battery or altenator

hi wounderd if anyone can help . this morning on the way to work the battery / altenator flashes up on the dash got to work and when i finished started the car and nothing came up there was no mailfunction then on the way home after about 15 miles another mailfunction light came on saying device...

Hiya, i am new to this site as i have only recently purchased my merc, i was driving it last night when the battery/altenator light come on and it was saying "visit workshop". The altenator belt is loose when the engine is running but seems tight and normal when the engine is off... Any idea for...