Normal Charging Voltage?...

[jon_iz]

At the begining of the the year suffered the "weak battery" problem and ended up getting the battery replaced.
Subsequent to this, I have changed my Sat Nav, to a Garmin unit which also has an adaptor that plugs into the OBDII port (Ecoroute HD) to read off additional data.
One of the items the Sat Nav will display is battery voltage.

I have noticed something odd, which may be normal, or may be not - so it needs input from someone who knows the charging system well.

Conventionally, I understand that the alternator/regulator output about 14.2~14.4V when charging, and this should remain relatively stable.
On my B8, I have now noticed that after the vehicle has been running a while (and this depends on how long and I guess the state of the battery after starting), so after 20 mins~1 hour, the indicated charging voltage drops back to 12.8V while driving on a open throttle with engine under load, irrespective of electrical load (i.e. lights, heated rear window, etc. on or off).
If I lift off throttle and remove engine load and the car is on over-run, then the charging voltage rises to 13.6~13.8V. As soon as I apply throttle again, the voltage falls back to 12.8V.
I initally wondered if the Garmin Ecoroute Adpator was getting false information, so I have checked the charging voltage with a volt meter and I see exactly the same thing.

If I stop and start the car at this point it doesn't make any difference.

I know the car has a battery manager.
My B8 is a 2.0TDI 170, made in 2008, so it doesn't use "Start-Stop" technology to kill the engine when in neutral and stopped.

So is this normal behaviour for the charging system?

[thescouselander]

IIRC the A4 has an intelligent power management system that removes load from the alternator under full throttle conditions. I think you are seeing real information here and it is as it's supposed to be. Have a look here at the recuperation description: http://www.audi.co.uk/audi-innovation/efficiency.html

[phil miller]

I think your car has a smart charge set up, where by the ECU tells the reg in the alternator how much to put out, i was always told at TECH that it takes about 20 mins of the engine running to put back what the starter takes out, now 20 yrs ago i can understand this, i think from what you have said its normal, but if you are worried have the output tested by a local garage

[John140]

I was wondering for ages what the Audi energy recuperation actually did, but this is 100% logical, use max alternator load on the overun and cut back load when a lot of power is demanded from the engine, it must help a bit, but it does make things complicated.

[Alan G H]

Lots of cars, vans & trucks have similar systems, it saves a small amount of fuel (every little helps!)