ChargeCap

ChargeCap helps you increase the lifespan of your battery. It does so by helping you avoid charging to 100%.

Modern batteries are expected to have about 500 charge/discharge cycles before they start to degrade noticeably. This figure can be increase substantially if certain conditions are met. One of the most important conditions is to prevent the battery from getting fully charged to 100%. This is where ChargeCap can help.
When the device is charging, ChargeCap constantly monitors the battery level. When the battery charge reaches a predefined level (by default this is 80%), ChargeCap alerts you so you can disconnect the device from the charger.
Ideally ChargeCap would stop the charging automatically by itself when needed, however this is impossible without rooting the device. This is why ChargeCap only alerts the user when it's time to stop the charging.

Note: ChargeCap is designed to start automatically a few seconds after charging starts, however this functionality is not available on all devices. When monitoring is active there is a small icon on the top status bar, and a notification in the pull-down curtain. If monitoring does not start automatically, you have to start it manually by opening the app and pressing the "Start Monitoring" button.

ChargeCap is a free and open source application. It does not display ads or have any other monetization mechanisms.
Source code is available at https://github.com/liorhass/ChargeCap