Alright so I've been through this same thing a few months ago and never found the solution, the Jeep just decided to start back up last time.
I removed the alternator and had it tested a month or so ago, it tested good, so did the battery. Fast forward to this past Friday. I went out to start it up and there were no interior lights. Hooked my dad's vehicle up to it with jumper cables... they did nothing. I let it sit for about ten minutes and went back out to check it. I opened the door and the interior lights came on and it started right up first try. It died again 15 minutes later at a gas station and I let it sit for a few and it started again. As I got back on the road, the volt-meter on the gauge cluster spiked all the way to 19 (max) and stayed there. When I got back to the house later that night, that was it for the XJ. It now has no life. All fuses/relays are good and so are the battery connections.
I removed the alternator and had it tested a month or so ago, it tested good, so did the battery. Fast forward to this past Friday. I went out to start it up and there were no interior lights. Hooked my dad's vehicle up to it with jumper cables... they did nothing. I let it sit for about ten minutes and went back out to check it. I opened the door and the interior lights came on and it started right up first try. It died again 15 minutes later at a gas station and I let it sit for a few and it started again. As I got back on the road, the volt-meter on the gauge cluster spiked all the way to 19 (max) and stayed there. When I got back to the house later that night, that was it for the XJ. It now has no life. All fuses/relays are good and so are the battery connections.