Charging problem

Well me and sum friend went muddin today and on the way back my battery gauge dropped to 0 and check guages light came on..but starts up just fine and will sit at around 12 then drop down to 0 what could cause this???

vehicle- 99 XJ 4.0


*UPDATE* Alright well today I had to jump it and took it to o'reillys and they tested a bad battery so I was headed to where I work so I could get a discount and it started missing VERY VERY VERY BAD..and then died so we jumped it again and it didnt even wanna run but i managed to get it to a parking lot then we put a new battery in it 650CCA and 800CA-NEW
OLD-630CCA 730CA and still only stays at 12volts then drops to 0 so I guess its just running off the battery charge since the ALT isnt charging it my friend said try and spray carb or electric cleaner into the Alt to try and get sum mud and stuff out of it..a New alt is around 200 everywhere iv checked*UPDATE*

my friend has a rodeo and he said his did the same thing and he replaced both alt and batt and still does it...

It sounds like a gage issue. Start by disassembling and cleaning all your battery connections and grounds. The next step would be to pull your dash pod and clean the connectors there. If you really had 0 volts, your XJ would not run.