Qi charging review

Despite initially being enthusiastic about 5 watt Qi, it proved to be too slow except for overnight use. USB-PD quick charging can yield charging in 20 minutes for 50%, full charge in an hour. Thankfully, Qi EPP such as the Pixel Stand can yield 10-15 watt charge with appropriate devices, which is close enough to wired USB-PD to be quite useful.

The Qi standard allows up to 5mm separation. The closer the phone (thinner the case) and better aligned the phone is, the faster it will charge up to the Qi limits.

Charging rate

Assume a phone with 3500 mAh battery discharged to 10% energy. Using a recommended wall AC adapter, the charge time is limited by the Qi 1.1 standard limit of 5 watts or Qi 1.2 EPP limit of 15 watts or so. Misalignment, heating and other nonidealities reduce actual charge rate. While non-EPP Qi can take 4 hours to fully charge a depleted phone, Qi EPP can take just over an hour, comparable to USB-PD.

If the device is getting too hot, it will backoff the charge rate to maintain appropriate temperature. Note the thermal insulating effect of the phone case retains extra heat from charging.