In my own car, I sometimes see it hovering from 13.5 down to 12V with engine running idle. I haven't got the faintest idea what happens then. Increasing the engine RPM brings it back to 13.5 V.
Try Address 71: Battery Charger 3D0 915 181 C.Where would you pick up and log system volts with VAG-Com??
From the advance measuring blocks (I think), it is Group 1 - Field 2 , Terminal 30 (Comfort battery). There should be a “Current - Term 30 (Battery)” as well. It would be interesting to see what that current is reading. Mine is always zero.
Interesting. I was pretty sure that the dash volt meter would be a class II device, i.e. 2% of range (+/- 0.3 V). I think it is an analogue instrument, connected to a DAC. This DAC would convert the battery voltage as given by the CAN bus back to an analogue signal.I've certainly been measuring 11.5V at the battery with a branded digital multimeter,whilst the dash voltmeter is showing 12.5V.
An infamous problem of needle type of indicators is the viewing angle. The difference between the viewing angle as is the case when we look at the meter, versus the optimal angle (slightly at the right of the steering wheel) is almost +0.5 Volts!
I think you mean to say that your multimeter shows the same trend? I.e. reads 11.5V versus 12.5, and 13.5 versus 14.5 on the dash meter? That makes sense. When it is really showing 14.5 on you multimeter, then it would be way too high. To be honest, even the often reported 14 and 14.2 Volts seem to me rather optimistic. The only time that 14.2 Volts is ever reached, would be with an external charger such as the one you have. Smart chargers “measure” the status of the battery prior to charging it. Then they do a bulk charge, then top off the charge up to 14.2 Volts. But in a car battery management system, it cannot work that way, because the many consumers blur prevent an accurate measurement of the charge status of the battery. The float charge principle always limits the charge current to a safe level in order not to damage the battery. So it is only logical that the voltage of a discharged battery cannot be 14.2 V shortly after you have started the engine.. I have not yet ruled out the multimeter.. but as it's showing readings I believe for the conditions.. and it seems to follow the battery volts up to 14.5V when appropriate, I think it's OK.
This gives food to the thought that the dash meters aren’t that accurate as I assumed. My own dash meter usually is hovering around 13.5 and only reaches 14 on long trips.
That’s the spirit!I'll keep researching it!
It would be a small effort to calibrate (determine the deviation) of the dash voltmeters on a next GTG.