THEOMAST1X

Hi all,

I recently bought a second hand Audi A4 S-Line B9 that is 2.5 years old and had 20k miles on the clock when I bought it.

It has refused to start twice and both times the battery voltage was 11.8 volts and 11.7 volts respectively.

To investigate I put a logger onto the car battery and did a 1 hour journey primarily motorway driving.

Attached is the log from this.

As you will see the battery voltage is all over the place for the first 20-25 minutes before somewhat equalising.

This seems highly irregular and I would suspect a faulty alternator.

As it's still under warranty I took it to Audi for investigations and they've said everything is fine.

I was wondering if this battery activity seems normal or whether you've had Audi refute any claim that you have an issue, and if so how you fought it.

Any advice is greatly received,
Theo

SDakota

I charge my battery with an external charger at least monthly via the terminals under the hood and I have never measured my battery below 12.0V prior to charging (usually it's 12.1V or 12.2V). In order to get an accurate voltage reading, the car should be turned off and the battery should be allowed to rest for at least a couple of hours. My charger reads about 14V while charging the battery, but after disconnecting and allowing time for the battery to rest the battery generally reads around 12.5V fully charged. I don't know if the spikes and valleys of your driving log are normal, but it seems like your alternator can put out 14V which should be enough to charge your battery above 12.0V. If I had to guess I would say you have a bad battery. I'm surprised that the dealer thinks any reading under 12.0V is normal, but maybe when they tested the battery it was at least 12.0V and they don't believe your story about measuring lower voltage.

THEOMAST1X

Hi SDakota,

Many thanks for the quick response.

I’ve had roadside assistance out twice and my father take a look at the battery with a specialised tester and all three have said the battery is oddly fine and has no issues.

Which then leads me to believe the issue is elsewhere and likely the alternator.

I’ll leave the battery for a few hours tomorrow and check the voltage and see what it is.

I have a voltage reading that fits into the cigarette lighter socket and every time I get in the voltage that comes up when I start is below 12v.

Thanks again for the information much appreciated,
Theo

Mesquite77

Why create 2 separate logins for same post?

audi8k

The battery monitoring system includes a feature that, when it decides that the charge is "enough", only charges the battery when you let off the gas. If the first part of the log included such driving conditions it could explain the behaviour. The loaded battery voltage shows a decreasing trend during this part and when the monitoring system decides that the battery needs charging it would let the engine charge the battery constantly. If you could monitor with OBDeleven or VCDS it could tell you what the car thinks that the charge level is and current into/out of the battery and also the voltage it measures.
Battery voltage dropping to 11.7V is low. Have you tried to fully charge the battery with a charger? Driving for an hour is not enough to charge a discharged battery.
I don't know if the B9 has the battery monitor readouts at the same address but in B8 it's module 19 (CAN-gateway) measuring blocks 18,19,20.

Raf99

I'll add to this. I have a voltage meter plugged into the cigg lighter. (I was curious when watching a movie with the car off how low the volts would go). I will say, the battery voltage reported is the same as your graph. ALL over the place when driving around the city and using accessories. I've seen 11.8V and also 14.8V. But my car has always started fine. Maybe an idea for you, as you can buy them cheap off ebay.

gooberman

Maybe a bad ground? Try disconnecting the battery overnight and compare the before and after voltage. Then try with it connected normally and compare before after voltage. Could be something is bleeding it.

agentphish

FWIW I purchased a NOCO Genius 10 and it’s been great at caring for the AGM battery in our MK1 VW. Constant power feature is also very nice if you need to do an ECU or TCU flash tune since you want to be 100% sure you have adequate power in those types of situations.

https://no.co/genius10

Imprezer

At the dealership I work at, we always have these GTs dying... What causes it, I cannot say other than clients opening and closing doors and playing with MMI. First time it happened, we had to literally tow the car out of the showroom into the shop onto the rack, take the belly pan and open the hood with a long poker stick. We got pretty good at it now that we can do it on the floor. Just jack up one corner, take front part of the belly pan off use a poker and open the frunk. Then you are all set. One of these days I will write up a how-to. In the mean time, I am more than happy to walk anyone through this if they are stuck somewhere. All you need is a jack, jack stand, torx to remove 8 screws holding the front of the belly pan and a poker with a 90' tip. Whole process takes 5 min if you have all the things I mentioned.

rennen-affe

What happened at 16:25 or after a bit, seems to stabilize.

Anything below 12.6 is a static battery with degradation. 12.4 isn't uncommon, but you are much lower than that. You shouldn't see that low.

The 14v+ is the alternator kicking in.

Why has the battery not been swapped to test / rule out a bad battery?