Defective second battery in Passat 3C5 (AGM)?

Hi everyone,

I've since gotten rid of my old car and bought a new one – unfortunately, the joy didn't last as long as I had hoped. The auxiliary heater was defective, so the dealer has now installed a new one.

When I now start the solar inverter using the remote control, it turns off again after 15 minutes and stores the following error code in the fault memory (+ MFA): 01408 - Undervoltage shutdown (fixed value).

Longer trips (2-3 hours) were taken without using the SH system, and each time the result was the same.

It is the original VW factory-installed battery (68Ah AGM, 380A DIN). I have basically accepted that I will need to replace it, but I wanted to do some research beforehand. Unfortunately, the service manual doesn't provide much information about AGM batteries in general, and I found a lot of conflicting information online.

What's worrying me is whether the battery is actually charging properly. Everywhere I read that an AGM battery needs up to 14.7V to reach its full capacity, and also requires a specific charging profile for optimal "health."
Since there's only one relay, there's no specific characteristic curve other than "as much as possible." However, the voltage drop could be problematic: I only have 14.09V at the starter battery; and at the rear, it drops to around 13.7V.

Question 1.1: Is that enough to properly charge the AGM battery?
Question 1.2: Is it even worth installing an AGM battery again (cycle life) or will it inevitably be "destroyed" by the lack of electronics?


Now, I can't imagine that Volkswagen made such a significant mistake with the Passat and that the principle doesn't work.
Question 1.3: Could it be that my voltage regulator is broken? In my previous car, I had 14.35 - 14.4V directly at the battery...



Please provide the text you would like me to translate from German to English. I will then give you only the translation, without any explanations.

Second topic: Today I had the starter battery tested using a high-frequency tester (Argus AA500p), and the results I got were implausible.
Battery type: MF (maintenance-free); Battery standard: DIN (printed on the battery); Reference value: 340A (also printed on the battery).
The state of charge was determined to be 80%, with a voltage of 12.3V.
Now comes the part where I : Capacity: 418A, Lifespan: 100%.
The battery's ability to start the vehicle is currently at 62%, with a voltage of 7.79V.

The device recommends continuing to use the battery. I haven't experienced any problems with the battery yet, but it is starting to weaken (the lights go out when starting, for example).

Question 2.1: The charging level is only at 80% - could this be due to my battery (due to age) or could it be related to the power supply?
Question 2.2: How is it possible that the measured/calculated capacitance is so drastically higher than the manufacturer's specified value? Is the device malfunctioning?
Question 2.3: A starting capability of 62% sounds okay, but I find 7.8V to be far too low. Should I also replace the battery in advance to survive the winter? I really don't want to miss work on a cold morning .


If anyone has good sources for more information about AGM batteries (I've already checked manufacturer websites and various forums), I would appreciate it, as the keyword "AGM" is not often found in this forum .



Thank you in advance for your responses!

Marco.

/edit: Another thing that might be worth mentioning: CBBB, 180A Generator.

Hello,

The value you specified as "capacity" is likely the inrush current.

The capacity, which indicates how much electrical energy the battery can store, is always specified in Ah (ampere-hours).

You could check when the undervoltage shutdown kicks in; it might be possible to change that value.

Apart from that, I think the battery is borderline for operating the auxiliary heater in that vehicle.

Please perform an auto-scan and post the results here.

Best regards, Rainer.

anywhere wrote:

I have read everywhere that an AGM battery needs up to 14.7V to reach its full capacity, and also that it has a specific charging profile for maintaining its "health". Since there's only one relay, there's no specific characteristic curve other than "as much as possible." However, the voltage drop could be problematic: I'm only getting 14.09V at the starter battery; and at the back, it drops to around 13.7V. Question 1.1: Is that enough to properly charge the AGM battery? Question 1.2: Is it even worth installing an AGM battery again (cycle life) or will it inevitably be "destroyed" by the lack of electronics? If anyone has good sources for more information about AGM batteries (I've already checked manufacturer websites and various forums), I would appreciate it, as the keyword "AGM" is not often found in this forum

.
http://www.microcharge.de/forum/index.php?page=Thread&postID=73&highlight=vlies#post73

Hi there,

Thank you for the answers!


I went back to Volkswagen today and had both batteries tested using a conservative tester (with a load resistor). The results were very interesting: Both batteries scored at the upper limit of "Very Good."


Now, let's talk about Rainer:
The last time, the undervoltage cutoff occurred at 11.2V according to the error memory; now it's at 9.68V. The auxiliary heater is programmed with:

Wednesday, December 21, 2011, 16:59:44:29087
VCDS version: RKS 10.6.4
Data as of April 18, 2011.


Channel number adjustment: 001:
Saved: 113
Undervoltage trip.
13.59 V Voltage (Terminal 30)
Rest Level 0 Current Shutdown Level
11.30 V - Undervoltage cutoff threshold.
manual undervoltage shutoff.

Wednesday, December 21, 2011, 17:03:24:29087
VCDS version: RKS 10.6.4
Data as of: 20110418

If I were to think logically right now, I would say, "9.68V << 11.3V - something is not right here..." Shouldn't the SH have shut down instead?


Best regards,

Marco.
/edit: The code seems to be messing up the colons...?!

Hello,

Unfortunately, I have no experience regarding the reliability of the undervoltage shutdown feature.

Is there perhaps another reason why the battery is draining so quickly? Please measure the current being drawn from the battery in the scenario you described, for at least 10 minutes.

Best regards, Rainer.

I suspect that the voltage at the STH (spark plug heater) will drop relatively abruptly from >>11.2V to the stated 9.8V as soon as the STH glow plug is activated.

Have you ever observed the voltage of the second battery during the first two minutes of the STH start-up process? And, if so, until the system was shut down?

It could also be a transition resistance between the battery and the STH (corroded contacts?).

Alternatively, try jump-starting the vehicle using jumper cables connected to both the starter battery and the second battery, and see if that allows the system to proceed.

Quote:

Überall habe ich gelesen, dass eine AGM bis zu 14,7V braucht um die volle Kapazität zu erreichen, sowie für die "Gesundheit" eine spezielle Kennlinie.

And have you also read what capacity is achieved with a lower charging voltage? I don't know, but I imagine that even with a charging voltage of "only" 14V, it's possible to reach well over 80%, and I think that's acceptable, because with 68Ah, you can run the heater for a very long time (certainly better than constant overcharging).

Quote:

an der Starterbatterie habe ich nur 14,09V; Hinten gibts nur noch ~13,7V.

From front to back, there is only 6mm² cable, two 60A fuses, and the charging relay. Copper has a resistance of 0.0178 Ohms/meter/mm², which is approximately 20 milliohms (rounded for a cable length of about 6 meters). Even with just 10A of charging current, the voltage drop across the cable alone is already 200mV = 0.2V. Since you measured a 0.4V drop, roughly 20A of charging current must have been flowing. After a longer drive, the voltage difference between the front and rear should decrease, and the overall voltage should be higher.
What is your typical driving/STH usage profile like?
Have you ever fully charged the second battery with an external charger and checked when the error reappears?

Greetings.
Guste (also 3C with STH).

Now it's getting really freaky .

Thank you for the response; I've only just been able to look into this now.


Experimental setup:
- Voltmeter at the front.
- Rear voltmeter.
- Ammeter at the rear.


The auxiliary heater started at 17:06. The starting voltages are negligible because the battery was charged shortly before.
Front: 12.20V
Rear: 13.07V + 0.00A

Video 1: 8-20A during the initial phase, Climatronic "lights on," blower off or at a low setting.

1708
12.33V
H 12.42V 2A

1710
12.34V
H 12.41V 2A

1712
I'm sorry, but I can't translate that because it appears to be a sequence of characters rather than a complete sentence or phrase. Could you please provide more context or clarify what you would like me to translate?
H 12.3V 10A (The Climatronic system has activated the fan).

15 seconds after the fan is turned on: Deactivation of the standby function – without a voltage drop!

1713
12.27V
H 12.6V 0.06A

Please provide the text you would like me to translate from German to English. I will then give you only the translation, without any explanations.

Now I was quite surprised that...
a) the voltage of the starter battery has increased, and...
b) that the heating system only ran for a short time and that the battery still had plenty of charge.


Okay, another start (video 2) – with a result that was quite surprising to me: The battery in the back dropped to 10V after just a few minutes, while drawing approximately 10 amps.

Please provide the German text you would like me to translate into English. I will then give you only the translation, without any explanations.
a) Could it be that something connects the two batteries as soon as the auxiliary heater is activated?
b) Is it possible that the battery is actually dead, even though two testers said it was good?
c) Still no undervoltage cutoff, even though the battery is already showing a voltage significantly below 11.3V?
d) Is it normal for the STH (Subject of Interest) to be moving around so much (just before I point the camera towards it)?



I'm now leaning towards replacing the battery in the rear again, as the voltage drop seems "wrong" to me. However, I'm still curious whether something might have been incorrectly wired somewhere in the car.

Bitte gib mir den deutschen Text, den du übersetzt haben möchtest.
[1] http://www.youtube.com/watch?v=2FXwQS6MvqU
[2] http://www.youtube.com/watch?v=1YKaIGBjaxM



Thank you again.

Marco.

Hi!
So, if the voltage drops significantly at 10 amps, it's probably broken.
Why don't you disassemble them again for a final test, also connect an external power supply and hook up a defined load? An H7 bulb with 55W should draw just under 5A of current, which would be perfect for testing.

Dirk.

has been happening since 1830, but with 12V / 20W .

Let's see how long it lasts...


/edit: It seems the issue is resolved - after less than 2 hours, it's only at 10.8V...

Thank you very much to everyone, nonetheless!