i own a old 1991 bmw. when i first got the car the radio and clock kept reseting and i would have to charge the battery every time i wanted to start it up. i just thought it was the battery so i replaced it. the new battery fixed the problem for a couple of weeks until a couple of days ago when it was slower to crank and start, but i didnt think anything of it until when i turned the a/c on, the car turned off then wouldn't start back up, and the clock & radio were reset.

it cant be the starter because it starts after i charge the battery, right?

the only thing i can think that could be the problem is the alternator, because if im correct the alternator recharges the battery (right?? or am i wrong?)
please give me some guidance.  

You might have a parasitic load. That or it could be that your alternator went bad. To check that out you need a volt or ammeter and check things like the battery (which you shown is not the problem), engine computer, clock, radio, and things that use electricity to keep the memory.

Here's a good website to check things out like these.

http://dacooley.com/RamStuff/1997/parasitic%20load%20explanation%20and%20test%20procedures.pdf