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If you're outside with a Apple iPhone 8 or iPhone X that needs a quick recharge, you can simply connect it to a supported phone like the Xiaomi Mi 6, if there's no AC power source around. The Mi 6 has the advantage of charging in both directions which allows it to act as a power bank and charging other phones as well as being able to be charged itself. This is unlike the case for the iPhone 8 and iPhone X which automatically just allows charging in when connected to a host device.  Here we use the iPhone 8 to illustrate this unidirectional charging behavior.

Since both the iPhone 8 and Mi 6 conveniently support USB Power Delivery fast charging, you can use the Mi 6 to supply power to the iPhone  through USB Power Delivery over the iPhone's Apple Lightning connector. However, this also comes with the price of draining the Mi 6's battery charge in the process if not plugged to any AC power.

To see how well the iPhone 8 works over USB Power Delivery with the Mi 6, we first link them up using a Apple USB-C to Lightning Cable in between when the iPhone is 30% charged while the Mi 6's battery is 89% full. When connected to the Mi 6, the iPhone only consumes power provided over USB Power Delivery meaning it can't output power to other USB Power Delivery based devices. Under the Mi 6's phone settings, we then select the option to enable USB charging.

When observed using the Granite River Labs GRL-USB-PD-A1 analyzer, a single 7.5 watts (5 volts @ 1.5 amps) power profile is repeatedly displayed by the Mi 6 over a Data Role Swap during the initial stage of USB Power Delivery communications.

After this 7.5 watts declaration goes on for a while only then the iPhone 8 finally starts to request this power, which is accepted by the Mi 6.

The iPhone is again seen to still establish the same 7.5 watts power contract once completing Discover Identity and SVIDs transactions with the Mi 6.

Once connected to the 89% charged Mi 6, the iPhone 8 starts pulling around 7.5 watts (5 volts @ 1.5 amps) when the iPhone is off or asleep at 30% battery. When reaching the first 30 minutes, while the iPhone is active, the current starts to form a 1.3-1.5 amps band which then gradually drops down to around 0.8-1 amps after an hour of charging at 5 volts.

In the next 6 minutes, the current pulled can be seen to decrease further to around 0.7 amps, while still at 5 volts.

When inactive, the iPhone 8 can get around 7.5 watts charging with the Mi 6 which is a bit faster than using the Apple 5W USB Power Adapter (5 volts @ 1 amps) originally shipped together with the iPhone.

Interestingly, this 7.5 watts of charging from the Mi 6 is higher than the 4.5 watts of charging that the iPhone 8 gets from the Google Pixel phone as seen in this review.