Warning message: “Error in the electrical system” battery or is it missing?

Hello dear VOLVO municipality, since yesterday I have the following warning message in the display of my V50: “Find out errors in the electrical plant – workshop” (not exactly quoted!). It also lights up the battery symbol in red and in the middle a red warning triangle. Today it went under & over: failure of radio, ESP, ABS and lastly even the power steering (the vehicle hardly allowed itself to be steered!!). Can this easily lie on an empty/defected battery or can actually the electronics be defective? I ask because I would change the battery myself. Otherwise the car goes naturally into the workshop. I have a V50 BJ 12/2004 with original battery. Incidentally, is the first problem with this vehicle. VG Tobias