As outlined in a support document, Apple says that the update will recalibrate the maximum battery capacity and peak performance capacity on the iPhone 11 models to address inaccurate estimates of battery health reporting that some users have encountered. Symptoms of this bug include unexpected battery drain behavior or in some cases, reduced peak performance capability. Apple says that the inaccurate battery health reporting does not reflect an issue with actual battery health.
Once the update is installed, iPhone 11 users will see a message in Settings —> Battery —> Battery Health about the recalibration process, which Apple says might take a few weeks.
“Recalibration of maximum capacity and peak performance capability happens during regular charge cycles, and this process might take a few weeks. The displayed maximum capacity percentage will not change during recalibration. Peak performance capability might be updated, but this might not be noticeable by most users. If a previous degraded battery message was displayed, this message will be removed after updating to iOS 14.5.”
When the recalibration is complete, the maximum capacity percentage and peak performance capability information will be updated. If the recalibration indicates that battery health has indeed significantly declined, users will see a battery service message.