I used to work on buses doing electrical repair, have also worked on cars for a long time.

if the alt is BAD the car will NOT run very long at all. Headlights take quite a bit of power too but the starter is by far the biggest.

my Boss was building a derby car, they were gutting a lot of things, and everything worked except the car would not start.

I FIGURED out the problem, guess what it was. The alternator? NOPE it was a bad ground, they grounded the battery to the body of the car, but it was not enough to get the starter to work, I ran a 4 gauge wire to the block, it popped right off.

I guarantee you you could not drive for an hour if the alt was that bad. what is drawing power while its running: the fuel pump, coils, computer, and whatever else you happened to have on, like heater, radio, lights. the battery would run down very fast.

maybe there is not enough of the story yet to be 100% sure what it is. but from what I have read, would not be an alternator issue. He said he just did some work to the car, and then thats when it happened.

and yes you need 12 volts to run a radio, lights, wipers and everything else. its not the volts. its the AMPS that matter the most.

are the lights dimming while driving, does it run poorly, and CEL on? we need more to go on.