As of August 8th Apple activated a 'doormat software lock" on the iPhone Xr, Xs, & XS Max & may spread to other devices. The goal of this lock is to deter the service of Battery replacements that aren't serviced by Apple themselves. The issue the phone shows is that the battery needs to be serviced, & it also shows if you pull a genuine battery from an iPhone that is in working order. The proper way to check weather your battery is good or not is to fully charge it & document how long it lasts after fully charging it while you use it normally, from there you can decide weather a battery replacement service is necessary for you.
-- Below are some tips so you can both diagnose & extend the life of your battery when it is new. (Steps work for phones, tablets, & Laptops!)
So you got a new battery on your shiny new gadget, well first we need to do a few steps in order to ensure maximum capacity!
All our batteries are controlled by a microcomputer, which means they have an intelligent management system for the capacity.
1. After getting your new item back from us, Let the battery die, & get to 0%.
2. Only after is has been dead, go ahead & plug in the device, & leave it plugged in (preferably not disconnecting the charger) until it has reached 100%.
3. Only after it has reached 100% you can unplug the charger & use the device as normal until the battery has reached 0%
If you have done all the steps above & you still have poor battery life, it looks like you may need a battery serviced, which you can click here to find the price for your device!