battery died yesterday, read please

[fiveohpatrol]

i went out to my car yesterday and tried to start it with no success, it clicked one time and then nothing after that, it didnt even have enough power for the dome light/horn/radio/lights, nothing, so i got it jumped and went to autozone, they said they cant go outside after dark to test things so i took my battery out and brought it inside, it tested out fine, this morning it started fine, as well as after my classes, then i went back and had them test my alternator and it tested out fine, atleast thats what he said, i saw about 12.5 volts at idle (i do have pulleys so is that about right?) then he said when he put a load on it at idle it went low, but i think thats because of the pulleys, i dont know where to start, my positive battery cable is fairly worn, could that cause this problem?
again, my car was fine all day today, but i know that something had to cause it the first time and it will probably happen again soon, i just wanna catch it before that happens

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88 Notch, B&M Ripper, 3.73's, Ported E7's, Explorer int. w/ ported lower, pulleys, 1.7RR's, BBK headers, off-road H-pipe, flowmasters, subframes, 65mm TB, March ram air ,MSD coil, U/L control arms, 155lph f/p, Crane Adj.FPR, Nitto drag radials, best 1/4--> 13.76@101.4 on a 2.17 60ft