Qi2 vs Qi2.2: what the 25W upgrade changes for accessory brands

Two profiles, one magnet ring

Qi2 arrived with a magnetic power profile: a ring of magnets in the phone and a matching ring in the charger, so the two coils sit on top of each other without the user aiming. That alignment is why a Qi2 charger is more efficient than a flat pad, and why it holds a phone at an angle on a desk.

Qi2.2 keeps the ring and raises the power ceiling to 25W. The receiver still sits in the phone, the transmitter still sits in the charger, and the negotiation between them still decides the rate. What changed is how much energy the pair is allowed to move, and therefore how much heat the charger has to get rid of.

25W needs four things at once

A 25W number on a box is the product of four conditions, and missing any one of them drops the pair to a lower rate. The first is a phone whose receiver supports Qi2.2. The second is a power supply that can hold the input, which in practice means a USB-C PD supply rated above 25W. The third is a charger whose power stage and coil can carry the current without cooking the housing. The fourth is alignment: the ring has to hold the phone where the coils overlap, because coupling loss at 25W matters more than at 15W.

• Phone: the model list decides whether 25W is reachable at all.
• Adapter: a 20W brick makes a 25W charger a 20W charger.
• Cable: a thin cable with a high resistance drops the input voltage under load.
• Alignment: a thick case or a metal plate moves the coils apart.

Heat is the real story

Wireless charging loses energy twice: in the conversion on the way in, and in the coil gap. Those losses become heat in the charger and in the phone. Doubling the delivered power roughly doubles the loss, and the charger has to move that heat through a housing that a brand wants thin.

The visible result is derating. The charger measures its own temperature, reduces power, and the phone reports a slower charge or a temperature warning. None of that is a defect. It is the product protecting itself, and it is why the derating temperature on a specification sheet explains the user experience better than the peak wattage on the box.

What changes for an accessory brand

Four things move when a programme goes from 15W to 25W, and three of them are cost.

• The power stage: a larger controller, more capacitance and a bigger coil. The bill of materials grows before the housing does.
• The housing: more internal height for the shield and the spreader, and a surface that can convect. A sealed thin shell is the wrong shape for 25W.
• The adapter: either bundled and rated above 25W, or stated on the box as a requirement. Both are defensible, silence is not.
• The claim: name the phone models the 25W figure was tested on. A claim without a model is a support ticket.

Certification is the fourth. Qi2 and Qi2.2 are separate entries per model, and a 25W build needs its own certificate. A certificate file names the model, the profile and the power, and the model number can be checked on the WPC product database before a purchase order is signed.

How to test a 25W claim in an afternoon

Take a phone that supports Qi2.2, run it to about 20 percent, and put it on the charger with a USB-C meter between the adapter and the charger. Log the input power for 30 minutes and note the surface temperature of the charger every five minutes. Then repeat with a phone that does not support Qi2.2.

The first run shows the ceiling and how long it holds. The second shows the fallback, which is the behaviour most buyers will actually experience. A supplier who can send both traces has done the work; one who sends only the peak number has sent a marketing figure.

Where 15W is still the right answer

A mixed phone fleet, a bedside stand, a car mount in a hot market and any product where the charger is an accessory rather than the hero. In those cases 15W is cooler, cheaper and easier to certify, and the retail claim is easier to defend because every Qi2 phone can use it.

The upgrade from Qi2 to Qi2.2 is not a firmware decision. A 15W board does not become a 25W board with a setting, and a brand that plans to sell both should tool them as two products from the start.