This has happened twice over the past 2 months. We have come to the vehicle after about 45 minutes of being parked, and everything seems to work fine until you actually begin to start the engine. The engine doesnt even try to turn over like there is a completely dead battery, but all the lights are bright and nothing has been left on that would reflect or suggest a dead battery. There are no warning lights other than the normal lights prior to starting the vehicle. The first time this happen I did everything I could think of....even jumping off the battery and nothing worked. After about 20 min. It started fine with no issues for about a month. Yesterday it happened the same way but jumping it off worked. I had the battery and alternator checked and they showed 13 volts and 100% on the battery. What could this be?