I have watched this play out in the office more than once. Someone drops a wallet case, cards and all, onto a wireless charger overnight, and the next morning the access badge simply will not open the turnstile. Then come the reissue forms, the registration desk, half a day gone. So let me put the conclusion first: bank cards are almost always fine. The ones that actually die are hotel key cards, access badges and older transit cards.

Question: will the magnets in a magnetic wireless charger erase a bank card? In everyday use, no. Magnetic stripes come in two grades. Bank cards and driving licences use the black high-coercivity stripe, which needs a field on the order of 4,000 gauss to wipe. Hotel keys, parking tickets and gift cards use the brown low-coercivity stripe, and 300 gauss is enough to scramble that one. Magnetic chargers and magnetic power banks typically measure 50 to 200 gauss at the surface, an order of magnitude short. On top of that, a modern bank card runs on its EMV chip and NFC radio, neither of which a static magnet can touch.

Question: then why do cards genuinely fail? The culprit is not the magnet but the alternating field generated while power is flowing. Sandwich a card between the phone and the pad and you have placed it squarely in the energy path. A reporter ran this comparison back in 2017. Cards left beside a powered charger, or even lying flat on the pad on their own for ten-odd minutes, all survived. Move them into the gap between phone and charger and after just fifteen minutes the cards were noticeably warm; the transit card and the stored-value card stopped reading altogether, and only the bank card still worked. A few millimetres of difference, an entirely different outcome.

Question: what does the manufacturer actually say? Apple's support document is blunt about it: do not place credit cards, security badges, passports or key fobs between the iPhone and the MagSafe charger, because the magnetic stripes or RFID chips inside can be damaged; if your case holds any of those, take them out before charging. That warning is not brand-specific, and it applies to virtually every phone that supports wireless charging. Apple marketing vice-president Kaiann Drance made much the same point years ago: most credit cards and ID cards will not be demagnetised, but disposable hotel cards deserve real caution.

Question: why do ID cards and access badges fall into their own category? Because they have no magnetic stripe at all. An ID card, most access badges and transit cards contain a coil and a chip, drawing power inductively from the reader to exchange data. What threatens them is not demagnetisation but an induced current large enough to damage the chip when they sit inside a charging coil's field. Chip damage is irreversible: the card looks perfectly intact, yet no reader will ever acknowledge it again, which is exactly why so few people trace the fault back to the charger.

Question: can a coin or a key on the pad actually start a fire? A properly engineered product has foreign object detection watching for that; a no-name one is a gamble. Metal entering an alternating field heats up through eddy currents like a miniature induction hob, and a coin absorbing energy for ten minutes can push past 80°C. FOD works on two lines of defence. Before charging, a Q-factor pre-check sends a faint probing ping and refuses to start if the coil's measured quality factor falls below threshold. During charging, the transmitter keeps an energy ledger, subtracting the power the receiver reports from the power it sent; when that gap widens abnormally, commonly at a few hundred milliwatts, it cuts output. Clause 9.6 of IEC 62368-1 Edition 4 pins the test down precisely: a steel disc, an aluminium ring and aluminium foil, each run in contact and at 2 mm and 5 mm spacing, with temperature rise held under defined limits. Asking a supplier whether that test was run tells you far more than the wattage on the box.

A few habits I stick to, feel free to copy them. Take the cards out of a wallet case or magnetic card holder before charging; those three saved seconds are not worth it. Keep hotel keys, parking tickets and gift cards, the brown-stripe family, in a separate pocket from the phone. Do not let keys, coins or paper clips pile up on the pad, and that includes foil-backed chewing gum wrappers. Well-aligned magnetic charging is actually the safer option here, since the phone sits flush and there is simply no gap for a card to slip into. Anyone with a pacemaker or ICD should keep magnetic devices at least 15 cm from the implant site, and beyond 30 cm while wireless charging is active, which is where the American Heart Association and Apple's own guidance agree. Finally, pull the magnetic power bank off before you tap to pay: the metal and magnets block the NFC signal. Nothing is broken, it is just being shielded.

Volume deployments deserve even more forethought. Once wireless charging goes into hotel lobbies, restaurant booths and shared-office desks, nobody reads the instructions; guests routinely leave a key card pinned under the phone, and the complaint lands on the operator's desk. Which is why FOD sensitivity, the alert behaviour when an object is detected, and a simple printed line on the panel reading "no cards on the surface" all belong in the design and customisation brief from day one. It costs next to nothing and beats replacing cards one by one. The same logic applies to buying a wireless charger, a magnetic charger or a three-in-one charging station for yourself: a certified unit whose foreign object detection has genuinely been tested may cost a little more than a no-name board advertising a big wattage number, but the difference is measured in the cards you would least like to replace.

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