Check your Voltage regulator brushes. They are a consumable item.
Just pull the VR off of the alternator and look at them and you'll know.
See Art Benstein's nice photo's.

The local alternator shop here quit soldering in brushes in Volvo (Bosch)
voltage regulators since they can get the whole unit so cheap.
They gave me a set of brushes free for my 740 alternator, and sold me a new one
for about $10.00 They say they need the Bosch part number and there's about six
different options for Volvo Alternators. Compare that to the wild prices at your favorite supplier for Volvo VR's.

Are you measuring battery voltage at the battery terminals?
Not to be confused with measuring at the battery cable connectors.
Put your meter directly on the battery anodes and come back with your
voltage readings with the engine off and with the engine running 2500 RPM
and the alternator charging.

Not all batteries are un-serviceable. I got an Allstate for my 850 two years ago from a good old fashioned local auto parts store and it had the usual features where you can check the electrolyte level and each cell with a hydrometer.
Fill it when needed only with distilled water and keep it from freezing when discharged and expect maybe eight years service life.

Good luck, Bill