99 Ford van

I have a 1999 Ford 15 passenger van. My wife was driving it the other day and states she heard a noise but could not describe the noise. The battery went dead and the van died. We had the van towed home. I jumped the van and it started right up.
I drove the van down the road and the voltage gauge read normal. I then turn on the air conditioner and the voltage gauge dropped down significantly. I turned off the air conditioner and the voltage gauge returned immediately to normal. As I got closer to home I turned the air conditioner back on and the battery drained and the van died as I rolled into the driveway. I jumped the van with the air conditioner off and it fired right up.
What would cause this?

A complete check of the charging system is needed including the battery, all cables and clamps and the alternator. May be a poor connection, acid soaked cable, cracked batt clamp or the battery itself. Alternator mounting, wiring, alt output is also suspect.