Alternators last for decades if the electrical system is not abused .
The No 1 cause for alternator failure is running a mower with a flat battery
The bit that usually fails first is the rectifier / regulator .
Your blades stopping could be either not enough power to energise the clutch coil or a faulty clutch
If the battery is good it should show 12.6 V at rest
So as Rivets mentioned if the system is working properly you should see the battery voltage rise to between 13V & 15 V when the engine is running at full speed
When you turn the blades on the voltage should remain the same .
If it drops then further investigations are needed
And in order to give you good information we need to know which engine is fitted to your mower
