[Barnsnake]

When you got the 18 volt reading was the motor running?
Exactly what did you touch with the meter when you got the 18 volt reading? Were you touching the battery posts, or the cable ends, or something else?

If the alternator was turning and the battery cable was not "connected" to the battery post (due to corrosion, etc.), you could get a high reading everywhere in the car but on the battery itself.