[Schulisco]

Originally Posted by wazzz

So it sounds like what I said above, battery is good but charging system is bad and when running the engine, only the battery is used, hence the voltage drop after some time.
It could be as simple as a dead bulb in the gauge. If that bulb is blown or missing, the alternator does not "boot" (can't find the word for it but it needs this bulb and some current flow through it, otherwise it does not supply voltage).

^^^ This! ^^^

That little current going through that bulb drives the excitement of the alternator. Without that current the excitement won't work and therefore the alternator won't produce any or only little electrical energy. That's also why an LED as replacement for the alternator control bulb won't work too as the current is much lower through that LED.

https://www.hella.com/techworld/uk/technical/car-electronics-and-electrics/starting-and-charging-system/checking-the-alternator/#in--2041489067

Section 5.4 and

https://forums.pelicanparts.com/bmw-6-7-8-series-technical-forum/550712-alternator-idiot-light.html

Thomas