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About 12 months ago I wrote a blog post “Is A Euro 6 Engine Killing Your Leisure Battery?” and it got a few comments and generated a number of emails. However 12 months on I’m getting a lot more questions relating to problems around smart alternators and I’ve brought forward this blog post by quite a few months from my planned posting date following a couple of long email exchanges with two readers and a few others.

Right, I think the best way to explain this is to set the stage so to speak.

Bob stores his brand new caravan at home and it is regularly plugged in to the house to run the internal battery charger and a dehumidifier. It’s got a new 110Ah AGM leisure battery and a new motor-mover fitted. The caravan is plugged in at least 24 hours before any trip to get the fridge down to temp prior to stocking it up. Bob also has a brand new car, Euro 6 diesel with a smart alternator. The car is a few months younger than the caravan and he’s never towed a caravan with this car. The tow bar and tow electrics were all fitted by the dealer (or dealers agents) prior to it being purchased. He did tow this caravan on 4 trips with his previous car, same make but 6 years older.

The caravan suitably prepped, fridge down to temperature and stocked. An early morning departure and six hour journey with a couple of stops to catch the Euro Shuttle over to France followed by a couple of hours driving in France to their first destination. On arriving, Bob sited the caravan on to the pitch not using the motor-mover and went about setting up. Mrs Bob knowing a request for a cup of tea was imminent went inside to put the kettle on where she discovered everything in the freezer had defrosted. Bob checked the fridge, it was still set for travelling. For what ever reason he also checked the leisure battery on the caravan’s system… 12.1 volts.

Over the next few days there was a couple of phone calls to their caravan dealer along with a number of emails. For the next three weeks and 2 other camp sites the fridge worked perfectly. On the return trip a similar distance and travelling time to the outward journey ended up when Bob returned home he only managed to get the caravan part way up the drive using the motor mover. He had to plug the caravan into his house overnight to charge the leisure battery enough to allow him to use the motor mover to finally put the caravan into its ‘home’ at the rear of the house.

Within a couple of weeks the caravan was returned to the dealer for extensive checking and the dealer could not find any issues with it or the fridge. They put forward the idea it must be an issue with the car. Bob tended to agree with this as he had taken this caravan on four trips towing with is old car and never had any issues. Three relatively short trips, the other a longer two week trip from the North East down to Cornwall, a similar 8 or 9 hour journey and everything seemed fine.

After a conversation with the main dealer who agreed to have the vehicle towing electrics checked over the next couple of days. The verdict from the main dealer was they could find nothing wrong with the vehicle, everything was working as expected.

Now, this is where I got involved. The above is actually an amalgamation of two very similar emails asking for thoughts and advice. The people involved had vehicles from different manufacturers and the caravans were also from different manufacturers. The only common element was the vehicles were new and had Euro 6 engines with smart alternators. I am not going to mention the vehicle or caravan manufacturers for a couple of reasons…. one, I don’t want to end up on the wrong end of a legal letter and two, I don’t actually think the manufacturer of either is relevant.

What’s Going On?

Luckily for me Bob is recently retired but knows his way round an AVO 8 being an apprentice TV repair man for Redifusion back in the day when valves were king. (Just as an aside, I bought my first AVO 8 back in 1976 and paid £8 for it. It was Ex REME in a leather case. I purchased it from MAZEL RADIO on London Rd in Manchester. Anyone from Manchester of a certain age will know Mazel Radio).

To cut a long story short over a period of a couple of weeks we proved that when hitched up, as soon as the smart alternator went into eco mode, there was a current flow up to about 4.5 to 5 amps FROM the leisure battery TO the car. This only stopped and reversed when the electrical load in the car caused the vehicles ECU to turn on the alternator’s output. Furthermore we determined that with the caravan hitched up, the period the alternator was in eco mode was also longer than when unhitched. Although we could not prove this with definitive evidence, this would also support the reverse current flow from the caravan to the vehicle that we were seeing as the caravan leisure battery was now supporting the vehicle battery.

Now there were a few questions that were buzzing round my brain….

  • Why didn’t the caravan habitation relay drop out when the smart alternator went into eco mode to stop this reverse flow?
  • Why didn’t the dedicated tow electrics strop this reverse flow?
  • Why didn’t the vehicle ECU knowing a trailer was hitched stop the alternator going into ECO mode?
  • If the ECU put the alternator into eco mode why didn’t it drop the fridge supply and by default release the habitation relay?
  • Why did the fridge defrost/not work even though in theory there was a voltage supplied to it?
  • Was this the reason I had seen an increase in emails relating to motor mover issues and batteries not holding their charge?

What Next?

I don’t really know. I don’t know how wide spread an issue it is or its going to become. I also don’t know the details of how specific manufacturers implement eco modes in the ECU programming or how the tow bar electrical interface manufacturers could work round the potential issues while still being able to get their products certified by vehicle manufacturers.

I think it might be down to the caravan manufacturers to come up with a solution. I know that there are a couple of after-market products available that provide a solution – We have one I installed in our caravan and in the short term I think this will be the quickest route. Caravan manufacturers may have a reluctance to respond and they can legitimately say “Well it is designed to work correctly to the relevant standards.” I think that maybe it will require the two main caravan clubs to look further into this and if what I have outlined above is proven by them to be the case, bring pressure to bear on all the parties concerned to come up with a way forward to resolve the issue.

For my part, I have contacted a few manufacturers asking for information and clarification or even acknowledgement of an issue. Unfortunately no one seems to want to talk about it. I do know individuals have emails details of their own problem to manufacturers and have received less than helpful responses.

Let me know in the comments below if you have had anything that might be related to this. I’d like to find out more.


The thought just crossed my mind…. what affect, if any, will this have on AL-KO ATC (if fitted) when the vehicle is in eco mode and the alternator shut down? Does is mean that there is a possibility that the AK-KO ATC may not work correctly in all circumstances?