I installed the beta on my iPhone 6 Plus. The battery died on me from excessive use. I plugged the phone into a wall charger for 2 1/2 hours To charge it. I went to pick it up, and it was still flashing a drained battery image at me. I moved my phone to the bedside table and plugged it into my MacBook Pro. After another almost hour, it finally came back on.
All I can say is, "don't let your phone die."
Also, I want to know what might have caused this serious power struggle for my phone to boot back up.