Just to add a casual observation. I have a Xenon running with a solar panel charging the battery and it will have been out there for 2 months on Friday. While it did have some sort of hangup and needed a poke last week, it has otherwise been fine.
What I did notice is the battery charge started at 4.04 vaults. Over the first 3 weeks, that gradually reduced down to 3.98v. (I only ever trust the battery level reading at night when there is no power being generated by the solar cell.) After that, the battery power stopped going down, making the graph I output rather dull.
3.98v seems to be the spot where it will not charge the battery any higher. I have seen this with both a 3.7v LiPo and an 18650.
2 months of running and seemingly stopped using battery power is pretty impressive. I take a few readings then deep sleep for 5 minutes to conserve battery life.