Trailer Socket 12v EHU - How I've Done It

[roadtripper]

So following some experiments over here I thought I'd write up making an adaptor to connect a charger to a 13 pin trailer socket.

Caveat: This is presented as info for what worked for me, make your own judgement about if it works for you ​

What is this?

In a fully wired 13 pin trailer socket there should be a permanent live circuit direct to the vehicle battery using pins 9/13. Traditionally this was used to supply power to the towed trailer/caravan.

However it can also be used to connect a maintenance charger quickly and easily to a locked van. This can be used to support the battery if the van is parked for any length of time but also if you have a wholly 12v fit out a charger that is happy to also be a supply can make a straight forward 12v EHU approach.

Can I use it?

In my case I have the OEM towing module fitted from the factory in a MY21 T6.1 Caravelle and the approach below I've tested works to at least 5 days with the van locked.

If you have the OEM towing module fitted later some people have found that some of the live supply might need enabling using VCDS or similar.

If you have the OEM second battery then pins 9/13 will likely connect to your leisure batteries - still useful but be aware.

If you have an aftermarket a fully wired solution should have pins 9/13 connected.

If you are not sure the cost of a 13 pin plug and some grain of wheat lamps is not huge to do a quick trial before investing further

What do I need?

A decent 13 pin trailer plug, I used this one from Amazon.
5 grain of wheat lamps, try your local model shop, I used these ones from eBay.

I also used this bootlace crimper kit to make a neat finish and avoid wire whiskers.

This is all you need to test but if you want form a complete system...

A maintenance charger with a quick connect system, ideally semi waterproof. I already use a number of CTEKs so I had an MXS 5.0 to hand and simply bought a new quick connect. If I was using this for EHU I'd buy one rated for supply as well like the MXS 7.0. There are equivalent chargers from NoCo and Victron that you might choose. Personally I'd stay in the 5-10 amps region.

A waterproof box to hold your charger and extension lead, I have several of these.

How do I build it?

What we are going to do is to build a "virtual trailer" using the grain of wheat bulbs, this will trigger trailer detection.

Trailer detection uses both indicator and both side light circuits and they must be incandescent lamps.

1. Strip about 1cm of insulation from 4 lamp leads - I find with the fine leads it's best to fold the exposed wire back over the insulation to give the crimp something to grip as they are thing.
2. Crimp 4 leads together as common ground, this will go in pin 3 (lighting earth), crimp the other ends individually and connect to pins 1 and 4 then 5 and 7. Pins 1-4 tend to be on a raised central bit so easiest to do these first.
3. If you want to test then crimp the leads of a 5th lamp and connect this to pin 9/13 - mark this lamp as if it lights it proves your permanent live is present and working.
4. If you want to dive straight in then cut off any existing terminals on your quick connect, crimp them and then connect ground to pin 13 and live to pin 9 - make very sure you get these the right way around!

Now assemble your connector but leave the lamps hanging out of the cable entry so you can see them and go test your setup. If you fitted the 5th lamp once you are happy it lights connect the quick connect and try that.

When you are happy you will want to open up the connector and tuck the lamps inside. You will then need the seal the quick connector cable and clamp it to keep things waterproof and provide some strain relief. I built up a "collar" of electrical tape and then tightened the connectors cable entry down on to that.

For use pop the charger and extension lead in the waterproof box. This has a small padlock loop on the closure so if you're not in a trusted area you might want to consider a padlock and maybe a short bike lock cable through the spare wheel cage to discourage tinkering. Tuck the whole lot under the rear of the van.

 

[roadtripper]

Some questions and thoughts:

You get a lamp warning about brake lights, why not connect a grain of wheat there?
Because it's not needed for trailer detection and if anything the warning is a useful reminder you have it connected before you drive off. Note that when the van was locked for several days I didn't get a warning so don't rely on that, stick a note on the dash or something too.

Aren't the grain of wheat lamps taking power?
Not really as you're never going to really have those circuits live, and the wattage is very low compared to what the charger is supply anyway.

Why not just connect the minimum 2 for trailer detection?
You could but grain of wheat are not the most robust. There is room for 4 and it gives some redundancy was my view.

The OEM circuit seems to be fused at 30amps - could I use the really big 25amp Victron charger?
I'd be wary, after all we are using the circuit in a slightly unusual fashion and in normal use this would not be used for days at a time. I think the "normal" 5-10 amp range of a maintenance type charger should be fine, and maybe up to 15 amp chargers if you are happy then. High current chargers should probably go directly to the battery as normal.

Should there be a fuse somewhere?
I did consider crimping on a couple of insulated blade sockets so I could add a "normal" 15 amp blade fuse inside the connector but in the end I decided that the circuit was fused in the vehicle anyway so it added another point of failure without really adding anything.
This was also the reason I used a "plain" quick connect and not one of the ones with an indicator. The indicator is useful but I didn't like the idea of the inclusive fuse holder sitting in the rain with all the water draining over it.
Obviously if you use a quick connect direct to a battery you want a fuse in it.

What do campsites think about this approach?
Honestly I have no idea as I've only tried on my drive. I think so long as you are sensible things should be fine but it is a bit unusual so if they have specific requirements, like an RCD, it may rise eyebrows.

• Have a waterproof enclosure, don't rely on the waterproof rating of the charger. Live lit up electronics on the ground are going to attract attention even if safe.
• You might want to consider putting a 16amp Commando type plug on the charger so there is no 13amp connection in the chain.
• You might want to consider a waterproof cable connection and permanently connecting the charger to a decent orange EHU cable.
• You might want to consider fitting an inline RCD instead of that waterproof cable connection.

Doesn't this take up space?
A bit, yes, but you don't have to have any mains EHU in the van so you get that space back.
Practically I think you could get the charger and a decent length of lead along with a padlock and security cable all in the box to make a neat package.

 

[roadtripper]

So some useful insight from @mmi over on my other threads.

What happens with a factory second battery?

Well it turns out that it is the Pin 10 feed that is moved from fusebox SA to the second battery fuse box SG. VW seem to label this as the "charging" feed (rather than "fridge" in generic 13 pin wiring diagrams) so it makes sense to move into the circuits under control of the second battery.

What does this mean in general?

This would appear to mean that the Pin 13 permanent live is always to the starter battery with the OE towing module.

That means for the starter battery maintenance use case an adaptor made up for Pin 13 should always charge the starter battery.

For the 12v EHU use case for a single battery a Pin 13 adaptor will work. I suspect that for a second battery vehicle making up an adaptor for Pin 10 will not work as I strongly suspect it will not be live unless the engine is running.

Can this behaviour be changed, what if I have different behaviour?

MMI has kindly added the VCDS screenshots to my other thread showing the options for behaviour that each pin can be set to in the OE module in a T6. I don't at this time have VCDS to confirm if these are identical in a T6.1

 

[roadtripper]

Added a waterproof cap to protect the plug when ready but not connected, and I can also pop the bright plug on the dash to prevent me driving off...

They're about a fiver from Amazon or eBay.

I find I'm using this at the minute as day trips are not happening because of other reasons and it's nice to know the van is not going flat.

 

[roadtripper]

Just a bit of an update. For various reasons we've not had the van out for several weeks, but the 12v EHU has not shutdown at all with the CTEK dropping into its long term maintenance step.

Needed to take the van out today at short notice and it was totally fine battery wise.

The bright green anti idiot cap has also been a lifesaver at least once

 

[Cnud]

Thanks for sharing this. I’ve made a couple of these up now, one for my Ctek and one for my Noco. Thought I’d try it out this weekend as I knew I wouldn’t be using the van. My van has a non oem second battery with a Ctek dcdc charger with a solar panel and victron controller. I also have non oem towbar electrics, so a lot of variables! But, positively the outcome seems to be (with the Noco initially) that the leisure battery is charging at 13.4v and the starter is maintaining at a steady 12.3v. I can live with that to be honest. Will try the Ctek charger next time.