The protocol is the conversation between the charger and the device. Most wireless charging complaints trace back to a part of that conversation rather than to a component: the device asking for a level the charger does not offer, the charger failing to detect an object, or the two settling at a rate neither intended.
Wireless charging protocol: the five exchanges that have to work
Detection, identification, power negotiation, ongoing control and foreign object detection.
The exchanges happen in that order, and a failure in any one of them shows up as a symptom the customer describes in their own words: it does not charge, it charges slowly, it stops, or it gets warm without charging. Working through the five is how a symptom is converted into a test.
The protocol is implemented in the transmitter and in the receiver, so the behaviour of a product is only half the conversation. That is why the device list belongs in the specification, and why the test plan names the reference device it uses.
Exchange 1: detection
The charger has to notice that a device is present.
Detection is a low-power exchange that wakes the charger when a receiver enters the field. Failures here appear as a product that does not start charging until the phone is repositioned, which customers describe as a faulty unit. The test is a placement sequence across the usable area with the reference device, recorded rather than eyeballed.
Exchange 2: identification and capability
Each side tells the other what it can do.
The receiver reports the power it is able to accept and the profiles it supports, and the transmitter decides what to offer. A mismatch here does not produce a fault; it produces a lower rate, which is the quiet failure that packaging claims tend to overstate. The test is a recorded negotiation with a named device, showing the level that was agreed.
Exchange 3: power negotiation
The agreed level, and the conditions attached to it.
Negotiation settles the operating point, and on a multi-surface or multi-coil product it also settles which coil is active. The behaviour should be recorded as a table: the level agreed for each device and case combination, and what happens when a second device arrives. Without the table, the design has a behaviour but not a specification.
Exchange 4: ongoing control
The charging loop continues after the negotiation.
The receiver asks for small adjustments as the battery fills, and the transmitter responds. This exchange is where thermal behaviour and efficiency meet: a design that cannot hold the agreed level reduces it, and the reduction appears to the customer as a charge rate that changes by itself. The derating threshold and the recovery condition are part of this exchange.
Exchange 5: foreign object detection
The safety function that decides whether charging should continue at all.
Detection identifies an object in the field that is not a device and stops or reduces the transfer. Its threshold balances two failures: refusing to charge a legitimate device, and continuing to heat a metal object. The behaviour should be recorded with the objects tested, because the objects a market encounters include items the design team may not have considered.
| Exchange | What it decides | Test | Symptom if it fails |
|---|---|---|---|
| Detection | Whether charging starts | Placement across the usable area | “It does not start” |
| Identification | What level is compatible | A recorded negotiation with a named device | “It charges slowly” |
| Negotiation | The agreed operating point | A table per device and case | “It stops when I add a device” |
| Control | Whether the level holds | Temperature rise and derating | “The rate changes by itself” |
| Foreign object | Whether charging should continue | The objects the market presents | “It gets warm without charging” |
How to specify the protocol behaviour
Four items, and each is a document rather than a claim.
The profiles supported, with the level each delivers; the device and case list used for the negotiation tests; the behaviour when a second device is added; and the foreign object behaviour with the objects tested. With those four, the marketing claim can be written from evidence, and a support query can be answered by a test rather than by an opinion.
What the production test can cover
Two exchanges per unit, three per build.
Detection and the delivered level at the alignment position are usually verified on every unit, because they are what the customer notices first. The negotiation table, the thermal behaviour and the foreign object threshold are maintained through sample testing and characterisation, with the plan stating the frequency and the limits.
Where protocol behaviour goes wrong
Three patterns, and each has a documentation fix.
A claim that assumes the highest compatible level for every device; a foreign object threshold set once and never tested against the objects the market presents; and a derating behaviour that is not stated, so the customer experiences a rate that changes without explanation. Each is corrected in the specification rather than in the firmware.
FAQ
Why does my phone charge slowly on a charger that claims a higher output?
Usually because the negotiation settled at a lower level: the device supports a profile the charger does not offer, or the case and position reduced the coupled power. The agreed level for that device is a measurable result rather than a specification figure. Usually the negotiation settled at a lower level because the device supports a profile the charger does not offer, or the case reduced the coupled power.
Why does charging stop when I put a second device on the pad?
Because the power budget is reallocated or the second device exceeds what the surface can deliver. The behaviour should be stated as a table so it is predictable rather than surprising. The power budget is reallocated or the second device exceeds what the surface can deliver, and the behaviour should be stated as a table so it is predictable rather than surprising.
What does foreign object detection protect against?
It stops or reduces transfer when an object that is not a device is in the field, which prevents heating metal items such as keys or a card holder. The threshold has to be tested against the objects the market actually presents. It stops or reduces transfer when an object that is not a device is in the field, which prevents heating metal items such as keys or a card holder.
Can the protocol behaviour be changed after launch?
A firmware change can alter the negotiation or the threshold, and it changes what the customer experiences, so the packaging text and the test limits follow it. A quiet change produces a product that differs from its documentation. A firmware change can alter the negotiation or the threshold, and it changes what the customer experiences, so the packaging text and the test limits follow it.
How is the behaviour verified in production?
Detection and the delivered level at the alignment position are checked on every unit; the negotiation table, thermal behaviour and object threshold are verified on samples with the conditions recorded. Detection and the delivered level at the alignment position are checked on every unit, while the negotiation table, thermal behaviour and object threshold are verified on samples with the conditions recorded.
Next step
Send the device list, the surfaces and the target markets to Wecent Group through the contact page. The reply provides the supported profiles, the negotiation table, the multi-device behaviour and the foreign object tests with the objects used. Related reading: wireless charger applications, OEM and ODM charger manufacturing and charger quality control. External references: WPC knowledge base, WPC Qi standard, Wireless Power Consortium, IEC standards, UL standards and certification and TUV testing services.
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