BMW 525 TDS Starter/Battery Problems

I have a 1992 deisel BMW 252 TDS and I have been having considerable problems with the starter/battery. The car won't start if I don't drive it for two days and if I drive it for three days, on the third day it will not start. I recently replaced the battery and have noticed the problems since that time. I was told that I need to drive the car at least 10 km/20 miles a day to keep the battery charged. This sounds strange. I have also been told that I should check the crank capacity on the battery to make sure it is at least 750. Does anyone have any suggestions as to why the car won't start after not being driven for a few days at a time? Thanks for your time.

maybe the alternater is bad? the battery should hold a charge everyday if its driven even on short trips