Either way if you have a 4 or 6 cyl, you could have gotten a bad alternator, which could work for a short period of time. I have had this happen before. Take it off and have the parts store test it where you bought it. If it comes back negative, you have a bad ground somewhere which is drawing off the battery, but I think it is the alternator by the sounds of your problem.
85' CJ7 Laredo, 383 GM, Turbo 350, NP 203. SOA, 2" frame lift, 34" TSL swampers, Dana 30 open, AMC 20 w/ detroit locker, 4:10's, 1 Piece Mosers, 35" boggers to abuse when needed.