This very could have been me today, but something told me (I read somewhere on RCU you should) to plug my ESV into the plane for a quick verification check even though I just charged it last night and it was first flight of the day. It read dead, no wait good.. no wait dead.. (Bad connection)
Apparently its been working loose over time, and my aircraft has a closed cell foam "battery box" that I fabricated to fit into the fuse where the original equipment used to go. No stress on the plug connection at all, in fact the battery wires fold back along the battery pack to fit in place. (original was soft foam padding similar to what I have and a plywood framer bolted over it to secure it all. Framer is still there, I just refabbed the foam insert to fit my new radio.)
Lesson learned, check EVERYTHING every time! (I Zip tied the plug together to be sure it cant pull out.) Almost lost it for another reason, but that will get its own post tomorrow when I am up to typing the story..