However, running the SE's battery down to zero is probably not good for it. Have you tried recharging normally since the Energizer experiment?
The inReach SE never got all the way to 0% in real life. The unit shuts itself off at some minimum charge; you can power on, and after booting, it says it doesn't have very much charge left and shut itself down again. That tells me there is still some charge left in the battery. Lithium polymer batteries do OK will full discharge cycles, but I wouldn't leave it fully discharged for extended periods. I keep mine on the charger between trips.
I've recharged normally since the experiment, but don't have any experience with battery life since then.
In the field, I would advise topping up the SE's battery nightly, a gentler approach.
You are probably right, but I worry about not knowing how much juice I have left in the Energizer charger.
Is it necessary to run the inReach SE day and night continuously? It seems that you could easily stretch the battery life by two by simply switching it off when you make camp in the evening and switching it back on in the morning.
Turning off the inReach SE at night will stretch battery life, but not by a factor of two. The unit goes to sleep after detecting no movement for 4 hours or 100 meters, and a power-on boot cycle seems to use about 1% of charge. Lots of settings and practices to play with to extend battery life: Bluetooth off, extended tracking interval (or no tracking), reduced screen brightness, sounds off, good sky view, turning the unit off at night, etc.