2001 Mitsubishi Galant Alternator

Hello, I recently changed out the two drivebelts on my 2001 Mitsubishi Galant. This was my first time changing belts. On the Galant there is no tensioner so I had to loosen the alternator to get the belts off as my Hanes instructed. I then replaced the belts and drove to work. When I drove on the Highway everything was fine. When I got off on the side streets all my dash board lights came on and went dim and my gauges all dropped to zero. I thorught that it might be the alternator so I had it tested and it passed. I then took my battery in and was told that I needed a new battery. Everything seemed great until I returned to the side street and the battery light came on and dimmed. I got on the highway and everything returned to normal. Once I got off the highway the lights came on again and began to dim. Luckly I made it home. Is this an issue with my belts? Night the right tension? Thank you so much for your help?