I have a '97 Saab 900 SE turbo. When it made a new rhythmic knocking noise under the hood I brought it to a few mechanics and here’s what they found: the alternator’s bearings are loose (that was likely the source of the noise) and it also needs the tensioner assembly (it’s vibrating visibly) and idler pulley replaced. The battery was also bad, with a 17.6 meter reading (handheld meter). The battery was still under full warranty and my mechanic said it probably had one, maybe two dead cells. Never had a problem starting or running.
My question is this: how likely is it that the bad battery caused the alternator/tensioner/idler problems?
Battery is not going to make the bearings in an alternator go bad.
And “17.6 meter reading” sounds like you mis-heard something. Battery voltage (which is about the only thing they would measure) is nominally 12.6 volts. “One, maybe two dead cells” would take it down to around 10.5 maybe 8.4 volts.
So the battery is dead. And MikeInNH is right: cannot have caused your particular alternator problems.
The battery is completely unrelated to the failure of the tensioner, idler, and alternator bearing failures. Had the alternator failed electronically rather than mechanically, there may have been a connection, but age and wear are the causes of this type of failure.