https://www.theverge.com/circuitbreaker/2017/9/22/16349480/apple-watch-series-3-qi-wireless-charging

At its annual hardware event this month, Apple announced a new wireless charging mat called AirPower that can charge either an iPhone 8, 8 Plus, or X at the same time as AirPods and an Apple Watch Series 3. At the time, we wondered if this meant that the AirPods and Apple Watch Series 3 would also support the Qi wireless charging standard, like the new iPhones. Japenese blog Mac Otakara tested the new watch on multiple Qi-compatible wireless chargers and found that it might be able to charge on certain, non-MFi devices.

The blog says newer chargers that Apple promoted at the event, including Belkin's Boost Up Wireless Charging Pad, didn't work. However, random chargers, like the Sharllen 10,000mAh Qi portable battery, triggered the watch's inductive charging. The blog doesn't know why certain chargers work, although it does mention that the watch has to be carefully arranged on the pad. We noticed a similar requirement with the iPhone 8 and 8 Plus when we reviewed them.