Mk5 conversion - second or auxilliary battery?

[triffic]

My AS camper (based on a 1999 mk5) has two batteries in the engine compartment - as far as I can establish factory-fitted by Ford.

What I'm trying to work out is whether battery number two (on the passenger side) is a "second battery" (i.e. meant to be permanently connected to the main vehicle battery) or an "auxilliary battery" (only supposed to be connected to the main vehicle battery by a "decoupling relay" when the alternator is running).

Please can anyone advise where the "decoupling relay" would be located if mine is the latter?

I should add that the two batteries currently appear to be connected together at all times - what I'm trying to do is figure out whether they should be!

[ake]

How thick are the wires on the second battery? As thick as starter cables? Or thinner?

[triffic]

Thinner, by quite a lot.

I'd guess at 7mm diameter, but that's with 2 layers of insulation (it seems to be sleeved in black heatshrink over the original red insulation).

Hmm... yes I think I can see where you're coming from. If it's connected all the time it should have the same size cable as the main vehicle battery to deliver power into the starter motor? Which ~suggests~ that I do have a problem. Fortunately it's a diesel that fires up pretty much on the first turnover, so I've never had to crank for a long time. But that thinner wire could I guess get pretty hot...

Trouble is I have no idea where to look for a relay to check whether it's welded itself closed!

[ake]

If the cables are thinner it's not Ford fitted, and will have been done by the converter company, follow the cables and see if you can find the relay

[triffic]

The thing is that the cable enters the Ford loom (or at least appears to do so) which I doubt the converter would have bothered to do - they typically don't tape round, but just ty-rap their cables onto the side of the Ford loom. The fact that it vanishes into the loom also makes tracing the cable very difficult.

Thanks for the input though - I'll have another look tomorrow. If I strip part of the heatshrink off at the battery I should be able to accurately measure the diameter, and then see where else that size of cable is terminated.

[robbie.cee]

Probably split charge to run beacons etc
Was it ex BT or cable tv?

[ake]

Can you post a pic of the batteries and looms?

I presume it was converted to a camper when new?

[triffic]

I presume it was converted to a camper when new?

Yes, converted from new and not ex-BT - it would have gone straight from Ford to the converter. Both original batteries were identical and Motorcraft branded - hence my thinking that neither they nor the wiring were added by the converter.

I am starting to think though that the thinner positive lead means that it's an auxiliary battery, which should not be supplying the starter, but actually is...

[ake]

Check under the bonnet for relays, also check behind the glovebox / below the pull out cubby hole
I doubt it would be in the main fuse box, but worth a look as well.

I think you are right it's an Auxialary battery, not a start battery

[triffic]

The problem there is that it's swarming with relays under the bonnet, because it's got a diesel cab heater, plus fridge relay, plus interior services cutout relay (none of the interior 12V electrics other than the fridge must function while the engine is running), additional rad fans, etc. None of these are documented anywhere.

But it sounds like I need to focus on original Ford-fitted relays, which should narrow it down a lot. I'll give that a go tomorrow. Thanks!

[triffic]

Well... It seems I was wrong. Been out and traced the wiring and it appears that

1. The positive wire is thicker than I thought (I think it only has a single layer of insulation)
2. The positive wires from main and secondary batteries seem to join at the starter motor solenoid

Here's a (slightly fuzzy - the camera always wanted to focus on something else) photo:



The place where they join is arrowed (or would be if I could work out how to put an arrowhead on a line in Photobucket!). The cable coming towards the camera goes to the main battery, and the cable going away out of the same connector tube goes to the positive of the second battery.

Does anyone recognise that type of starter solenoid? It ~looks~ as though the two battery positives are at two ends of the same cable, and that this connector is crimped or soldered in between. In which case I was wrong - it's a second battery, always connected to the main one.

Anyone care to agree or disagree???

[ake]

Yeah that's a factory twin starter battery set up

[crazylegsvan]

i have a two battery ex bt van that seems to have the normal battery on the passenger side and another on the drivers side. I thought the drivers side one was a leisure battery as its now a camper. i jump started it with the left battery? it this right. (sorry to hijack but it seemed you had your answer.)

[ake]

The driver side would normally be the start battery, but it appears most MK5 just have the two batteries connected in parallel so you can use either

[crazylegsvan]

any reason for this? it seems odd that they would double up.