We have a 2000 Oldsmobile Alero and about a year ago we had a tow truck driver tell us that the battery would probably need to be replaced. I believe he said it was at 35%.
Yesterday we were driving and the battery light came on. We kept driving and all of the lights on the dash board came on and then flickered. The car kept going, but then all of the dashboard and radio lights and meters (speedometer and rpm and temperature) went off. We were on the freeway so we were trying to get to an off ramp and the temperature meter went above the highest heat setting. We finally pulled over and turned the car off and of course it wouldn't start back up so we got it towed to our house.
We took the alternator to o'reillys to get it tested and they said it was good. So we bought a new battery and put it in and then everything worked.
Today we were driving for only a few minutes and the battery light came back on.
We turned around because we didn't want to experience the same thing we did the day before.
What could this problem be?
Thanks for any ideas.

If you were driving along and the dash lights came on and flickered, you may have a bad connection.
I would check Battery connections,
Battery Ground, Starter connection from the Large Battery Cable.
And Check all Alternator connections.

Disconnect the negative battery cable, clean the battery cables and both ends of the alternator "B" wire with abrasive cloth. Verify that the regulated battery voltage is 14 to 15 volts, when the engine is idling. If not, replace the alternator. Check all under hood and under dash fuses with a test light or multimeter--this includes any "mega" fuses between the alternator and the under hood fuse box.

Check the ignition switch for wear, pitting, or melting, if the dash lights go out again.

I suspect the alternator is bad (brushes and/or voltage regulator).