Coconut Battery

I've used that app for several years. You do not have to synch your phone to the Mini for it to work. You simply connect the phone with a USB cable.

FYI: The useful history of your phone's battery condition that Coconut Battery maintains will be stored on your Mini, not the phone.

well their website suggests that being the functionality of the app

you may have to

If an app is not from the Mac App Store - Apple Support