electrical problem? on 1991 Buick Park Avenue

I just replaced the alternator 2 days ago, was going fine till this morning.
The voltmeter on the car was staying around 9-10 for about 10 miles on the way home today. When I drove it a couple of miles earlier about noon. the voltmeter said about 9 then it shot to 18 and stayed there till I shut the car off a couple of minutes later/. Not sure If I just have a bad alternator or if it could be something else. Thanks for any help. Jason

1 answer 2 comments
Make sure your battery cables are clean and tight, because a loose terminal will cause a variety of problems
I just brought the new alternator I just bought back to Auto Zone and they said it was bad. I got a new one again and, so far so good....
Good job as even a new or rebuilt alternator can be defective right out of the box