battery or alternator?

[gittins]

hi everyone. question on my car... 82 300 sd. the first day i had it, it ran fine. the next day it wouldnt start up- i found out that it had a dead battery, one of the cells had dried up. my dads friend helped me put another working battery in the car. after we put it in, it started up and ran fine for a couple of days. i drove it around the block and parked it-thjen tried to start it up and it wouldnt go. my friend and i jump started it, and it started right up. i drove it home and parked it and it wont start now. the battery that we put in it was also from an old mercedes, and it was an old battery...but still worked, and it ran fine for a few days. do you think it was just a bad battery, or could it be an alternator problem? how could i tell? input would be appreciated. thanks!
zach