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OK… so that title was a bit dramatic! However here’s the thing… I don’t think everything is peachy with Euro 6 engines and charging leisure batteries.

If you are a regular reader, you know I have recently installed one of Sterling Power’s Wildside units (and so far I’m super happy with it!) but I did get an email from someone who had read all my postings about it and asked me if it might cure his problem. Here’s the gist of his email….

Note: I have edited this down a bit…. and withheld the name of the person and vehicle.

“I have recently changed my car to a new 2017 xxxxxxxxxxxx and after several trips with it, on arriving home there is never enough charge in the motor mover to manoeuvre our caravan up our drive (which is on an incline) and park the caravan round the back of the garage. I have to plug the caravan in overnight before I can use the mover.

This only seems to have started happening since we changed to the xxxxxxxxxx. I had the caravan’s battery tested at two garages and they said it is OK and it is only 2 years old. The local caravan service man said he could not find anything wrong with the motor mover.

As we are on mains at the caravan sites we visit for a few days the caravan battery should be fully charged.

Can you help?”

This did have me stumped for a bit. Battery tested OK, no issues with the mover, so what was going on?

While doing some of the prep work for writing about my installation of the Wildside unit, I had spent a few days prodding about our VW Amarok with a multimeter and making a few notes about voltages etc. One thing that I did cotton on to was the cyclic way the vehicles ECU seemed to turn off the alternator…. well I should really say put the alternator into “idle” mode. Now with the caravan attached (pre Wildside unit install) I did notice that initially it seemed to take longer for the vehicle’s alternator to go into idle mode but also it seemed to stay in idle for a lot longer initially and I was in the process of trying to work out why as initially I’d have thought it would have been less as it was running the fridge and charging the leisure battery.

A picture is worth… you know the rest. Here’s one of my excellent drawings!

Euro 6 Problem 01

Right, here we go… The drawing shows a caravan plugged into a tow vehicle that has the engine-turned off. Pin 9 is live as it should be, and the habitation relay in the caravan is effectively off allowing the caravan’s leisure battery to connect and power the caravan’s internal 12 volt systems. As the vehicle engine is off, there is no power on Pin 10 the fridge circuit, as this is controlled by the vehicle’s ECU.

Really this could be any vehicle with any engine. Now let’s have a look at what happens with the engine running…

Euro 6 Problem 02

Pin 10 is live, turned on by the vehicles ECU and this powers the caravans fridge. It also operates the caravan’s habitation relay which now disconnects the leisure battery from the caravan’s 12 volt systems and connects it to Pin 10 so that the vehicle can start to charge it.

Again, this could be any vehicle with any engine. This is how our Land Rover Freelander works with our caravan. All straight forward.

Now lets look at what happens when the Euro 6 engine puts the alternator into “idle” (or Eco mode etc.)

Euro 6 Problem 03

This is where it starts to get interesting. I have taken a few liberties here and made a few assumptions. I have shown the vehicle’s alternator disconnected. In practice the ECU doesn’t disconnect the alternator, it will reduce the field voltage and hence the output, not really disconnecting it but reducing the output to a negligible amount.

The ECU will also monitor the vehicle’s battery voltage and continue to allow the vehicles general electrical system to drain the battery to somewhere around 75% charge (this may be a bit of an arbitrary figure) The ECU will then turn on (or up) the alternators output to recharge the vehicle’s battery to about 80%. Why 80% well it needs the remaining 20% ‘free capacity’ so that when you brake, the excess energy of engine braking (regen) can be dissipated into the vehicle battery. Remember that on a Euro 6 engine the alternator is capable of generating round about 2Kw.

Now at this point it dawned on me that something could be happening here, but the idea was a bit ridiculous…. guys with far more agile grey cells than mine must have worked this out and I dismissed the idea. I must have missed a trick somewhere.

I did a bit more checking. I was using two 17Ah sealed lead acid batteries as my “leisure” battery simply because it was quicker to charge or discharge them than a 120Ah battery. For a fridge load I was using 3 x 50 watt light bulbs and it was all jury rigged to a 13 pin plug so I could just plug it in to either the Freelander or the Amarok to make comparisons. I was using a trusty old AVO 8 meter, a couple of digital multimeters and a clamp meter to measure current so really the whole set up was super sketchy for anything that I could write about. I thought that I must have been missing something somewhere and I actually kind of put it to the back of my mind. I just got on with installing the Wildside unit and writing it up.

“DING” You have mail……

I received an email from Charles Sterling with some very interesting information. During testing he had come across exactly the same issue I was pondering over but had put off further testing. I guess by now you have worked it out. Quite simply you can get current flow in the opposite direction… from caravan leisure battery to vehicle battery. In testing Charles had measured a current of around 6 Amps.

It dawned on me that maybe during my initial testing with my jury rigged set up I hadn’t missed something and the readings I had seen were correct. Both Charles and I quite separately had (in my case ‘stumbled’) on a potential issue with Euro 6 engines and caravans.

Back to the original email earlier. It now made sense. The sender of the email was setting off from their campsite to travel home with a fully charged battery (being on EHU while they were on site) and during the course of the drive home, the vehicles ECU was actually reducing the fully charged leisure battery down to 80% charge as it actually thought that the ‘vehicle’ battery was at 100% charge. Hence when he arrived home, the caravan’s leisure battery didn’t have enough charge to run the motor mover long enough to put the caravan away.

So what does this mean in practice?

Well effectively (give or take a bit of loss due to cabling) the caravan’s leisure battery will only get charged to about 80%. You can now think of the vehicle battery and caravan leisure battery as being one battery bank because that is how the vehicle sees it. If the leisure battery is fully charged its voltage will be higher than the vehicle battery so the vehicle will turn off (or down) the alternator so that the vehicles electrical system can drain it to about 75% ready for accepting the excess energy from regen braking. The caravan’s fridge helps the vehicle by draining the battery bank that bit quicker.

This also answers another question. While I was testing, sometimes I’d plug-in my jury rigged setup and if my two little 17Ah batteries were fully charged I’d get a low current drain indicated on my clamp meter. I’d dismissed this to a certain extent, but now I realise that as my two 17Ah batteries were fully charged and the vehicle battery would be at about 80% charge, there would be enough of a voltage difference for a short while, that the two 17Ah batteries would try to equalise with the vehicle battery by recharging it slightly..

Whats the answer?

Well thankfully I solved my problem when I installed the Wildside unit a few weeks ago.


If you have had any electrical issues with a Euro 6 engine and towing please drop a comment below. I’ll try to help.

As an aside, I am trying to arrange attending a tow bar installers electrical course with a couple of the OEM electrical equipment manufacturers and one of the approved bodies so I can hopefully increase my knowledge base and widen the number of vehicles I can cover. It’s a bit up-in-the-air at the moment as it would appear its going to cost a small fortune! (Sponsorship deals gratefully received!)