Ford Focus 2006 zx4 lost all power

Good Afternoon,

So for a long while my car battery light has been on. I had a friend do a continuity test on it to see if the alternator was charging the battery and he advised me that the amperage (or voltage) showed that it was. Today while i was driving all the electronics in my car shut off while driving, i was able to drive the car home and park it. Once i turned the car off i tried to turn it back on and the battery was fully dead. I was hoping some of you may be able to give me a hand diagnosing this issue before i take it to a shop or purchase parts.

Thanks