Towing electrics ex RAC

[Euge]

If you mean the engine run signal that drives the split charge relay it's just a thin wire from BCM to the relay socket terminal that I had the blue wire in the photo earlier.

Thanks I pulled the relay out and sure enough there’s another wire that’s ignition positive. Testing the engine battery supply cable it’s positive. Oddly I’m getting nothing after the relay and 80A fuse, even with engine running.

Seems I can relatively easily disconnect the relay and use the cables into a new Viktron dc dc.

 

[n10n]

I think it's a very different situation infrequently carrying a near to the limit load (a few hours of towing) versus carrying that load every day - particularly around heat induced degradation of the insulation over time.

Yes. However a) VW cannot control how their customers use the connections - basically there are *so* many different uses for these vans that all of the spectrum will be covered and b) if they thought there was any kind of risk with this they would have fitted a smaller sacrificial fuse nearer to the trailer socket

In the end I think I'd be more worried about 30A over the cheapest dodgy eBay CCA 10mm2 versus 30A over decent marine grade 2.5mm2

Indeed, especially CCA is very evil. And I'm pretty sure OEM cabling is really good quality, it certainly is difficult for us normies to get our hands on equally good quality - hence I would never just copy VW wiring gauges directly for equal loads.

Anyway enough off topic I think...

Yes, but interesting!

 

[n10n]

Oddly I’m getting nothing after the relay and 80A fuse, even with engine running.

Well, if you have the engine run signal and battery feed in the relay socket there's really not many options left, you either
a) have a borken relay - unlikely
b) have ground wire stripped away from relay socket - more likely

So, check ground on terminal on opposite side of engine run signal

 

[roadtripper]

In the OE arrangement it's not strictly an engine run signal I thought, more a secondary battery charge signal. I thought it did load shedding as well and if you are testing shortly after starting it may well leave the secondary battery disconnected for a while to prioritise charging the starter?

 

[Roodedoo]

With 12vplanet calculator it's actually less, used 30A here as you mentioned fusing dc-dc with 30A so that's the maximum. Note that you really don't need to account for ground return so I used only 2.5m length since calculator doubles the distance automatically. The van body is a gigantic busbar, basically corresponding to a wire of hundreds of mm^2.

View attachment 226833

I did this same calculation with 5 meters.
I didn't read enough to discover it doubles it automatically

 

[n10n]

In the OE arrangement it's not strictly an engine run signal I thought, more a secondary battery charge signal. I thought it did load shedding as well and if you are testing shortly after starting it may well leave the secondary battery disconnected for a while to prioritise charging the starter?

Could be, I don't think I've ever seen any definitive specs on this. Being VW internal implementation details it's not likely even really known - empirical testing would be pretty much the only way to know for sure. What I do know is that I've never observed the relay not being on with engine running and I'm pretty sure it's status is not on CAN. With some luck might be somewhere in 09 central electrics blockmap. Testing would be pretty difficult as well as it's not known which conditions, if any, would result in that signal to be off while engine is running.

Interesting topic in itself but maybe not worth a deep dive, there's not much to win except little bit of guru karma.

 

[Euge]

Well, if you have the engine run signal and battery feed in the relay socket there's really not many options left, you either
a) have a borken relay - unlikely
b) have ground wire stripped away from relay socket - more likely

So, check ground on terminal on opposite side of engine run signal

View attachment 226845

I didn’t worry about checking that as I’m going to remove the relay and mount. Did you keep the cable in the OEM loom from starter battery? It doesn’t look like 16mm2 more like 10mm2.

 

[n10n]

I didn’t worry about checking that as I’m going to remove the relay and mount. Did you keep the cable in the OEM loom from starter battery? It doesn’t look like 16mm2 more like 10mm2.

Yes it’s 10mm^2 and yes I kept it. I only have 20A charger fused at 30A, 10mm^2 is plenty.

 

[Roodedoo]

I decided to use the RAC jump cable for my split charge. I don't know the gauge but it's much more heavy duty and starts out with a mega fuse holder by the starter battery. Plenty of length to reach behind the driver's seat, which is where my leisure setup with be. It'll be easier to tidy away the RAC stuff too.
Obvs it'll go straight into a victron or similar non isolated charger and a kill switch.

Pictures show how I pushed enough back so that I could grab it where it comes through the bulkhead.

 

[roadtripper]

Ideally fuse it by the charger slightly lower than the fuse at the starter battery end. The OE setup uses 100A at the starter battery end and 80A at leisure battery end.

This way of the worst happens it should be the easy to reach fuse by the leisure battery that goes first, rather than the one you have to get the battery out to change...

 

[Roodedoo]

Ideally fuse it by the charger slightly lower than the fuse at the starter battery end. The OE setup uses 100A at the starter battery end and 80A at leisure battery end.

This way of the worst happens it should be the easy to reach fuse by the leisure battery that goes first, rather than the one you have to get the battery out to change...

This is ex-RAC and not their split charge. It's their jump-start setup. So it goes from the top of the battery with a mega fuse straight after, and there are no worries about access.
When I do the interior, I'll no doubt add a smaller fuse and kill switch before the intelligent dc-dc charger.