Power System Failure message - 2006 Volvo XC70 (165,000 kms)

[AcadienenOntario]

Good evening,

A "power system failure" message (in brigth red colour in text message display) shown up early this morning (1st time, no prior trouble). I turned the car off and restarted it and the message did not re-appear. My volvo dealership told me that the alternator was not charging and the batteries was not keeping it charge - they both had to be replaced for the tidy sum of about $1500 ($750 for new alternator, 3.5 hours labour, batteries, coolant, drive shaft has to be taken down, getting it ready for fligth into space - kidding here). The service department indicated the car was likely running on battery and when this goes, the car will shut down. How do I know they are not blowing smoke up my you know what? Could it be a belt or tensioner, perhaps the voltage regulator? Any input would be great!

Thank you!