How can I preserve battery lifespan on an always-on harvester/charging embedded device?
LiIon battery lifespan killers are temperature and voltage.
What cellphones do, which is stick the battery right next to the hot CPU and charge it to the max, is pretty much the opposite of how to maximize battery lifetime.
In fact, stopping charge at a bit lower voltage will reduce capacity slightly, but you'll get a lot more cycles out of it (table from above link).

If you don't require 100% capacity, this is an easy solution. Note partial charge/discharge cycles as you're doing don't cause &quot;memory effect&quot;, they're easier on the battery than full cycles. See link above for lots of details.