There is a paid app called Accubattery pro on play store and it is undoubtedly the best app i have used so far. It is rated 4.7 out of 5.
What it does is it shows you stats of your smartphone battery based on your use, like screen on time, screen off time, charging time, wear and tear per charge, etc and gives tips to keep the battery healthy.
From research, it's been proved that charging beyond 80% has adverse effect on the battery, so it emphasises on that and lets you set a battery % at which, once charged smartphone will beep to let you know it's time to disconnect.
It works for me and i recommend this app.