Hi recently bought a 530i Sport e39 2002, having a problem with battery going flat (or at least wont start car). There is some battery there locks,lights work etc. The battery is 18 months old (Bosch 90 amp)had it checked said it was ok. also the alternator was checked out. Not doing big mileage with the car,if not used for a day or so will start first thing but will not start later on. Heater motor seems to work ok. The resister was changed in 2008 also alternator. Any suggestions.