Foreign Object Detection, usually shortened to FOD, is a safety feature built into Qi wireless chargers that cuts power when it senses a metal object sitting between the charging pad and the phone. The charger constantly compares the energy it sends out against the energy the phone reports receiving. When too much energy goes missing, the only sensible explanation is that something metallic is absorbing it and heating up, so the charger stops. FOD is required by the Qi standard, which means every genuinely Qi certified charger has it, and it is the single feature that keeps a coin, a key or a metal backed card from becoming hot enough to burn a desk or a hand.
How Foreign Object Detection is used
FOD runs automatically in the background of every wireless charging session, so users never switch it on. The most common real world trigger is a card left in a phone case, especially a metro card, an office access card or a hotel key card, all of which contain a metal antenna coil. Coins are the second frequent trigger, particularly on a wireless charging desk mat where loose change can end up sitting on the charging zone. Metal ring holders and pop sockets stuck to the back of a phone case trigger it constantly, which is why phones with those accessories often refuse to charge wirelessly at all. On multi device chargers, a stray paperclip or a metal watch strap draped over the pad is enough. In each case the outcome is the same: charging halts, and on better chargers an indicator light turns amber instead of green.
Key characteristics
FOD is mandatory under the Qi specification, so any charger carrying genuine Qi certification implements it. It works by power loss accounting, comparing what the transmitter sends against what the receiver acknowledges, rather than by literally imaging the object. Some newer chargers add a quality factor check that measures how the metal disturbs the coil's magnetic field before charging even begins. Detection happens both at handshake and continuously during charging, so an object dropped onto an active pad is caught too. False positives are possible, and thick cases, metal flecked case finishes and misaligned coils can all trigger a stop when nothing dangerous is present. Response is a full power cut rather than a reduction, because a partial current would still heat the metal. Cheap uncertified chargers frequently omit or weaken FOD, which is the main safety reason to avoid them.
How to choose a charger with reliable FOD
Look for genuine Qi or Qi2 certification from the Wireless Power Consortium rather than a listing that merely says Qi compatible, because certification is what actually guarantees the feature was tested. Prefer a charger with a clear status indicator, since a light that changes colour tells you the charger stopped on purpose instead of leaving you to discover a flat phone hours later. On desk mats and multi device stands, check that the charging zone is marked or raised, because a defined zone makes it far less likely that coins and clips wander onto it. Magnetic alignment helps too, as a MagSafe style magnet ring holds the phone in position and reduces the coil misalignment that causes false trips. Buy from a brand that publishes certification details, and treat unbranded high wattage claims with suspicion.
Common confusion: FOD is not a fault
The most common misreading is that a charger which keeps stopping is defective. In almost every case the charger is working exactly as designed and is telling you something metallic is on the pad, usually a card in the case. The fix is to remove the object, not to replace the charger. The second confusion is between FOD and overheat protection. FOD is preventive and looks for metal that should not be there, while thermal protection is reactive and throttles or stops charging when the charger or phone itself gets too warm. A phone that charges slowly on a hot afternoon is hitting thermal limits, not FOD. Finally, FOD is not the same as device detection, which is simply the charger noticing that a phone has been placed on it.
Frequently asked questions
Why does my wireless charger stop when my card is in the case? Access cards, metro cards and hotel keys contain a metal antenna coil that absorbs charging energy and heats up. Foreign Object Detection sees the energy shortfall and cuts power to prevent that heating.
Can I turn off Foreign Object Detection? No, and you should not want to. FOD is a mandatory part of the Qi standard and cannot be disabled by the user, because without it a coin on the pad could reach burn temperature.
Does a phone case block wireless charging? Most plastic, silicone and vegan leather cases up to about 3 mm are fine. Cases with metal plates, magnetic ring holders or metallic paint finishes will trigger FOD and stop charging.
Do cheap wireless chargers have FOD? Certified ones do, uncertified ones often do not. A listing that says Qi compatible rather than Qi certified has not necessarily been tested, which is the main safety argument for buying a known brand.
How do I know FOD triggered rather than the charger failing? Look at the indicator. Most chargers switch from a steady charging light to an amber or flashing light when FOD trips. Remove any metal from the pad and charging should resume within seconds.
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