Audi A6 Battery Problem: Weak Battery Warning Light

I replaced the battery in this car 1.5 years ago.
For the past few days, the "low battery" light has been illuminated in the car.
Two days ago, it stopped starting.

I just took a quiescent current measurement using the BEM (Battery Energy Management system), and the result was 0.00.
I then measured the battery charging current in the MWB 4/1 while the engine was idling, and here the reading fluctuates and even goes into the negative range.
I then measured directly at the positive terminal with pliers and got around 60 amps there. At the cable under the cover in front of the water pump sensor, I still read 55 amps. Directly at the positive cable on the battery, I measured between 1.5 and 6 amps (possibly the meter is too slow).
It can sometimes go into the negative.

I've now run a temporary cable from the jump-starting point to the battery, but it doesn't make any difference, even though I'm measuring a constant current in that cable.

The alternator itself might not be the problem, but why is the battery receiving so little power?

According to the log, the battery charge level is 100%. Perhaps the battery is simply defective.

Battery level was at 60%.
Then I adjusted the battery settings again, and that's when it showed 100%.

VCDS and ODIS perform different types of coding.

And the battery is charged.

It's not the first time this has happened; we have this problem every 1.5 years.

/viewtopic.php?t=26290&highlight=battery_status+vcds

Wonder where the difference between the current measured by the LIMA and the current arriving at the battery is going. The measuring device must be faulty.

If the battery lasts for 1.5 years before repeatedly experiencing the same issue,
Could there be a slow-acting component that leads to sulfation and causes the battery to fail prematurely?

I left the car locked with the tailgate open for 2 hours and then used a clamp meter (which is probably not as accurate as an inline multimeter) to measure the current draw, and it showed no power consumption. Of course, it's possible that this isn't always the case.
I've now measured the power consumption for the BEM again. For circuit breaker 4/2, I have 0.00A (average quiescent current). For circuit breaker 5/2, I have 0.23A (average total current of the last operating cycle).

Hello!

Back in the days when there wasn't as much electronics around, people would simply measure the voltage by connecting a device directly to the battery.
Battery, after resting overnight: 12.2V, quite discharged; 12.7V, fully charged.

With the engine running at idle, you can measure the voltage at the battery terminals while the high beams and hazard lights are turned on. An ideal reading is 14.4V, a tolerance range for a discharged battery is around 14.0V, and anything below 13.8V indicates a faulty alternator.
So, it's only operating on 2 out of the 3 phases, which means it's only delivering two-thirds of its power.
You don't notice it in the summer, but you might be surprised when autumn arrives and you find that the battery is constantly running flat, especially if you use your lights more often.

So, the alternator, under heavy load, is no longer able to provide the necessary voltage that the battery needs to charge properly.
It's a common problem.

It's not necessary, but if you have a measuring device available, just take a measurement.
You have 5 minutes.

Autoservice wrote:

Ask yourself where the delta between the current measured by the LIMA and the current arriving at the battery goes.

The measuring device is probably malfunctioning.
I would say that the remaining power goes to the active consumers. Especially shortly after starting the engine, there might be some afterglow. Otherwise, it could be things like the entertainment system, seat heaters, lights, interior fan, and engine fan (if the climate control is set to "auto"). I'm not really surprised by a total of 50A / 600W. (I once measured values well over 100A in our Galaxy during the peak of summer – just the two interior fans and the engine fan running at full speed were already exceeding 80A).

Without knowing the voltage and battery status, the values don't provide much information initially. I would only rely on a multimeter. In this error scenario, I wouldn't trust the ECU (Engine Control Unit) installed in the vehicle, as a fault there could both cause the problem and distort the troubleshooting process. Therefore, only rely on external measuring devices.

According to the log, the battery shows 100% charge all the time. If that were true, the readings would be plausible. However, I couldn't clearly determine from your statements whether you charged it with an external charger and therefore if it was actually fully charged at the time of the log. Therefore, my suggestion is: deliberately discharge the battery (by leaving on headlights, radio, ignition, etc.) until the power cut-off point. Then, start the engine and record the values during charging using a clamp meter and voltmeter.

Greetings.
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

Quote:

I would say that the remaining power goes to the active consumers.

Okay, in order to check how many amperes could actually benefit the battery, I assumed that all consumers were switched off...

If the amount of electricity being drawn by the devices connected is greater than what's available, then the battery won't actually be charged at all.

Otherwise, I fully agree with the further test proposals.

When I received the car, the battery was at 60%.
I then had a qualified technician reset the battery using the original diagnostic tool (which uses a different coding procedure than VCDS), and after that, I achieved 100% charge. I then charged the battery and installed it, and the log in the first post is from that point.

I've tried the diagnostic suggestions you provided today. I used a voltmeter and ammeter (and these readings seem to correlate with the data from the control unit in the '61).

Engine running at idle without any electrical loads: 14.10-14.12V directly at the battery.
With light: 14.00-14.07
With lights and hazard warning flashers: €13.90 - €14.05.
With light and rear window: €13.95 - €13.98
With LED lights, heated seats, and lumbar support, the price range is €13,85 - €13,96.

At the foreign starting point, 14.1 volts.

Then I discharged the battery to 70% (11.8V), started the engine and let it idle, and measured the voltage directly at the battery (without any loads connected).

13.50V 70A
13.60V at 40A at the auxiliary starting point; 117A (with low beam headlights on).
13.75V 60A
13.80V 55A
13.85V at 42A at the auxiliary start point; 14.1V and 105A.
13.90V 32A
14.00V 30A
14.05V, 13A, 90% charge level. Now the display is fluctuating between 6 and 13A.
"The display showing 14.10V fluctuates between -1.5 and 8A."

Difference between battery voltage and jump start point: 0.1-0.15V.

The longer you let the engine run, the display will eventually show a negative value.

I'm not sure which alternator is installed, but I doubt there's a problem with the stator winding if it's producing 120 amps.
I also need to test a comparable car to see if I can achieve 14.4V. According to the specifications for the '61 control unit, the alternator voltage should be 14.3V.

Hi!

The measurements are right on the borderline... I can't make a definitive judgment about them at this point; it's difficult. I've also had instances of misdiagnosis before, so I'm being cautious.
As far as I know, it's often one of the power diodes in the alternator that's causing the problem.

Okay, find a comparable car to compare it with, but make sure it's tested under the same conditions.
As far as I know, the on-board voltage is likely or very probably regulated in a way that depends on temperature. For example, it might be 14.4V when cold and 14.15V when warm.

Technically speaking, the battery temperature should be used. In some older cars, the regulator temperature is used instead, and perhaps in newer cars, even the outside temperature.

It should also be noted that the alternator's power output is speed-dependent. Depending on the belt ratio, it can be assumed that the alternator delivers approximately 90% of its maximum power at a medium engine speed.
At idle, it's probably around 65%, but I can't really tell accurately, just a rough estimate.
Therefore, it is quite possible that the alternator cannot maintain the desired voltage at idle when all available electrical consumers are turned on, simply because it is overloaded.

But generally speaking, it seems to me that you should really look into the alternator's performance and voltage output in more detail.

Unfortunately, the time sequence of the measurements is not clear. Overall, it appears that the battery is being charged significantly.

Can you roughly summarize how many Ampere-hours were charged into the battery during that time? Alternatively, can you roughly indicate what the time interval was between your measurements?

It's not clear whether the alternator is still working properly at 120A! Our defective 120A alternator also produced currents between 60A (idle) and 95A (medium speed). The used 140A alternator that was installed afterwards already delivered around 110A at idle. Therefore, it would be interesting to know what you have installed. If you measured the current at idle, an alternator up to 150A would certainly be fine. I would be skeptical about a 210A alternator. It's hard for me to say anything in between.

Did I miss something, or haven't you measured the quiescent current yet with a multimeter connected?

Greetings.
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

It has a 150A Lima (alternator) installed. If it's only producing 120A under load, I can rule out a faulty alternator.

The measurements were all taken in the LL (likely referring to a specific lab or location).

Approximately 20 amp-hours were charged into the battery within about half to three-quarters of an hour.

I recorded the results every 5-10 minutes after the voltage increased.

I didn't measure the quiescent current with a multimeter connected. (And I can't do that right now either, since I no longer have the car here.)

chli1976 wrote:

It has a 150A Lima alternator installed. If the output is only 120A, I would rule out a defective alternator.

I agree with that regarding the alternator's ability to supply power. The alternator does not have any burnt-out diodes or similar issues.
The only thing I can't definitively assess is the voltage level. I would have expected around 14.4V here, like Steffen did. However, I'm not sure if this is normal for vehicles with battery management systems. But even in conventional vehicles, a battery at "only" 14.1V should still be over 90% charged. This shouldn't be the sole cause of your problems.

chli1976 wrote:

Approximately 20Ah were charged into the battery within about half to three-quarters of an hour.

This roughly corresponds to the displayed 30% discharge level.

chli1976 wrote:

I did not measure the quiescent current using a multimeter connected in series. (I also can't do that right now, as I no longer have the car here.)

Apart from the unlikely possibility of bad luck when buying the battery, that would be my last resort. If this were also acceptable, I have no other ideas. Unfortunately, even with the average quiescent current of the last usage period being 0.23Ah (over what time period?), the BEM test didn't show any obvious problems.

It's possible that a control unit might occasionally malfunction and then drain the battery. That will be difficult to find.

Lass mal wissen, wenn du eine Lösung erfahren solltest. I am curious.

Greetings.
Sure.

Here is the translation of the text from German to English:

"Guste" means "gust" in English. It refers to a sudden and strong gust of wind.

Measured on an Audi A6 3.0 TDI today.

13.8V, 15-25A at 80% battery charge level.

And that one definitely has a more powerful battery than my gasoline-powered car.

Today I took measurements on a 2.7 TDI engine. The motor control unit (ECU) parameter 4/1 also ranges from -3.1 to 8A.
And the voltage is between 13.95 and 14.05.