01
First choose Source and Sink
Source supplies power; Sink receives it. Two DRPs resolve their attachment roles using their behavior and role preferences. There is no general rule that the device with the larger battery must become Source.
02
Roles can change after attachment
PD includes PR_Swap for a power-role exchange. DR_Swap changes a data role instead. Being a USB host does not always mean supplying power. Inspecting role messages can explain the actual outcome.
03
Use the controls that the products provide
Use an output-only bank port if the product offers one. Check any phone setting for reverse charging or powering a connected device. Reconnecting does not guarantee the desired result; consult both devices’ instructions and power indicators.