the main one

What current demands are you putting through it

[steve6367]

It feeds 3 standard cigarette lighter sockets in the boot, running a cool box, charging phones and occasionally an inverter. Normal load is around 4 - 5 amps Max (with the inverter) has been around 20 amps.

It then charger using the tow bar / caravan supply in the boot, but with  a voltage sensitive relay installed so it can only be charged and the load in the boot can't flatten the main car battery.

Steve

[05omegav6]

So you effectively have the tow bar power socket, and have tapped into the charging supply for the second battery

[steve6367]

Thats it - in the tow bar socket there is a permanent live which I have used. That is why I need the VSR or it will just flatten the main car battery when the engine is off.

I can get a photo tonight and post here if that helps :-)

[05omegav6]

Why not use the caravan charge supply or is the power from a trailer lighting connection

Or have I missed summat...

[steve6367]

The supply I have connect to is for charging a caravan battery / running a fridge - however it relies on the caravan having a suitable  VSR or other device to stop the main battery being drained.

There is no switched / ignition feed in the standard VX wiring (certainly not on my car anyway)

[Entwood]

The supply I have connect to is for charging a caravan battery / running a fridge - however it relies on the caravan having a suitable  VSR or other device to stop the main battery being drained.

There is no switched / ignition feed in the standard VX wiring (certainly not on my car anyway)

Agreed, there is a permanent 12V, on a fairly thin wire, but nothing ignition switched.

For the 'van (in my case) I have put ignition SWITCHED relays in, near the existing battery, and run substantial wires through to the boot to connect to the 13 pin socket for the van, in your case to the "extra" battery. The advantage of this is all the fuses are very near the "battery" ends of the cable runs.

Be aware there are two types of control relays ... one is ignition switched the other is voltage sensitive (VSR). The switched units are more reliable and give less of a voltage drop, but do require the extra wiring of the ignition source feed. The VSR are easier to fit but can be prone to problems, the "switching" voltage seems to be unstable on many IME.