Cranks the batteries die while driving
This morning my truck cranked up fine, it was a chilly rainy morning here in East TN maybe low 40's, drove to school just fine. My truck sat there all day and when I got in this evening to leave the batteries were dead. Had a professor jump the truck off and started on my way home. I got a little ways down the road and noticed my battery gauge dropping and the airbag light came one. So I turned my lights, the radio, wipers, and everything else that uses electricity. I got to the autopart store and the gauge was below 8 when I got there. I asked someone to check the batteries and alternator. The batteries were low but still good and the alternator checked out too. While I had the truck running while it was being checked, everything ran fine. I left the store and continued on my way home and after 20 minutes of driving the batteries started dying again! What is going on? Any best guesses? 99 F250 2wd with a 6spd.