According to Apple, you can charge your iPhone and the MagSafe Battery Pack by attaching the battery pack to your iPhone and connecting your iPhone to a power source.
iPhone 12’s reverse wireless charge doesn’t work with everything…yet
This is solid proof that the iPhone can reverse charge the battery pack wirelessly. We tried to reverse charge our Apple Watch and wireless earbuds by placing them at the back of the iPhone, but both devices did not charge at all.
This means the battery pack is the only compatible accessory for the iPhone 12’s reverse charging as of now — many are hoping that Apple will ‘activate’ the phone’s ability to reverse the AirPods or Apple Watch in the near future.
MagSafe Battery Pack charges the iPhone faster when using a higher watt adapter
On a side note, you can also charge the battery pack and your iPhone simultaneously by plugging the former into a power adapter. If you use an adapter with at least 20W, your iPhone can get up to 15W of charging power. Without a power adapter, the battery pack only charges the iPhone at 5W.