2002 Other Jaguar Models Overcharging

I have a 2002 Jaguar X Type 3.0 with automatic transmission. My battery died so I took out the battery and put a new one in (I had the battery tested at the local autoparts store and it was bad). When I put in the new battery I took the whole car to the autoparts store to be tested again (to make sure I did everything right) and the technician told me my car was overcharging at around 17 volts. I then took out the alternator and took it to one store that said it was bad. I took the alternator for a second opinion and they said it was fine. What else could be causing my systems to charge so high?