Spotted by @horwitz, the iPhone's SAR Simulation Report reads:
In addition to being able to be charged by a desktop WPT charger (puck), 2020 iPhones also support WPT charging function at 360 kHz to charge accessories. Currently the only accessory that can be charged by iPhones is an external potential apple accessory in future.
The charging session only occurs when the phone is connected to an AC power outlet. However, due to the potential apple accessory in future and the phone being held in place by magnets, it is envisioned that customers may use the charging function in a portable use condition, for example, charging the battery while making a call or texting. Therefore, to be conservative we evaluate iPhone WPT transmitter as a mixed mobile/portable device. Future designs and accessories may support true portable use condition, with the host-client pair able to be placed in a pocket or backpack. In those cases, a body-worn exposure assessment would be conducted.
The most obvious future accessory that could benefit from reverse charging is Apple's wireless AirPods. Bloomberg recently reported that Apple is working on a new design for AirPods Pro that eliminates the stem. Additionally, Apple is said to be working on third generation AirPods that could arrive in the first half of 2021.
Reverse charging could also be useful to charge an Apple Watch, another iPhone, or even Apple's rumored AirTags.
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