Battery not charging on 89 ford mustang even with a battery charger?

about a week ago my battery went completly dead overnight first thing i assumed after i checked to see if all the lights we off was that there was a short but before i jump into all that hide and seek for the short i had the starter check because it was lagging well the results came back fine so i figured id try a new battery well that worked for about 24 hours and then went dead again so i had a friend jump the battery and the ground was messed up so we had to ground directly to the block and it charged right up so i figured at that point it was the ground connection to the battery and replaced the wire and cleaned all the terminals and yet again same thing is happening and not sure what else could be wrong except maybe the alternator but i did not think that could affect it unless the car was cranked any ideas ??? and thanks in advance