Citroen Xsara Picasso battery light on blowers won't work

Hi all have had this problem for a bit my battery light comes on when you start car saying fault battery charge !! With this the blowers won't work fag lighter and buttons on digital display it will only turn Airton off and on after a bit the battery light goes off and blowers come on for a bit then usually go off again .. have had battery and alternator checked both doing what they should also run a obd2 and no faults came up checked fuses too help much appreciated x