A customer left a set of keys on the charging pad and found them warm an hour later. The product had passed its certification tests and its foreign object behaviour had never been described, so nobody could say whether it had worked as designed or failed to.
Wireless charger foreign object detection: what has to be specified
Four items: the objects the function has to detect, the threshold it applies, the behaviour when it engages, and the recovery condition.
The function exists because a metal object in the field absorbs energy and becomes hot, and the design cannot see what the customer has left on the pad. The threshold balances two failures — refusing to charge a legitimate device and continuing to charge through a metal object — and the behaviour decides what the customer experiences in each case.
Specifying the four items turns a safety function into a testable behaviour, and it also gives the packaging and the manual something accurate to say.
Item 1: the objects to detect
The list should name what the market actually carries.
Keys, coins, a card holder attached to a phone case, a metal-bodied accessory, a paper clip and a ring are the common items; the combinations are what make the test interesting, because a card holder on a phone that is charging is a legitimate device with metal present. The list is written from the market and the use case rather than from the laboratory’s standard set, and each object is tested against the product’s behaviour.
Item 2: the threshold
A threshold that is too sensitive fails ordinary use; one that is too loose fails the safety function.
The threshold is set from the energy that would make an object hot enough to be a problem, with a margin that reflects the variability of the coupling. The design consequence is that the threshold interacts with alignment: a phone placed precisely couples more strongly and absorbs more energy, while a metal object in the same position may or may not cross the limit. The threshold should therefore be stated with the conditions it applies to.
Item 3: the behaviour when it engages
Stop, reduce, or continue with an indication.
The three behaviours are all used, and each has a different customer consequence. Stopping protects the object and can confuse a customer whose phone shifted; reducing output protects the object and keeps charging slowly; continuing while indicating gives the customer the information and puts the decision with them. Whichever behaviour is chosen, it is stated, because it is what the manual will describe.
Item 4: the recovery condition
What has to happen before charging resumes.
Recovery may be automatic when the object is removed, or may require the device to be repositioned or the charger to be power-cycled. The condition belongs in the specification because it decides the customer’s next action, and it also affects the test: a recovery that requires a power cycle is verified differently from one that resumes when the field clears.
| Item | What to state | How it is tested | Common gap |
|---|---|---|---|
| Objects | The items the market carries | Each object on the active surface | Only a standard test object |
| Threshold | The limit and its conditions | Objects at the alignment position | A threshold with no condition |
| Behaviour | Stop, reduce or indicate | The response recorded per object | Behaviour left undescribed |
| Recovery | The condition to resume | Removal and re-placement | An undocumented reset requirement |
How the function interacts with the rest of the design
Three interactions, and each is a design decision.
Alignment, because the energy absorbed by an object depends on where it sits relative to the coil; the thermal path, because the behaviour of the function is one of the things that keeps the surface within its temperature limit; and the protocol, because detection is an exchange between the two sides rather than a sensor reading on the charger alone. A design that answers the threshold without those three has specified a limit rather than a behaviour.
How to verify it for a retail SKU
Three levels, and the third is the one that protects the brand.
The certification test covers the standard objects at defined conditions; the characterisation covers the objects the market carries, including the case combinations; and the production plan states which of the behaviours, if any, is verified on a sample per build. Triggering the function on every unit is impractical, so the evidence comes from characterisation plus the delivered-power test that would reveal a design that had drifted.
What the packaging and the manual should say
Two sentences, and both should match the behaviour.
One states that metal objects should not be left on the charging surface, which is a user instruction rather than a technical claim. The other describes what the product does when an object is present, using the behaviour actually implemented. Where the behaviour is a reduction rather than a stop, saying so prevents the customer interpreting a slower charge as a fault.
FAQ
Does the product need this function to be certified?
The requirement follows the specification the product is certified against and the market it is sold in, so it belongs on the market list. Where it is required, the test and the threshold are part of the evidence. The requirement follows the specification the product is certified against and the market it is sold in, so it belongs on the market list.
Why does my charger sometimes refuse to charge?
Usually because something on the surface or inside the case is being detected as an object. The behaviour and the recovery condition are specified, which is why the manual can tell the customer what to do rather than describing it as a fault. Usually something on the surface or inside the case is being detected as an object, which is why the behaviour and the recovery condition are specified.
Can the threshold be adjusted after launch?
A firmware change can adjust it, and it changes the behaviour the customer experiences, so the manual and the test limits follow. A change made without that documentation leaves the product differing from its description. A firmware change can adjust the threshold, and it changes the behaviour the customer experiences, so the manual and the test limits follow with it rather than later.
How is the function tested in production?
Triggering it on every unit is impractical, so the evidence comes from characterisation across the object list and from the per-unit delivered-power test that detects a design that has drifted. Trigger it on a sample and during characterisation across the object list, and rely on the per-unit delivered-power test to detect a design that has drifted from its limits. Ask which test detects a drift in that behaviour.
Next step
Send the device list, the use case and the target markets to Wecent Group through the contact page. The reply states the objects tested, the threshold with its conditions, the behaviour when it engages and the recovery condition, with the packaging wording that matches. Related reading: wireless charger applications, charger quality control and OEM and ODM charger manufacturing. 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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