Proper alternator testing really requires an ammeter, preferably inductive. Testing should not be done at idle; it should be atleast 2000rpms. The previously mentioned voltages should be achieved. As pointed out, a weak battery will absorb output (give a lower reading) but this is why the ammeter is important. The ammeter should show the rated output at test speed after correcting for load. Load is determined by turning key on engine off and viewing the reading. This reading must be added to the viewed output under load to get the actual output.
Your test do indicate that the battery is low and your charging is inadaquate at idle. Depending on the car this may not directly indicate anything. Early models charge poorly at idle.
Bosch Master, ASE Master, L1
33 years MB technician