Why use the Mac?
iOS already shows battery health in Settings on supported devices. The Mac becomes useful when you want the raw capacity, cycle count, temperature, voltage, live power, or one history that also includes the Mac and iPad.
A normal sandboxed iOS app cannot read all of those private battery fields. A trusted Mac, however, already has a diagnostics relationship with the device through Finder.
The one-time trust step
Connect the device with a data-capable cable, unlock it, and tap Trust. This creates the pairing used by Finder and other Apple development tools. The first pairing cannot be completed wirelessly.
Reading over Wi-Fi
In Finder, select the device, open the General tab, enable “Show this iPhone when on Wi-Fi,” and choose Apply. When the Mac and device are on the same network and the device is available, the trusted connection can work without the cable.
What WhatBattery can show
WhatBattery Pro's connected-device view maps the device battery into the same model used for the Mac: health, cycle count, charge, temperature, voltage, live power, and time remaining when the device reports it.
It also records long-term health and cycle history while the device is available, which makes it possible to compare more than one snapshot.
The compatibility trade-off
The diagnostic relay is an undocumented Apple interface. Its fields may change between iOS versions. A responsible reader should validate the returned capacities and health range rather than assuming every dictionary is still shaped the same way.
WhatBattery reports “Battery not readable” when the response is missing or implausible. That is less exciting than inventing a number and much more useful.