With a volt meter across the battery and the car running, it says it is putting out 21V ? With it of the battery is reading 17V. Doesnt that seem kinda high?
Either your meter is screwed up or your going to fry that battery quick. First I'd find a fresh 9volt battery and check your meter.
Measure the fresh 9v battery with the meter. A low meter battery should not alter the voltage readings.
Yep new meter says 14.4V.. Whew I have had the other one for like 12yrs.. Think its the only tool Ive been able to keep that long...