Hi - and apologies in advance for my first post being a request for help.

I have a 2008 slk 1.8 kompressor (r171). The battery light is on BUT the battery is showing a healthy 13.5v+ on idle. Nevertheless I changed the voltage regulator, no difference. I then changed the voltage rectifier, no difference.

I changed out the alternator for a new one, no change.

I added an additional earth from the engine to the battery, no change.
Checked all grounds and cleaned, no change.

Scanned with OBD reader.. no errors

Checked the alternator signal wire - has 0v on ign off and 12v+ on ign live.
Checked main power lead off back of alternator which is showing the same as the battery does

Battery is NOT brand new but is healthy and has been trickle charged off the car. I've also tried another battery I have, still the light persists. Battery is 12.6v with gin off.

Car starts as you'd expect (not lazy) and after around 10-15seconds the battery icon displays in red in the top panel of the instrument cluster.

Any advise welcome?! I'm really stumped