charging problem

charging problem for 1.6 diesel golf
when my battery is fully charged it will only start the car once maybe twice. when it is running the red alt. light gets brighter and dims when you slow down would the cause of this be the battery or the fan belt/alternator.

the light means your alternator isnt charging  

if it was your fanbelt youd hear loadsa squelling. so it sounds like the alternator firstly make sure all the wires are tight the replace the voltage regulator pack (about a tenner) its the small or big black plastic thing on the back and if that dont work a new alternator im afraid