Polo 6C high and constant charging voltage.

Chat about your 6R/6C model Polos here!
Post Reply
RUM4MO
Bling Bling Diamond Member
Posts: 5859
Joined: Sat Dec 30, 2006 9:12 pm
Drives: B8 S4 & 6R/6C1 1.2TSI 110
Location: Mid Lothian

Polo 6C high and constant charging voltage.

Post by RUM4MO »

Hi, has anyone got any experience of a high charging voltage level on a 6C Polo, in my case a 2015 1.2TSI 110PS SEL, I've made a habit of keeping an eye on the charging by plugging a cheap DVM into the 12V power socket. Up until this week, everything seemed okay with pulses of high charging voltage on the over run, roughly as high as 13.7V and under power as low as 12.6V or lower as the battery was not being charged due to its present charge level - all that seemed to make sense to me.
This week, the charging voltage has been fixed at between 14.7 and 14.9V and never dropping off on the overrun. As the battery is well charged, auto Stop/Start is still working well, and as usual is enabled within a mile or so.
By unplugging the battery dongle, the fixed voltage level drops back to 14.1-14.5V which to me makes it seem that the control circuit within the alternator is still working and that highish voltage with the battery dongle lead unplugged maybe that is the default voltage level , ie when feedback is lost.
Using VCDS reveals no fault codes, unless the battery dongle lead is unplugged, battery "useful charge" is showing as being 36Ah - that battery started life as being 59Ah and the battery "charge level" is showing as being 78%.
So basically I'm wondering what to check for next, maybe the battery dongle as it will be supplying the BMS with the info it needs - or is this just an apparently serviceable 6 years old battery dying in a way that is messing up the charging system - it is always very easy to point the blame at the battery, but is that true in this way as I've tested this battery this week using a CTEK battery charger and it still claims that the battery is "good" and has a CCA rating of 550Amps EN, which for a 6 years old EFB battery that started life being 625A EN, is not too bad.

Edit:- just to add, outwardly, without having actually checked the charging current over time, I would not have been any the wiser that anything was wrong, so this situation could exist on other cars as well, but that does not make it right or correct, something has changed over the past week.
heftyshooly
New
Posts: 32
Joined: Thu May 07, 2020 6:10 pm
Drives: Polo 6C 1.4 TDI BMT
Location: Portugal

Re: Polo 6C high and constant charging voltage.

Post by heftyshooly »

I've observed the same intermittent 13.7V <-> 14.7V (ish) behavior. I've measured it with an OBD dongle a few times and I can usually hear the speed of the air circulation fan increase on a few occasions - when letting off the throttle with a gear still engaged while the RPMs are above a certain threshold, that's when the voltage is raised.

I've never noticed the voltage being fixed at the high level, but I don't usually keep an eye on the voltage, maybe once a week when I plug the OBD dongle.

What I have seen is that the battery management system behaves differently when the car is not driven for a while. In my case, I always do longish journeys (> 40km), but sometimes I won't use the car for a full week. In that case, the start and stop will not activate for a day or two, even if the battery displays as 80%-90% charged. I don't remember measuring the voltage in this situation and this is possibly unrelated, but from what I observed I got the idea that the BMS takes a few miles of consistent driving to work "properly". Maybe in your case, it might go back to the old behavior after a few miles... that's just a wild guess though!
RUM4MO
Bling Bling Diamond Member
Posts: 5859
Joined: Sat Dec 30, 2006 9:12 pm
Drives: B8 S4 & 6R/6C1 1.2TSI 110
Location: Mid Lothian

Re: Polo 6C high and constant charging voltage.

Post by RUM4MO »

First, thanks for your reply!
My worry at the moment is, the constant high charging voltage and what it will do to the battery and maybe over time other modules, though I'd hope that every thing apart from the battery will survive this situation.
I've tried keeping the heated rear window switched on, but that is not having much effect in the voltage probably due to the alternator is rated as 140Amps 14V.
My next plan, as well as finding out battery prices, will be to use a CTEK smart charger to get the battery as far up to 100% as possible - and then check with VCDS how well charged the BMS thinks it is, and then if it accepts that it is fully charged, I'll use that car and see if the charging voltage drops and is now controlled in the way it used to be, I doubt that it will as my first check with VCDS today after a short 5 + 5 miles returned an initial charge level of 78-79% which is quite close to the system's target, I think, of roughly 80% charged.

It might sound illogical/wrong/stupid to panic and grab a new battery when in earlier times, batteries only ever, failed to hold charge and the cars would not start, or they would fail with an open circuit cell that only showed up after the car was stopped and then restarted an hour later - my wife experienced that with a previous car, a Ford Fiesta with a FoMoCo battery, but something has failed or is upsetting the smart charging system in this car. The car's own built in diagnostics claims that the alternator is fault free in term of Thermal issues, Mechanical issues and Electrical issues. The BMS dongle(current measuring resistor) connected to the -VE side of the battery might be causing this, but again that is just a guess, though I'd think that replacing that would be cheaper than buying a new battery - I really don't like just throwing parts at problems, but what else can I do other than let a specialist throw my money in same parts at it!
User avatar
iichel
Bling Bling Diamond Member
Posts: 6361
Joined: Fri Nov 14, 2014 2:14 pm
Drives: Polo 6R 2.0 TDI, Passat B8 2.0 TSI
Location: http://mypassat.nl/

Re: Polo 6C high and constant charging voltage.

Post by iichel »

What you are describing perfectly fits the behaviour of the 'Bluemotion Technologies' charging strategy.
The system will very often attempt to only charge on demand, i.e. when a high consumer is enabled.
And otherwise, it will only intermittently charge when the car is coasting in gear or braking to reduce it's parasitic footprint on engine power and ultimately, emissions.

You'll often hear the alternator while when you enable a high consumer or release the throttle.

I don't think there's anything wrong.
I've seen my Polo 6R 1.2 TDI Bluemotion drop down to 13.3V during normal driving and up to 14.8-ish territory when recharging.
RUM4MO
Bling Bling Diamond Member
Posts: 5859
Joined: Sat Dec 30, 2006 9:12 pm
Drives: B8 S4 & 6R/6C1 1.2TSI 110
Location: Mid Lothian

Re: Polo 6C high and constant charging voltage.

Post by RUM4MO »

iichel wrote: Sun Oct 17, 2021 9:14 am What you are describing perfectly fits the behaviour of the 'Bluemotion Technologies' charging strategy.
The system will very often attempt to only charge on demand, i.e. when a high consumer is enabled.
And otherwise, it will only intermittently charge when the car is coasting in gear or braking to reduce it's parasitic footprint on engine power and ultimately, emissions.

You'll often hear the alternator while when you enable a high consumer or release the throttle.

I don't think there's anything wrong.
I've seen my Polo 6R 1.2 TDI Bluemotion drop down to 13.3V during normal driving and up to 14.8-ish territory when recharging.
Thanks for replying, but while I completely agree with how the system is designed to work, I have been running with this cheap plug mounted DVM in place for maybe the past 18 months, and it has always, in that time either shown 12.3 > 12.7V when not charging or short bursts of 13.4 > 13.7V roughly when on the overrun or at other times when charging is required.
Right now, shortly after starting the engine, or a second or so, the voltage gets up to 14.7/14.9V and stays there at all time during journeys with no dropping back down to "not charging", so that means now there is never any periods of no charging, and when charging the voltage is always 14.7/14.8 - or if the battery dongle lead is removed, 14.2/14.3V.

I've checked the stats in the CAN Gateway and when the battery dongle is connected the battery charge level was 78%/79% and with the dongle unplugged it reverts to battery charge level of 100% - which is probably a default figure to prevent over charging if the dongle or its lead fails.

The trouble is, if I did not have this DVM plugged in, I'd not be aware that this was happening as there is no other indication of problems and no faults being logged - I have checked with a reliable DVM both at the battery terminals or +ve and chassis and these values do really exist.

The continued heavy charging is what is leading me to consider buying a new battery as I'm guessing that the smart charging system can not bring the battery up to what it considers is a "good" level of charge - although I believe that the target "battery full" for these cars is roughly 80% - and so far I've logged 78/79% using VCDS which is near enough 80% - so why is continuous charging now taking place - only over the past 7 days? Replacing the battery is not huge accidental spend as the battery is over 6 years old, the car's BMS reckons that its present useful capacity is 36Ah - this battery was rated as 59Ah when new, I have records taken back in February 2020 and after the battery was fully charged using a CTEK charger, its charge level was 92% and its usable charge capacity was 41Ah, a month later under the same conditions, its charge level was 91% and its usable charge capacity was 37Ah - okay the ambient temperature would have been a lot lower back then and the "battery at rest" voltage both times was 12.7V. With the battery dongle connected and without having used a smart charger to bring the battery up to its fully charged state, presently the battery charge level as I said is 78/79% and the usable charge 36Ah and the "battery at rest" voltage 12.5V. So what I am thinking is, maybe the present state of health of this battery has reached the point where the "Blue Technology" plan of only charging when absolutely necessary can no longer be used. I'll recharge this battery using my smart charger and see how the results compare with the Feb/Mar 2020 results so comparing like with like, and then might grab a AGM battery.

I suppose this situation, is concerning me more for what effect this higher voltage will be having on all the rest of the car's electrics, okay I know the front indicator bulbs I think are rated at 14V but I'd prefer to stop this happening now - than need to deal with the effects of it later.
Also I wonder if this is eventually what kills so many of these EFB, ie they "decline" into a lower state of health, and so then get continuously charged at this higher voltage of just under 15V and that causes them to fail
RUM4MO
Bling Bling Diamond Member
Posts: 5859
Joined: Sat Dec 30, 2006 9:12 pm
Drives: B8 S4 & 6R/6C1 1.2TSI 110
Location: Mid Lothian

Re: Polo 6C high and constant charging voltage.

Post by RUM4MO »

Today was the first time that we have used this Polo since I changed the serial number of the battery, the charging voltage started off today again fixed at 14.8/14.9V but after maybe 20 minutes driving, the charging voltage changed back to what I would expect, which was a surprise but a welcome one, as surely this indicates that at least the systems electronics and interconnection wiring is good/working. On the return journey the charging voltages alternated between what I had seen over the previous 6 years or so and the constant 14.8/14.9V - so could this be due to the BMS trying to adapt the charging profile to suit what it thinks is a new battery?
Time will tell, a new but slightly bigger AGM Bosch battery has been bought - "luckily" my source for car batteries, Costco,do not stock the 027 size Bosch AGM, just an EFB.
Post Reply