Intermitent Battery Light

[zzz4]

The car in question is a 2007 A6 C6 with 250k miles.

I've been experiencing intermittent battery light on the dash. The battery light comes on intermittently. When I drive with the battery light on, the battery level drops... When the battery light is not on, the battery charges.

To test quickly, to see if it's the alternator or the battery, I disconnected the positive battery terminal while the battery light was on. The engine immediately died. This is the normal test I have performed on other cars with good quick diagnostic effect. I tested it when the light was off and the engine continued.

Last night the battery was at 100% according to MMI. I started it, no battery light but the battery level showed 10%. As I drove, the battery reached 40%. Stopped to fill gas, and when I was pulling out of the gas station and was on a decline, the battery light came on. Quickly drove to a parking lot a mile away as the battery drained down to 10%.

What I have noticed is that if the car is parked on a level ground for a few hours, the battery light is off. If I park with the nose facing down on an incline, or am on an incline facing down, the battery light comes on. Once it's on, it doesn't go away when I am driving on a flat surface.

The battery is 7 years old and I will be replacing it anyway. But with an old battery, the issue should have been consistent, not intermittent.

What do you think?

[Avantly]

1) These cars are very sensitive to battery condition
2) The battery monitor module right next to the battery commonly suffers water ingress (especially on the avants) causing the pins to corrode, so I would definitely check this.