Battery light
 

I have a 2008 Mini Cooper base and the other day I had the battery light come on. Two days later, before I could look into it, the car lost battery power on the highway, no interior lights, no radio, no power steering. I pulled off the road and into a parking lot and muscled the steering into a stall but left it running because I felt it wouldn’t start if I turned it off. Shortly thereafter however, it started to misfire so I killed it. It wouldn’t start up again, just as I expected. The next day I walked a few miles to it to see if I could see a problem, but it actually started. I made it the few miles home before it did the same powerless things. I had the battery tested and they said it was fine, so I replaced the alternator. But the problem persisted so I have it sidelined while I try to find the problem. Anyone know what else it could be or what else to look for?