Usually this means that you have an alternator fault...check all of the connections first, then check with a voltmeter across the battery terminals.  You  should see around 14.2 volts with the engine running - slightly less with things like the lights and heated rear window on.  A motor factor will happily do this test for you, and be even happier to fix you up with an exchange alternator.
                 John
                
 January 2008