No power, no start

I have a 2001 XJ Sport 4.0 4x4 auto, 106,xxx miles. It's been running fine, no problems starting or idling or anything. I go to start it this-afternoon, and it clicks and nothing happens. All the gauges and electronics went dead. The light seemed dim, so I hooked up jumper cables and it started no problem. I let it run a sec, then disconnected the neg battery cable, and it kept running fine. So I hook everything back up and go inside to get a phone number (left it running), and the thing died. I hooked the cables back up, and it barely started, but then wouldn't stay running unless I kept my foot on the gas. If I let off it died immediately. Any ideas? I've never seen anything like that.

Thanks in advance

Your battery is toast. Go get a new one. I like my red top, but others will tell you it is an unnecessary expense.

wouldn't it run off the alternator though?

Your alternator may be toast also.

The alternator may be shot as well, but not necessarily. The jeep will not run off the alternator at idle if the battery is bad.

Ok, I will try the battery first thing and update

Thanks guys

Once you get it running, take it to autozone or whereever first thing and have them check the alt.

The odds are good (particularly good if that's still the OEM batt) that the battery just got old, but there's no sense in killing the new battery if it is the alternator.