I can't speak for BMW but we sell alot of car and truck batteries with very few returns of those that do come back the vast majority are faulty do to the use of them ie:not been used for long periods causing sulphation of the plates,vehicle does not start very well meaning longer than normal cranking times.
If a battery has a fault on 1 cell only it should be warranty,2 cells possible warranty anymore cells than 2 with problems the battery has been damaged and is not a fault in manufacture.