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When the cars running,the alternator should charge the battery to around 14 volts.If you then turn the lights on full beam(still with the engine started,then the alternator should charge to around 13 volts(engine ticking over).If the voltage gradually drops,then the alternator is suspect.
The battery needs to be tested under load to see if it is OK.With the ignition on and headlights on,check the voltage given by the battery.Anything less than around 11 volts,then the battery is U/S.
Also,check the battery connections(both ends),especially the earth connections.These have a tendency to corrode and cause problems similar to the ones your having.The tell tell sign is a 'greening' of connections.If this is present,then they will need to be removed and cleaned to ensure good contact.If you have a multimeter,you can do a resistance check between the engine and battery negative terminal.If there is any resistance present,then there is a connection problem.
Also,make sure the starter motor connections are good.Again,these have a tendency to corrode and cause starting problems.
Hope this helps,and keep us up to date with developments.
I'd rather push my Alfa than drive a BMW....
Currently in Lagdti's garage:
2004 Saab 9-5 HOT Aero and a 2001 Audi A3 1.8 Turbo