Hey guys,

Interesting problem which has had me trawling the internet & forums without finding a similar problem or cure.

Replaced both of my batteries ( 2013 X250 Sportbrake ), about 9 months ago, had the 'lost comms with door lock units' scenario so went the SDD + Mongoose route and managed to reprogam that fault myself. No problems up until last week when suddenly I have 'battery not charging' on the dash. Tested engine running voltage at main battery - zip, nada, nothing. Just reading what is in the battery itself i.e. about 12.5 volts or less as time goes by. The small battery however, merrily charging away at 14.7 volts. Is it possible for the generator/voltage regulator to fail on one battery? Logic tells me not, if it charges one it should charge both. Given that, I am assuming that it may be a contactor not closing or could it be the batt. monitor system causing problems?

Battery on trickle charge at present, not sure where to look next so any info or help greatly appreciated.

 

Since you have a working SDD I would bet there are errors in the charging system. Have you looked?
You got a dash light so I would expect that to register as an error somewhere?
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As an update I now have no charge into either battery today. I also have a carsoft scanner so will plug that in to read DTC's but I am expecting a generator failure code.

 

Yes the small battery is not charged directly from the alternator ( so 14.7V etc at the small battery does not indicate the alternator is functioning. ) For more info have a look at this thread.

 

Thanks for that Phil, I had no idea about that. My icarsoft scanner shows P065A which is of course generator failure. Thanks to all for input.

 

How does that work? It's all one electrical system so when the alternator is running then surely both batteries are receiving a charge? Otherwise you're saying that the small battery would only charge from the large battery when the car is turned off which would seem odd...

If you've measured 14.7V across small battery before then it must be on the alternator as nothing else in the car produces that voltage.

So it does suggest an alternator / charging issue if there isn't the right voltage across the electrical system?

 

The small battery is not directly wired to the alternator or large battery ( being much smaller it would overcharge if charged for the same length of time as the large battery, they would balance to the same voltage too ) The small battery is only connected to the cars main electrical systems when the car is being restarted from the "stop/start" stopped state, at that point it powers the cars systems other than the starter motor, so that the systems do not suffer the voltage drop on the main battery caused by the starter motor.

It's not the case that "If you've measured 14.7V across small battery before then it must be on the alternator as nothing else in the car produces that voltage."
The small battery is charged by the "Dual-Battery" module ( it's the module with the cooling fins mounted to the rear of the wheel arch near the Rear smart junction box ), it provides the correct voltage to charge the small battery, under command of the Gateway module ( mounted a bit higher ) so the gateway module can keep track of how many Watt/Hours of charge are available in the small battery. It should only charge the small battery when the engine is running, but needs to be able to generate a higher voltage than output by the alternator at the time if required, as the alternator output voltage of the alternator is controlled to correctly charge the main battery and provide power for the other electrical systems ) To put it another way the Dual-Battery module includes a DC-DC boost convertor/step up PSU, ( i.e. a better version of something like https://www.ebay.co.uk/itm/283757806256 )
It does seem that the designers of the system missed a trick, in that they didn't disable charging the small battery when the system knows the alternator has failed! ( But I have seen several posts from people who have measured >14V at the small battery while having a failed alternator, including the thread I linked above )

I suppose it's worth mentioning that preventing overcharging is required these days, both because of the use of AGM batteries ( which will have their lifespan cut significantly when overcharged, unlike old flooded lead acid batteries ), and also the drive to lower fuel consumption/emissions, which can be helped by not having the alternator put load on the engine when the batteries are already charged - I'm pretty sure they even do tricks like having it charge when you coast (i.e. when you'd be engine braking ), if the battery is fairly well charged to begin with.

 

Is this something the Diesel cars have but not the gas cars?
I have never come across a dual battery module except on Diesel powered trucks that use two large batteries for starting?
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It's the cars with "Stop/Start" ( in case you haven't come across it it's a feature that stops the engine when you pull stop at traffic lights etc, and restarts it for you as soon as you release the brakes etc ) that have it. For the UK in any case I think that is everything after 2013 and some 2012 models.

 

The X250 went to start/stop with dual batteries in either 2012 or 2013 as you say.
Most (all?) other Jags did as well, eg XJ and F-Type.
In 2016 the XJ and F-Type (both still with start/stop) went to a single battery system.
Dunno much about the X260 and even less about the F-Pace and E-Pace (except of course they all have the ubiquitous start/stop) but I believe they all switched to a single battery system in 2016, either that or they never had a dual battery system from the get go.