i have a98 chevy astro van, i start the truck and it runs fine, but if i only let it run for a few minutes and turn it off an hour later it is completely dead. if i start it and drive it work a 15-20 minute trip it will start after sitting for upto 9-10 hours, i have replaced the alternator and the battery twice under warranty, any suggestions would be appreciated.
Check both ends of the postive battery cable. Check the battery end of the cable for damage from a previous battery that leaked acid,check the end at the starter to see if the nut securing it is tight (these nuts often work themselves loose). You should also check the big wire on the alternator for tightness,these nuts also come loose.
Finally get a complete starting/charging system test complete with parasitic draw test and a battery load test. These steps are all just standard checks/tests that a pro. mechanic would perform.
First guess would be a voltage regulator if your vehicle has one. Along with the previous post.