Q: Car Losses Power While Driving

While driving on the highway, traveling roughly 65 mph, my radio suddenly lost power. Seconds later, my warning indicators (all) became illuminated while my gauges all dropped to zero. The headlights also dimmed. Quickly realizing that something was wrong, I attempt to get off the highway via an exit ramp. Once at the end of the ramp and coming to a stop, my vehicle lost all power and died. I attempted to restart the vehicle numerous times, but couldn't get power of any kind. It's important to note that I was experiencing similar issues the day prior to this occurrence. I went to AutoZone and had the battery tested. They informed me that the battery had a reading of 20 and normal battery should read around 600, thus it was extremely low. I replaced the battery at this time. Knowing that the battery is not the problem, what could be the issue? Your input is appreciated. Thanks.

My car has 98000 miles.
My car has an automatic transmission.