Is there a battery monitoring app that can send a notification when the battery reaches X %?
There is an app called Coconut Battery that does this but it is hardwired for certain values all less than 50%.  There is also a setting for number of minutes left. Load the app and go to Preferences under the coconutBattery menu item. This opens a panel and one option is Notifications. See image below.



CoconutBattery is free but there is a Pro version for $9.99. This will also report on iOS devices connected to your computer.