Thanks for the responses. My wife has been driving the X5 for over three years now with similar driving habits all three years. The problem just showed up for the first time about 3 months ago and started increasing in frequency. As my wife no longer trust the truck, I have been driving it for the past few days without incident. The road trip is this weekend which will be the acid test.

I understand short trips might not fully charge the battery, but that does not explain the battery going dead while driving. If the battery had enough charge to start the vehicle, the alternator should be sufficient to keep it running and provide excess current to the battery to at least maintain its currently level of charge. It was not happening, hence my decision to replace the alternator.