Related: If you have a Mac, you can check the health of an iPhone battery via the Console app when the phone is connected via USB. Search for the string "BatteryHealth", logged by the phone's Springboard process.
Stuff that I could see is the battery percentage, example:
``` default 08:44:30.015519 +0100 coreduetd CDDBatteryMonitor: received batterycallback, currentPercentage:94.555824! ```
You can install it via 'brew cask install coconutbattery'