In my case I'm running FKB now on 5 Fire HD 10" tablets and 2 Android smartphones. All are dedicated HD+ dashboards.
After 3 years with the devices connected to power and with the batteries at 100% 24/7, and after having battery issues with some other devices, I've grown scared of having swollen batteries, so I have installed Zigbee switches inside their power outlets to keep the batteries in the 40-70% range.
Of all 7 devices, only one smartphone has a built-in option to limit the battery charge. The other one almost jumped off the wall due to a swollen battery 
FKB is running in background, the Hubitat driver fetches the battery reading every 5 minutes and decides if it needs to cut or restore power.
In my experience, the events actively sent by FKB to Hubitat were not reliable, they stop working after some minutes, or don't work at all when the devices restart for any reason. To wake them up, I needed to manually click the FKB icon on the tablet and then click on "Start using fully" button. I have tried MANY combinations of tablet/FKB settings. Maybe it is because in my case FKB is in background. That was despite having no battery usage restrictions, and despite the fact that FKB always responds to the HTTP requests, so the app is always running.
Other than that, I have just improved my bedtime experience by turning the screen off on my bedroom tablet with the same automation that turns all lights off.
I hope your device is better than my Fire HDs, I've tried using this feature, but it needs more processing power than my devices can provide, so they would sometimes freeze for many seconds.
