A batch charged correctly on the bench in the factory and charged slowly in the customer’s car. The coil was inside its position tolerance, the delivered power measured correctly on the reference adapter, and nothing in the outgoing inspection was outside the limit. The difference was temperature: at a higher ambient the design derated, and the derating condition had never been written into the specification. Reading the symptom before the board is what separates a five-day fix from a redesign.
This guide is part of our Travel & Wireless Charging. Read the full topic guide for the decisions that come before and after this step.
Wireless charger factory faults: match the symptom to the cause first
Five causes explain most wireless charging complaints, and each one produces a distinguishable symptom.
Coil position tolerance, the alignment reference, thermal derating in the shipping enclosure, the foreign object threshold, and input negotiation with the adapter the customer actually uses. Established from the symptom, they can be confirmed with a measurement rather than by changing components.
The order matters because the fixes are not interchangeable. Replacing the coil does not correct a derating threshold, and increasing the input specification does not correct a misaligned magnet. Each cause below states how to confirm it and what the fix changes.
Cause 1: coil position tolerance in the moulded assembly
Symptom: charging works on one unit and drops out on the next, with no pattern in the electronics.
The coil sits in a moulded pocket, and the position depends on the moulding, the adhesive or the clip, and the assembly fixture. A positional error of a fraction of a millimetre moves the coupling coefficient enough to change the delivered power at the top of the range. Confirm it by measuring the coil centre against the alignment feature on a sample of finished units rather than on a prototype, and fix it in the assembly fixture rather than by tightening the coil specification.
Cause 2: the alignment reference and the magnet arrangement
Symptom: the phone charges when placed carefully and stops when it moves a few millimetres.
Where the alignment is left to the user, the usable area is set by the coil geometry and the phone’s own coil position, which varies by model. Where a magnet array is used, the holding position is defined, but only if the array is concentric with the transmitting coil and the receiving magnet ring. Confirm it by mapping the delivered power across the surface with the target phone, then comparing the coupling centre with the mechanical centre. A mismatch here is a mechanical correction, not a firmware one.
Cause 3: thermal derating in the shipping enclosure
Symptom: full power for a few minutes, then a lower rate that never recovers until the unit cools.
Losses at the coil interface and in the switching stage become heat, and the only exit is through the housing. A bench test on an open board shows a lower temperature than the same design inside the shipping enclosure, which is why the derating threshold has to be measured in the final assembly, at a stated ambient, with the intended phone and case. Confirm it with a temperature-rise test at full load and a repeat at the ambient the market actually experiences. The fix is a thermal path, not a lower rating, unless the rating is genuinely overstated.
Cause 4: foreign object detection and the threshold
Symptom: charging stops when a metal object, a card holder or a key fob is nearby, or fails to stop when it should.
Foreign object detection protects against heating an object that is not a phone, and the threshold balances two failures: refusing to charge when nothing is wrong, and continuing to charge when something is. Confirm it by testing the objects the market actually encounters, including a card holder on the back of a phone case, and record the behaviour rather than describing it. This is a safety function, so the fix is a documented threshold change with a re-test, never a field adjustment.
Cause 5: input negotiation with the adapter in use
Symptom: the charger delivers its rating on one adapter and a lower rate on another.
The delivered power depends on what the input negotiates, and the adapter in the box is often the smallest that will work rather than the one used in the test. Confirm it by measuring the input voltage and current with the adapter the customer will receive, at each output tier, and by stating the required adapter specification on the packaging. A charger rated at the top of its range with an input that cannot supply it is a specification fault that presents as a hardware fault.
| Symptom | Likely cause | How to confirm it | What the fix changes |
|---|---|---|---|
| Unit-to-unit variation | Coil position in the moulded pocket | Measure the coil centre on finished units | Assembly fixture and adhesive process |
| Stops when the phone moves | Alignment reference or magnet position | Map delivered power across the surface | Mechanics, not firmware |
| Falls to a low rate and stays there | Thermal derating in the enclosure | Temperature rise at full load at a stated ambient | Thermal path or stated rating |
| Stops near metal, or fails to stop | Foreign object threshold | Test the objects the market encounters | A documented, re-tested threshold |
| Different rate per adapter | Input negotiation | Measure input and output with the shipped adapter | Adapter specification and packaging text |
Preventing recurrence: the three records
A position record, a thermal record and a threshold record keep the same fault from returning.
The position record states the coil centre against the alignment feature with its tolerance and the fixture used to achieve it. The thermal record states the temperature rise at full load in the shipping enclosure at a stated ambient, together with the derating threshold. The threshold record states the foreign object behaviour and the objects tested. Together they turn a discovered fault into a specification item, which is the only version of the fix that survives a second production run.
What to ask a factory when a batch is disputed
Ask for the three records before asking for a remedy.
A factory that can produce the coil position measurements, the thermal result and the threshold test is describing a process; one that answers with an opinion is describing a batch. The records also identify which of the five causes applies, which decides whether the remedy is a fixture, a mould, a test-limit change or an adapter specification. Without them, the fastest available action is to change parts, and that is the most expensive way to find the cause.
FAQ
Some suppliers say a slow charge is normal for wireless chargers. Is that true?
Some loss is inherent to the interface, and a lower rate than the label claims is not. The delivered power should be stated with the adapter and the case condition, and it should be measured on the finished assembly. A claim without those two conditions is not a specification. State the delivered power with the adapter and the case condition, measured on the finished assembly.
Can the charging rate be raised with a firmware change alone?
Sometimes, when the limit comes from a conservative control threshold rather than from the design. Where the limit comes from the thermal path or the coil coupling, firmware changes the symptom and not the cause, and the unit will derate again under the same conditions. Where the limit comes from the thermal path or the coil coupling, firmware changes the symptom and not the cause.
Is foreign object detection required in every market?
The requirement follows the standard the product is certified against and the market it is sold in, so it belongs on the market list rather than in a general discussion. Where it is required, the threshold and its test belong in the technical file. Where it is required, the threshold and its test belong in the technical file, so the requirement is confirmed for each market.
Why does the same charger behave differently on two phones?
Because the receiving coil position, the case thickness and the phone’s own charging behaviour differ between models. That is why the device list belongs in the specification, and why a delivered-power claim should name the device it was measured with. The receiving coil position, the case thickness and the phone’s behaviour differ between models, so a delivered-power claim names the device it was measured with.
Next step
Send the symptom, the batch details and the records you already hold to Wecent Group through the contact page. The reply identifies which of the five causes the symptom matches, states the measurement that confirms it and describes the process change that prevents recurrence. Related reading: charger quality control, the factory floor and wireless charger applications. External references: WPC knowledge base, USB Power Delivery specification, IEC standards, TUV testing services, UL standards and certification and ETSI standards.
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