I prefer gSam monitor as you can set battery above alarm, battery below alarm and temperature above and battery overheated and battery over voltage.
I have used this app for over 5 years on all my android devices with captive battery`s, check in settings/alarm to set alarm levels, I use 80% high 20% low.
John.
Awesome John…I like your app even better 
Uninstalled mine and installed GSam
Thanks
Have your batteries lasted any longer than they would have if you had not used it?
(If you’ve compared battery life before installing it for example)
Devices with LiPo batteries like phones, laptops, and tablets actually never charge to 100% of the possible battery capacity. The manufacturers test and set the highest charge level to provide the best balance between use-time and battery life. When the device says 100%, that means you’ve reached the upper limit they have set, not that the battery can hold no more power at all 
It also bears remembering that a LiPo batteries lifespan is also related to the number of charging cycles it goes through- the fewer charge cycles the better for it’s lifetime. So if your undercharging results in more charge cycles you’re not likely to gain anything.
These folks have their entire business staked on how well their products perform for the average user and they’ve got top-notch engineers figuring this stuff out, so you’ll be hard-pressed to beat them at their own game. If you’re a non-typical user it may be worth trying.
Phil