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

Asked by for the 1991 Buick Park Avenue
Make sure your battery cables are clean and tight, because a loose terminal will cause a variety of problems
Good job as even a new or rebuilt alternator can be defective right out of the box
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....