If the battery is good, it should take longer than 2 days (maybe two weeks), before you lose all the charge (clock and other miscellaneous small drains) without driving it.
If you lose all the charge within two days, it sounds like there is a short somewhere and the battery is getting drained by it.
If you drive it after charging it, and it loses charge while you are driving (though I have read in an earlier post that you can probably drive 70 miles or so on a good battery/bad alternator combo), then I would say that it is probably the alternator.
