Help Ford Ka battery light on

My gf's 2003 1.3 70PS Ka's battery light is on.

On start up the light goes out, then comes on 1-2mins later. Apparently the light has gone off while driving too. The car runs fine, starts fine and the light has been coming on, then going off a few days now.

Where can I start the fault finding? The last time I had warning lights up was on my old Fiat which a quick disconnection of the battery sorted out Could this method work lol?

Thanks in advance for any help