You have probably done this: set your phone on the pad, only to find a coin or an access card sitting underneath, and the pad blinks and stops. The pad is not broken. That is it protecting you. The metal heats up by induction like a tiny resistor and can reach a dangerous temperature within seconds. The feature standing in the way is Foreign Object Detection, the most important safety function in wireless charging.
Before power flows, the charger checks itself
Before power flows, the charger runs a self-check. The transmitter coil sends out a simulated pulse and measures a value called the quality factor, Q, then compares it with the factory calibration. Q reflects how well the coil resonates; when metal gets close, the field is disturbed and Q drops. The standard — BS EN IEC 63563-7:2025, which adopts the WPC Qi 2.0 Part 7 verbatim — says a Q deviation past a threshold (around 8.76%) means a foreign object, and the energy is cut before charging even starts. One detail: if the phone itself is already on the pad and answers back, the charger will not misread that Q wobble as a foreign object, so you do not get a shutdown the moment you place your phone.
Once real charging runs, a second line of defense kicks in. The receiver (your phone) keeps reporting how much power it actually received, and the transmitter compares that with how much it sent. The gap is loss. If that loss clearly exceeds what the coil and "friendly metal" should normally absorb, and it stays above a threshold (say 500 mW for a sustained moment), something unknown is soaking up heat, and the charger throttles or stops. The finer method first builds a calibration curve at known power points (0.5 W, 5 W), subtracts the normal loss of friendly metal, and watches only the truly abnormal remainder. That keeps it sensitive without false alarms.
For buyers, the takeaway is simple: look for Qi certification. Only certified products have actually run this FOD and thermal test, with surface temperature held under 60°C. For higher-tier products a single method is no longer enough; Qi2's EPP and MPP tiers require both power-loss and Q-factor methods together. Good wireless charging is not about how fast it goes. It is about stopping the instant it should.
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