Your batt could be old and unable to keep the charge well. Happened to me, only my lasted 2 days. 3rd day I had to turn her on, otherwise she wouldn't crank without jumper cables. Replaced batt and all is well. The simplest test is what phitmein said, fully charge batt, disconnect (-) cable overnight or better two, and then see if it cranks. If not, the batt is goodbye. If it does, then ratmonkey's way - multimeter out & see who's missing the bedtime...
Beauty is in the eye of the beer holder
* 1994 ZJ (by VIN) with a 1995 engine (PO knows why)