need your alternator tested regulator in it could be shot ie not supplying enough voltage to the battery. [a good alternator can maintain a supply of 14.0 to 14.5 volts]
[put a D.M.M across the cars battery terminals and measure the DC output voltage,with the engine running] ANY LESS THAN 13V
REPLACEMENT alternator is required ,
most replacements from a motor factors carry a 12 months guarantee
wirecutterman
February 2009