You can do all the checking you want to the battery, but if the light is coming on, it has nothing to do with the actual battery. It's all alternator man. The battery light in the car gets triggered when you approach 12 volts. The alternator should produce 14 volts (or a tad less depending on if you're running head lights when you check it, or where you check it at). Odds are, your battery is still fine. The only reason it dies is because your car runs off of it alone while you're driving (instead of running off the alternator and getting charged, hence the battery light in your dash)

before you replace the alternator though, try a few things. First check all the connections coming out of the alternator. Next, when the battery light comes on, try pulling over and "tapping" the alternator with the handle of a hammer, or long screwdriver (with car still running) Sometimes, the brushes lose contact with the rotor in there, and will stop producing a charge. You can get it to start working again sometimes. Good luck.