either battery neeeds replacing or the bike isnt charging and flattening the battery as you use it,you need to check the bike is charging by checking the voltage accross the battery terminals while the bike is running on the dc setting on a multimeter it should be putting about 14 volts into the battery when the battery is charged up if you check it with a flat battery it will give you a false reading.
adam bike doctor (of milton keynes motorcycle centre), January 2011