Unhappy Mercedes Benz owner

In 2015 my wife bought a Mercedes Benz A Class 200CDI. With a 5year/100000km warranty. During July 2019 I took the vehicle in for the last service before the warranty expires (99 0000 km). I reported a certain problem at Stanmar Motors in George. The told me the problem is with the secondary battery. Because the vehicle was stil under warranty I was informed that they will replace it. Stanmar Motors told me that unfortunately they do not have that battery in South Africa and they will have to import it. They will let me know when the battery arrive. They never came back to me. In January 2020 I enquire. The request the km on my vehicle which at that stage was obviousel over 100000km. They now refuse to replace the battery which cost over R6000 . I bought the car new for more than R480 000. The was a written contract. Now Stanmar Motors as a sign of good gesture wants to give 50% and I must pay the other 50%. This is totally bad.