BMW 3-Series E46 - 03 3.0 Flat Battery - Recharge or Replace - R.P.
Is the car's system charging the existing one effectively ? You may need to diagnose that before splashing out on a new battery. If memory serves me right there's a dashboard cheat that lets you check the Alternator output on the E46. If you have a charger may be worth giving it a jolt. Can't remember whether you can check the battery's condition on these to see if it will take a charge. (