Alternator/Battery ?

Hi people, the following problem. Wanted to re-park my V50 2.4 l / 140 HP Automat Bj 06/04 today and then turned on stand light. Before I turned off the engine, the red exclamation mark lights up and the control lamp of the battery – BC meant “electric system – maintenance urgent”. Engine off, after a while again – error occurred after a short time again. What can it be – alternator or battery? Vehicle currently has about 135 TKM on the hump. Has stood the last 3 days in the carport – ran flawlessly so far … had the feeling that he was at the MO (moved it there the last time) in cold condition uphill on a bit later (as usual) gas – but could have tauted me also (step speed, automatic, kickdown etc) … because then he ran well 100km without intermediate flaw … Someone an idea? What costs could be connected to the lighting machine? Thanks in advance. Best Gruesse, //**-peter