The battery meters have shown a tendency to need a couple of full charging cycles to calibrate themselves. It's possible that yours
thought it was fully charged when it was nothing of the sort, and when you left it charging for 9 hours it figured out what a full battery
really looks like.
That happened some with the 500 model in the early days too. It's probably why we kinda fell into the habit of suggesting that folks charge the things overnight before they tried anything else.
