When wallets runs out of juice: NFC with external power

One of the common refrains of criticism about NFC payments on mobile devices is what happens when the phone runs out of power. The common response is users can always fall back on their “regular wallet,” with the implicit assumption that nobody is foolish enough to rely only on their mobile wallet just yet. But unless the payment options are identical between their plastic and mobile incarnations —which is rarely the case, because of tokenization and virtual cards— that strategy has limitations. For example refunds are usually required to go on the same credit card used in the purchase. Worst case scenario is public transit, when the same card has to be used on both entry and exit to the system as in the case of BART turnstiles in the Bay Area. If the consumer were to tap-in with their phone and then the phone runs out of battery before they get a chance to tap-out on the other side, they have some explaining to do.

But it turns out that running out of battery power is not necessarily an impediment to making payments. The trick is a feature known as “powered-by-the-field” or PBTF for short. This is not exactly a new capability: consider how ordinary  plastic smart-cards operate. There is no internal battery powering most of these devices, which also explains why they do not have persistent clocks to track time. Instead they draw current from the reader to power their internal circuitry for those few seconds when the card is being called on to perform some cryptographic task. Initially that power was conducted via the brass-plate which comes into direct contact with the corresponding metal leads inside the reader. Later RFID cards were introduced, which can operate without direct metal-on-metal contact with reader surface– hence the name “contactless.” These cards instead draw power from the induction field generated by the reader by using an antenna that is typically placed around the outside perimeter of the card to maximize its surface area.

Since NFC is effectively a specific type of RFID technology, it is not surprising that the same principle translates to contactless cards used for tap-and-pay purchases. What is less obvious is that this capability carries over to their mobile incarnation, with some caveats. We covered earlier how NFC-equipped phones can be viewed as having the internals of a dual-interface smart card embedded inside, called “secure element” in these scenarios. (This is an over-simplification: cards usually have a simple analog antenna, while mobile versions are equipped with more complex NFC controller chips that support additional NFC functionality.) That same hardware can draw enough power from the external field to operate the complete payments and similar NFC scenarios such as physical access or public-transit. In theory then running out of battery is not a problem– although consumers  still need a battery present because the antenna is usually incorporated into the battery itself.

So why isn’t this feature advertised more prominently? Because it is optional and typically disabled for good reason. (Google Wallet designers also opted for disabling PBTF mode.) Next post will go into why conducting payments this way can be problematic.


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