Christine Chan, iMore:

When you charge your iPhone in the Smart Battery Case, the iPhone will usually have priority when normal or fast charging. Once the iPhone reaches about 80 percent charged, the charging is split and allocated to the Smart Battery Case instead. However, if you use a power adapter that can provide more power, such as a MacBook Pro USB-C power adapter, then it can fast charge both the iPhone and Smart Battery Case at the same time. What has priority depends on how much power is being taken in.

Good to know.