sounds like the alternator is overcharging the battery, use a multimeter in volts dc and measure the batter voltage before you start the car. shouldn't be normaly 12.65 - 14 volts.
With the engine running the battery voltage should not be more than 14 volts if so then i'd say that the voltage regulator in the alternator needs replacing, depending on make of alternator the bosch ones just screw in, you may or may not have to take the alternator off to take the regulator off, depends on if you can get at it.
Some alternators require the whole alternator to be taken off and taken apart to fit the regulator.
jim (vw mechanic).
September 2007