Piggy-backing them on mains USB chargers has got me a long way, but what I want right now is something I can strap an 18650 to, wake the thing out of deep sleep once every five minutes to squirt a reading over MQTT, and recharge once a month (or whatever. As long as it's better than once a week). I haven't seen a board with the lipo management onboard to make it easy, but I also can't imagine it's that rare a use case.