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[Kevin Wood]

There are a couple of downsides to using the existing permanent feed to the towing connector, IMHO:

1) I believe it was actually provided to power the trailer lights through a relay module in the OEM towing kit rather than for charging batteries, running fridges, etc.

2) The fuse is shared with other devices (e.g. seat & mirror adjustment on the same fuse and many other things on the same fusible link) on some cars, meaning if you blew the fuse it would cause a nuisance by disabling items in the car, and the extra loads might cause a bit more voltage drop, exacerbating the situation when charging batteries.

3) Whilst it's fused at 30A, it's 2.5mm² wire. I wouldn't trust it at over 20A continuous current. Although, if a serious fault occurred, I'm sure the short term overload would blow a 30A fuse I'm not sure I'd say the same about a long term mild overloading of that wire. Load it to 29.5A for long periods of time, and my bet is that the wire would melt. In any case, the voltage drop would be higher than you'd want.

If you're going to use a voltage sensing relay, you need the bare minimum of voltage drop between it and the vehicle supply to prevent it dropping out erroneously. Best off to mount it up the front end of the car (but then you've got to run a dedicated wire down to the back end).

[Andy B]

.....
2) The fuse is shared with other devices (e.g. seat & mirror adjustment on the same fuse and many other things on the same fusible link) on some cars,  ....

Not on my MFL car is doesn't. I fitted the fuse to power my caravan fridge/charging and everything that should work was working before hand.

My trailer lights are now actually fed via a module that's powered directly from the battery (using a fuse  ) using an OEM kit but that's a completely different set up from the trailer lighting plug behind the rear boot trim

[Kevin Wood]

.....
2) The fuse is shared with other devices (e.g. seat & mirror adjustment on the same fuse and many other things on the same fusible link) on some cars,  ....

Not on my MFL car is doesn't. I fitted the fuse to power my caravan fridge/charging and everything that should work was working before hand.

My trailer lights are now actually fed via a module that's powered directly from the battery (using a fuse  ) using an OEM kit but that's a completely different set up from the trailer lighting plug behind the rear boot trim

That's why I said some cars (including the OP's).

[05omegav6]

For the sake of clarity, this is the GM part in question...

http://oemcats.com/oem-parts/93160931.html

http://www.ebay.co.uk/itm/VAUXHALL-TRAILER-COUPLING-ELECTRICS-KIT-12S-7-PIN-GENUINE-93160931-/221378263883

Which might well be what Andy has fitted

[Andy B]

For the sake of clarity, this is the GM part in question...

http://oemcats.com/oem-parts/93160931.html

http://www.ebay.co.uk/itm/VAUXHALL-TRAILER-COUPLING-ELECTRICS-KIT-12S-7-PIN-GENUINE-93160931-/221378263883

Which might well be what Andy has fitted

No, mine's a 13 pin 

I've still used the original 12 volt feed from the square socket on my car (I know the facelifts' are round  )

[steve6367]

Kevin,

I think its an expectaion thing - it certainly does work very well for me, but I only have limited charging requirements - fairly small single battery in the boot that is never really flat. Totally agree with you that if you want to achieve fast charging of several hundred amp hours of battery in a caravan you are going to need some much thicker wire.

Not sure which the OP is after.

Steve

[05omegav6]

Kevin,

I think its an expectaion thing - it certainly does work very well for me, but I only have limited charging requirements - fairly small single battery in the boot that is never really flat. Totally agree with you that if you want to achieve fast charging of several hundred amp hours of battery in a caravan you are going to need some much thicker wire.

Not sure which the OP is after.

Steve

An efficient way to get approximately 140-180 amps to the boot

Option 1. run a suitably sized cable from main battery to a distribution board in the boot.

Option 2. fit a battery in the boot with a short run to the distribution board, charging via the towbar wiring kit listed earlier.

Both options would employ a runlock for secure battery charging whilst on display...

[Kevin Wood]

An efficient way to get approximately 140-180 amps to the boot

'kinell! are you putting another engine back there?

[steve6367]

I think you need a wet battery or 2 in the boot to do that, for long life you are looking to not flatten them below 50%. The cable to the front of the car would have to be 50mm or bigger to supply that and you would be flattening the car battery very quickly as the alternator is only going to put out 100 amps.

You will then need a bigger cable to the front the re-charge your setup as the current you would get down the installed cable would take too long to charge that sort of setup.

Just being nosey, what do you have in the boot that requires that kind of current?

[steve6367]

I think you put that more succinctly Kevin!

[Bigron]

Are you sure that you need such an enormous current feed to the boot battery? Granted you might need a heavy load from battery No. 2, but not to charge it, surely?
Ok, assuming that you really do, why not feed your alternator to each battery via a 200 Amp Schottky diode. "Schottky, because they have a low forward voltage drop of around 0.2 volt, "diode" because you want current to feed to each battery only and effectively have them isolated from each other - i.e. current cannot flow from either battery to the other. I wish I knew how to draw diagrams on here.....
The minimal voltage drop due to the diodes should not affect even today's clever regulators on the alternator and it is fully automatic - no need to remember to switch anything!

Ron.

P.S. Don't forget to use a grown-up cable to the boot battery if you really do need that heavy a current feed.

[05omegav6]

I assume that is the current requirement based on the fuse ratings...

Front Blues 10A
Rear Blues 10A
Rear Reds 7A
Headlight flasher 15A + 3A
Taillight flasher 15A + 3A (though could drop to 10A as rear fogs draw less than main beam)
Siren Amp 10A
Siren control 10A
Take down/Alley lights 3A
Front Cruise lights 3A (cannot be used with front blues active)
Rear Cruise lights 3A (cannot be used with rear blues active)

That's 80A, plus video kit (?A), and could reroute the current ICE (50A) gubbins through it, as that is currently routed through the front seat feeds (suitably fused at connection) although if it ain't broke and all that...

[05omegav6]

If I drop the rear flasher down to 10A, leave the ICE as is and allow 20A for the video kit then that gives a total demand of 95A in the boot...

The lightbar power feed comes off the front battery, the boot is mostly switching current...

[steve6367]

Fuses are to protect the cables and prevent fire so the equipment should be actually using less than the fuse rating.

A bit of measuring required really.

[Kevin Wood]

Yep, firstly, the fuse ratings will be pretty conservative, and secondly, you won't be loading every circuit to its' full rating at the same time.