Same issue, and voltage is steady during the dive, but jumps back up post-dive.
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I had this same thing with a new perdix. First few dives it would go to about half or a little less battery and then rebound. Replaced with my own battery and had no dip on my most recent dive trip.The energizer lithium battery that game with my perdix 2 seemed erratic on voltage. Replaced with new and seems to have solved the issue. I would start there.