According to Apple, some Smart Battery Cases can experience charging issues, including cases that will not charge or charge intermittently when plugged into power, or cases that will not charge an iPhone or charge it intermittently.
Affected Smart Battery Cases were manufactured between January 2019 and October 2019. Apple says that this is not a safety issue and Apple or an Apple Authorized Service Provider will replace eligible cases for free.
All Smart Battery Cases designed for iPhone XS, XS Max, and XR manufactured between the above dates are eligible for replacement. Apple says that no other iPhone Smart Battery Cases are a part of the program, which means no iPhone 11, 11 Pro, or 11 Pro Max battery cases will be replaced.
Apple says that customers with an impacted Smart Battery Case can find an Apple Authorized Service Provider or make an appointment at an Apple retail location to have their case replaced. Smart Battery Cases will be examined prior to service to verify that they are eligible for the program.
Affected cases will be disposed of in an environmentally friendly way and the program covers Smart Battery Cases for two years after the first retail sale of the unit.
Since the Smart Battery Cases launched in early 2019, there have been complaints over various failures and issues with charging. At one point in mid-2019, the Smart Battery Cases were unavailable for purchase and backordered for over a month, a stock issue that could have been related to ongoing charging problems.
https://www.macrumors.com/2020/01/10/apple-battery-case-replacement-program/