Most of us plug our phones in to charge them. But there is also wireless charging available in some devices, which uses a charging station to transfer energy to the receiving device via electromagnetic induction.
So what is reverse wireless charging? As it sounds: the ability to turn the tables; for a wireless charging-capable phone to act as the charging station and be used to charge another wireless device.
However, it's a feature in its infancy, first offered by Huawei, with other flagships expected to follow in the future.
How fast is reverse wireless charging?
First thing's first, reverse wireless charging isn't quick. This is a low power solution that's for emergency top-ups really - like when your friend has forgotten a cable and is down into 'danger five per cent' territory.
Quick charge technology - i.e. when using a cable - has been progressing over the years. At best there's Oppo's Super VOOC, which can manage 10V/5A (50W). It'll charge a 3000mAh battery in about 30 minutes. Check out our rapid charging compared feature (and the accompanying video below).
Qi wireless charging isn't as quick, with 9V/1.67A (15W) possible - although most chargers at 10/7.5/5W, thus slower. Typically that'll mean around 120 minutes to charge a 3000mAh battery.
Reverse wireless charging doesn't have a set standard and Huawei hasn't released the specification. Needless to say, however, it's slower still. Plus the source device will have a limit on how much energy it can output from its battery before that depletes.
Which phones offer reverse wireless charging?
Huawei Mate 20 Pro
The phone debut reverse wireless charging and, to date, the only device to offer the feature. Huawei's Mate 20 Pro can charge any other Qi wireless device, irrelevant of brand, when the feature is set active and the two devices are paired back-to-back.
Samsung Galaxy S10 / S10+
Ok, so this is a bit of a cheat because the S10 series hasn't been announced yet. However, rumour suggests that it'll be the next handset line to receive reverse wireless charging functionality.
Which phones can be charged by reverse wireless charging?
If you don't own a Huawei Mate 20 Pro then you currently can't charge other devices using reverse wireless charging. If a friend or colleague does own this phone, however, then there's no limit on what can be reverse charged - so long as it's Qi wireless charging compatible.
That includes (among others not listed) the Apple iPhone 8, 8 Plus, X, XR, XS, XS Max; Samsung Galaxy Note 9, S9, S9+, Note 8, S8, S8+; Google Pixel 3, 3 XL; Huawei Mate 20, Mate 20 Pro; Xiaomi Mi Mix 3; Sony Xperia XZ3, XZ2; LG G7 ThinQ, V30; and Nokia 8 Sirocco.