UK-POLOS.NET - THE VW Polo Forum

Oops, yes, I missed that. All the same, if the dealer replaces the battery without finding out what caused it to perform badly in the first place, then the OP could soon have another misbehaving battery on his hands. Sometimes, dealers/workshops are unwilling to share causal information, so he might never find out the reason for the current status of his stop/start. I guess the weeks and months immediately following any such replacement would either start to provide some hard clues or alternatively would confirm that the original battery had simply been a one-off rogue with limited life.

I'm not too sure veteran about the need to connect the -VE side of the charger to the car chassis when supporting the battery from an external charger, the car's charging control system might/does need that info, but when using an external charger, that external charger has its own charge control system.

The original reason to avoid the -VE battery terminal was to minimise the possibility of a spark from making that connection igniting any local hydrogen gas that had come out of the battery, probably more at "disconnection when fully charged" time than at initial "partially discharged" time.

The issue to which I alluded is not one of 'charge control' (as you put it), it's one of charge monitoring by the battery management system (BMS). Unless you charge the battery - and here we mean, of course, using an external charger - by connecting the negative side of the charger to chassis, you will be bypassing the special resistor that sits between the true battery negative and chassis. The BMS has to constantly assess the charge state of the battery and it finds this out by periodically measuring the small voltage developed across that resistor and then performing a computation. (It's always trying to predict a battery failure of some sort, so as to be able to start shutting down various unnecessary current-users). If the battery is substantially discharged and you proceed to charge it directly across its posts then you'll be bypassing that resistor and consequently the BMS will lose track of the state-of-charge of the battery. It'll do no longterm harm, it's just that the data in the BMS module, about the state of charge of the battery, will differ from the actual state. However, from thereon the onboard software might decide to do something unexpected, depending on the degree of mismatch.

I can't recall where I first read about this as a warning; it might have been in the VW Driver's Manual. I think there must be some sort of software reset procedure do-able on this aspect of the BMS, in the event that the mismatch ever became substantial. I suspect a reset's done as a matter of course whenever a replacement EFB or AGM battery is fitted.

veteran wrote: Thu Jan 04, 2018 4:55 pm The issue to which I alluded is not one of 'charge control' (as you put it), it's one of charge monitoring by the battery management system (BMS). Unless you charge the battery - and here we mean, of course, using an external charger - by connecting the negative side of the charger to chassis, you will be bypassing the special resistor that sits between the true battery negative and chassis. The BMS has to constantly assess the charge state of the battery and it finds this out by periodically measuring the small voltage developed across that resistor and then performing a computation. (It's always trying to predict a battery failure of some sort, so as to be able to start shutting down various unnecessary current-users). If the battery is substantially discharged and you proceed to charge it directly across its posts then you'll be bypassing that resistor and consequently the BMS will lose track of the state-of-charge of the battery. It'll do no longterm harm, it's just that the data in the BMS module, about the state of charge of the battery, will differ from the actual state. However, from thereon the onboard software might decide to do something unexpected, depending on the degree of mismatch.

I can't recall where I first read about this as a warning; it might have been in the VW Driver's Manual. I think there must be some sort of software reset procedure do-able on this aspect of the BMS, in the event that the mismatch ever became substantial. I suspect a reset's done as a matter of course whenever a replacement EFB or AGM battery is fitted.

I'd be very surprised if that is the case, though as yet I have not re-read the Polo owner's manual, or my S4 owner's manual, and both these cars have either EFB or AGM batteries with "coded" batteries and a BMS aided/supported by that lump beside the battery -VE terminal.

Remember, if what you have read is true, if a car with this BMS had been left unused for a few days, and had a battery discharge fault, what you are saying cold mean that that car would ignore the battery charge change and maybe leave that car a bit short of charge after restarting. The BMS must always refresh its info as regards "present" battery charge state, it will do that successfully, I'd expect in the few milli seconds given to it after the ignition key is turned to the first position, the alternative might be that the BMS constantly monitors the battery condition even during "engine off" periods, so it would always have a true picture of the battery charge condition - and it would also get this updated information if you used an external charger, remember VW typically use CTEK chargers to "top up" their showroom stock as they get doors etc opened frequently.

The long term "coded" battery car users on the Audi forums, all seem to claim that while it is best practise to update the car's BMS system with the details of any new battery fitted during the car's life, in reality the car's BMS will have sorted things out without any issues after a day or two's motoring. I'd still always update the BMS with the details of any new battery I needed to fit though.

I can see we have a 'doubting Thomas' on our hands. But then that's fine by me, as debate and conjecture are healthy pastimes, in my book.

Since my last posting I've re-scanned the Driver's Manual but have been unable to find the paragraph or two to which I referred. I think I must have read it on a website instead somewhere, in the context of stop/start setups, possibly on the website of Exide or one of the other big battery names, or maybe in a different VW forum to this site's, on the Web.

Look at it this way, Rum4mo - the special resistor (it's always been referred to as a 'resistor', as far as I can see) and the two thin wires coming away from it aren't there for show, they're there for a specific purpose. Those two wires, which are directly across that resistor, can only be there for the measurement of a (small) voltage, from which, via simple Ohm's Law, the net current flowing through the battery, at any instant, can be calculated. The net current will be as a result of charge from the alternator, or charge from an external battery charger, together with the current actually used by the vehicle's electrical systems.

For many, if not the vast majority, of the electrical systems and components on this car and on others that now use stop/start technology, the return side of the battery is not the battery negative terminal (the post) but is the vehicle chassis. I don't doubt that if you were to find the appropriate wiring layout diagram for this or similar vehicles, you'd discover that there were dozens of 0v (gnd) connections on the car at myriad different 'chassis points' all over the car. Bearing that in mind, if you then take the trouble to draw out a simple block-type circuit diagram of the arrangement of the battery, you'll see that that resistor is always included in the load - unless you bypass it. Looking at the battery in situ, you can plainly see that that resistor sits between the battery's negative post and vehicle chassis, as the thick wire involved in that connects to chassis only about 3 inches to the side. If you connect a charger directly across the battery's posts, ie. use the battery's negative post instead, you'll be bypassing that resistor. Left to operate normally, the onboard system, as now devised, should be capable of measuring, at any time, the effective capacity (ampere-hours) left in the battery. I suspect that the designers found that a more accurate way of determining how much useful charge was still in the battery, rather than just measuring the difference in voltage at the posts themselves as the state of charge changes, particularly as the useful voltage range on stop/start setups tends to be more limited than on older, conventional battery setups. Incidentally, my estimation is that the voltage on the post side of that resistor will sit just a few tens of millivolts above chassis (0v), varying all the time according to the net charge going into or out of the battery.

If I come across those published words of advice again, about charging the battery, I'll post them here and you and others can make up your own minds. I must admit that when I first saw them I had my doubts, but when I drew myself some diagrams and thought about exactly how the charging works, it dawned on me that that resistor (if indeed it is a resistor) could well be acting as a means for the BMS to constantly measure the current going into and out of the battery (from whatever current source or load). Incidentally, what else could that component actually be, given that it's a non-encapsulated device and has a pair of thin wires connected across it that disappear off into the vehicle's onboard electronics? It could conceivably be a VDR (voltage-dependent resistor), designed to limit current if a charger with too high a voltage were connected. Or it might even be a TDR (temperature-dependent resistor) but, if so, with what purpose in mind? But my money's on it being a special, low-value resistor used by the onboard BMS to continually assess the state of charge of the battery.

Veteran, I don't think that I've ever, in this discussion disputed the existence of a current sensing device existing on these cars and I'd agree that it will probably just be a high wattage 0.1ohm etc resistor with a voltage "take off" at each end to provide the BMS with information, but all I'm questioning is, does it make any difference to the approach a workshop or user needs to take when fitting a "support" voltage source or when charging that car's battery.

I'd think that if it did, there would be big yellow notices placed under the bonnet to avoid anyone causing damage to that car when applying an alternative power source during maintenance or repair.

There are certain procedures that with time tend to be set in stone, if VW Group or any other car manufacturer suddenly changed how these common procedures now need to be carried out on their products, I'd suspect that they would be laughed out of court once the users started taking out small claims against them, so for what it is worth, I am thinking that VW Group and other car manufacturers will have made sure that any design changes made to cars due to the introduction of Start/Stop, would have been made with what has been done in the past taken into consideration.

Edit:- what I ran out of time or concentration, to say was, seeing as my S4 has a remote +VE and -VE dedicated "connecting post" located under the bonnet(chassis earth bond), like Polo GTI the battery is in the boot, I would find it more convenient to use them to fit any support battery/device - and so end up using the same in the Polo, which would be battery +VE and battery earth bond point, especially as we know that the Polo battery connections are well shrouded, although if using the removed battery terminal turned out to be easier, I'd still use them. Sounds a bit like a change of tune but I'm only really just clearing that point up.

Maybe connected in some way with this thread, I scanned my wife's Polo today for another issue, and discovered a fault logged in Address 19 : CAN Gateway (J533), that fault was 1282 - Supply voltage U1011 00 [008] - Voltage too Low, dated 2017.12.19 at 15:19:42 Terminal 30 : 7.9 V Terminal 15 : On!

Now on that day at that time, we had just returned to the car to begin our 40 mile journey home from visiting my MIL who was in hospital, so that car prior to that had just completed a 40 mile journey on a road with very little traffic and mainly duel carriageway. My thoughts are, this was just a "fake news" bit of information, sh*t in leads to sh*t out, not very helpful and means that I know that I can't believe what is being logged always, I have already had a "fake news" report on tyre pressure last May/June - so maybe it says it all for a poor quality product built using too many poor quality parts?

could have been the engine starting or waking up from start/stop, and while starting the voltage dropped under 8V.
or just a bit of fake news from a car that exists out of about 10.000 parts all manufactured and assembled by the lowest bidder :p

iichel wrote: Sat Jan 06, 2018 7:39 am could have been the engine starting or waking up from start/stop, and while starting the voltage dropped under 8V.
or just a bit of fake news from a car that exists out of about 10.000 parts all manufactured and assembled by the lowest bidder :p

WRT your comment "car that exists out of about 10,000 parts all manufactured and assembled by the lowest bidder" - yes, and that is a pity!
Seems a bit strange for a car company that is still trying to be the biggest in the world, just goes to show that you can fool a lot of people a lot of the time!
The term "race to the bottom" springs to mind here.
I did not feel good about using one of Donald Trump's "special" phrases there, but it did seem to fit the bill nicely!

Well, after 3 weeks - on Wednesday I received a call from AC to say that they have received a delivery and that I can drop the car in on Friday for new battery fitting, great!
But then yesterday I get another call from their parts department, turns out the wrong part had been ordered or delivered and they will get back to me when they know anything further.

Looks like I'll be waiting another few weeks at least from here, still driving on my 'faulty' battery. Giving them benefit of the doubt here, but starting to take the mick a bit.

Thinking back, your comment "battery is on back order" - so it sounds like either there is a lot of this going on across the VW Group Polo/Ibiza/Fabia and maybe A1 - if it is down to faulty batteries or acceptably incorrect fault diagnosis who knows, but it seems to be draining their reserves of EFB batteries.

In the absence of something like a cracked casing where electrolyte has leaked out, surely there are basically only one or two specific things that can spell the end of a wet battery: one of the cells going open-circuit; one of the cells going short-circuit; gross sulphation; all usable stored charge used up? I would suggest that if there's truly an issue across models with EFB's, then it's more likely to be with the charge management of the battery than anything else. A dealer workshop swapping over a supposedly duff battery under such circumstances will merely be providing a temporary solution, with the battery problem - if indeed this is due to a problem with the battery at all - re-emerging some months further on.

Bit of a bump, but having a Fabia now means I'm very rarely on here!

I'm having similar issues with my Fabia- it shut down last night for the first time in about 2 months, and only did so for about 5 seconds before starting back up again. I posted here over on Briskoda, one commenter said that he read about something which can be faulty, preventing full charging of the battery.

I need to book my car in for a check I think.

The past few days my stop/start hasn't been working. I've noticed the temperature outside has dropped significantly and my commute to work is only around 3.5 miles, so not sure if the car is trying to keep itself warm by not stop/starting on my short commute. Having said that, I've just been on a 15/20 minute journey and the stop/start still didn't kick in. I should also mention that the temperature has reached 90. Any thoughts on what could be the problem?

Hey mate, seems like quite a few GTI's had a batch of bad batteries, mine was replaced under warranty and it was sorted from there.

If you're still under warranty just have your dealer take a look.