Qi charging review

Currently, I use a phone without Qi. I have found that with modern USB-C/USB-PD quick charging–20 minutes for 50%, full charge in an hour, I don’t need wireless charging anymore. But here are notes from an older phone without USB-PD where I did use Qi.

These notes are from using the Choetech Stadium triple-coil Qi charger. I used a non-thin phone carrying case akin to Otterbox. The Qi standard allows up to 5mm separation. After establishing charge start, I can take my phone nearly 1 cm away from the charging base while maintaining connection (likely charging time is longer in this unusual deliberate excessive 1 cm separation case).

Charging rate:

Assuming the Blackberry Passport has a 3400mAh battery, and that I discharge it till 10% energy is left (that would be after 1.5-2 days of use), and that the Qi charger is 70% efficient, I would expect the charging time to be on the order of 8.5 hours, neglecting post-Qi efficiencies and need to taper battery charge rate as cell fills. Using a recommended 2 A charger, the charge time is limited by the Qi 1.1 standard limit of 5 Watts or 1000mA. Misalignment and nonidealities reduce actual charge rate to less than 5 watts. Looking at data from my Blackberry Passport where I hadn’t charged overnight, and so came to work with 50% battery energy, I charged to 100% in about 2 hours. This implies about 850mA is making it into the battery on average. This seems consistent with the phone backing off the charge rate slightly to maintain appropriate temperature–the phone case retains extra heat from charging.

Placement:

I can’t place the phone diagonally on the Qi base, but even having the phone hanging nearly half off, till it falls off the base, the Qi charger keeps charging, albeit at reduced rate. Maximum charging rate occurs when the phone is aligned with the Qi base and completely covering the Qi base.

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