I just had to replace my alternator as well. I ran on the battery alone for a few days, actually. The car completely died and the battery wouldn't take a jump but I brought it to an auto parts store who charged it and it worked fine. Replaced the alternator ($250 for Bosch re manufactured part or $60 for a used one from a junk yard, I went with the used one) and haven't had any issues keeping the same battery.