what does this wire do?

[67cst]

this is probably a longshot here but i know there is lots of traffic on the board so i thought id give it a shot, im having problems with my 78 datsun 620, my battery isnt keeping a full charge, at first i thought my alternator was hooped but then today i discovered i have a bad connection (the wire coming off the pos battery post)...does anyone know what it does? it disapears into the wiring harness and its hard to follow, could it be the reason why my alternator isnt keeping the battery charged? sometimes my truck is completely dead when i turn the key, if i pop the hood and jiggle the wire it fires up first try, so what im thinking is maybe this wire is working sometimes, like when i start the truck, then while driving its gets jerked or something and the connection is cut off causing the battery not to be charged..i know its a longshot but if this is the problem it will save 120bucks for a new alt....i have pics to show but right now i cant upload any of them for some reason, ill try later,

thanks guys