Greetings, I've got a 1980 245 with a B21F and M46, about 178K on the odometer. Yesterday the car worked perfectly. This morning, I went outside to start it, and it was completely dead; I would get a few dim bulbs inside the car, but it wouldn't even turn the engine over, and when I turned the key, everything died. Typical dead battery, I thought. So I popped my clutch and drove to work (20mi) without issues. However, when I arrived at work, I attempted to restart the car after that long recharge drive, and everything went dead again, as if the battery hadn't charged at all on the drive. Again, car was fine while driving, my voltmeter read in the middle like it normally does. I assume it can't be the alternator, considering once it starts, it's fine. I checked the connections between the alternator and battery, and everything looks normal. If it were a short, or seized starter motor, wouldn't a fuse have blown somewhere? My battery is about 5 years old, so it seems the obvious culprit, but can a battery go completely bad overnight? Yesterday the car started fine, turned over quickly like normal.

What are the possibilities? Thanks!