I just had a similar experience with the black spot in my battery: poor, to no crank over power, Black spot wouldn’t go away even after charging the battery with a 6amp charger.
Called the Automobile Association to come and diagnose the problem. They checked the charging system - it was fine, no real drain, battery showed it needed a charge but was ok - well then it had to be the battery - couldn’t hold a charge, only 4 ˝ years old. So looked for an after market one, but couldn’t find one in stock that would fit properly ( height problem), so went to my BMW dealer and was pleasantly surprised that the old battery type had been replaced by a new larger one 720 cold cranking amps from 650 (about an inch longer as well) No coding required they said- close enough to original. I had taken a couple of tools, so just bought it and changed at the lot - 5 minute job -then they took my old battery back. All is good-back in action.