I'm so sorry to hear about the loss of Charlie. He was really cute, and I love his babbling in the video that you posted.
Budgies do have a high body temperature, so it's always important to move them indoors of the weather is cold. Unfortunately, it's impossible to say why Charlie passed, but I am hoping that the good memories that you had together will help you, in time, to lessen your sorrows. We learn how to become better caretakers with each bird, and although it's hard to say bye to them, they really make us happier and vice versa!
Lots of hugs to you!