Ford Ka - charging light continually on?

Ford Ka has warning light on all the time. It is an '03 model with Duratec engine and power steering. I have checked the charging system and it seems to be ok. It has been like this for a long time -3 months and the battery has remained fully charged. The belt does not seem particularly tight, but it has an automatic tensioner with no adjustment. It has occasionally squeeled, when really cold but no more than that. Has anyone got any ideas please?