1994 Jeep Cherokee

I cant figure it out, I dont know my way around a jeep cherokee its my moms car and it keeps dying we have changed the battery twice and the battery cable connectors are new ( the cables are not) when they checked the alternator they said it was fine but im not sure they are right I keep going back to that. That new batteries seem to be ok and then all of a sudden nothing and not even a normal jump (with jumper cables) will do anything I actually have to boost it with a 50volt boost to get it started then it works good for a short time (two days or so) and then again out of the blue its dead. Im stumped it seems like something is draining the battery I dont know. Thanks