My V50 came up with a message 'Power unit in need of urgent attention'. on 2006 Volvo V50

the battery light also came on in red.
I stop the engine and restarted the car and all appeared fine.
the next day it happened again this time all the warning messages appeared then eventually the car shut down and would not restart. totally 'dead'
could it be battery or alternator?

Asked by for the 2006 Volvo V50
Hi, the same thing happened to me last year, it is the alternator that needs replacing. The car shuts down one system at a time in order to keep it running on a low battery, until it eventually shuts down completely.
A new alternator, and a charge to the battery, and all was well again. Only thing is that Alternators on the Volvo V50 can be quite expensive.
1 more answer
Exact same thing just happened to me in the exact same sequence.
Because I let the car die completely (like you did), I also fried the battery.
Looking at $1000 or there is a cheaper less trusted alternator that will put you at about $650 out the door.
Good luck.