Starting to second guess myself here...I was always under the impression that alternator output should generally be 14 to 14.5 volts....dependant on battery state of charge.
Two days and two different family vehicles with dead batteries. Alternator output 13.6 - 13.8 volts with little increase in engine RPM. Im choosing to think defective alternator In both ?? Thoughts?

 

They may be getting weak in their old age. Wouldn't hurt to have them load tested. (can be done on the car.)