How Does Wireless Charging Work in a Wearable?

Concept render

Wireless charging works by magnetic induction. An alternating current in a coil in the pad makes a magnetic field, and a second coil in the device turns that field into current for the battery charger. Qi, from the Wireless Power Consortium, adds a control link so the device tells the pad how much power to send and when to stop.

Most guides explain wireless charging for phones. A wearable has a smaller coil, a smaller battery and skin right next to the hot parts. This guide covers the physics, what Qi and Qi2 mean for a small product, and how to choose between Qi, a magnetic puck, pogo pins and USB-C. Figures come from the Wireless Power Consortium (WPC), Texas Instruments and the FCC.

This is an engineering summary. The latest Qi specification is available to WPC members, so we quote no threshold from it.

How does wireless charging work?

Wireless charging uses two coils. The WPC describes it this way: an alternating current in the transmitter coil makes a magnetic field, and that field induces a voltage in the receiver coil. The device uses that voltage to run or to charge its battery (WPC, as of November 2026).

Coupling: why distance and coil size matter

Not all of the field reaches the receiver coil. The share that does is the coupling factor, k. The WPC says k runs from 0 (coils independent) to 1 (all flux passes through the receiver coil). It depends on the distance between the coils, their relative size, their shape and the angle between them. The WPC calls values from 0.3 to 0.6 typical, and says an axial offset lowers k (WPC coupling factor, as of November 2026). Transfer efficiency depends on k and on the quality factor Q of the coils (WPC).

The control link inside Qi

A Qi pad does not push power blindly. The receiver sends data packets back over the same coils by load modulation. TI's 2014 article describes the coil voltage and current being modulated at a 2 kHz rate, and says a loss of communication ends power transfer. TI also notes the Qi feedback loop is slow and may need up to 100 ms to change its operating point, which matters when your load jumps (TI SLYT570, Analog Applications Journal, 2Q 2014).

The receiver output is not the battery input. In TI's wearable reference design, the receiver output feeds a separate Li-ion charger IC, which sets the charge current. The design takes 5 V from USB or a Qi transmitter and sets a fast-charge current between 5 and 300 mA (TI SLUA748A, revised April 2016). If the device also charges through a USB-C port, USB-C power delivery for device design covers when a plain 5 V sink is enough.

What do Qi wireless charging and Qi2 mean for a wearable?

Qi is the WPC standard for the link between transmitter and receiver. The WPC lists versions 1.0 in 2010, 1.1 in 2012, 1.2 in 2015, 1.3 in 2021 and 2.0 in 2023, and says the latest version, v2.3, is available to WPC members (WPC specification page, as of November 2026).

Qi2 is the magnetic version. The WPC says Qi v2.0 has two profiles: the Magnetic Power Profile, branded Qi2, and an enhanced Extended Power Profile without magnets that keeps the Qi logo (WPC news release, November 15, 2023). The magnets give the alignment that phones on flat pads often lack.

For a wearable, the power headline matters less. TI describes typical wearable batteries of 100 to 300 mAh and a receiver output in the range of 5 V at 100 to 250 mA, compared with up to 5 V at 1 A for a standard WPC 1.1 receiver (TI SLYT570). A ring or band needs far less than the 15 W of Qi2. The questions that decide the design are coil size, alignment, heat and certification.

How small can the wireless charging coil be?

Coil size and alignment

TI's figures show the trade. A standard Qi design delivers 5 W with a 50 mm coil. TI's wearable reference design uses a smaller 30 mm coil with adjustable output from 500 to 1,500 mW (TI SLUA748A). TI's article says smaller coils are possible if DC resistance does not rise too much. More turns of thinner wire raise resistance and lower efficiency (TI SLYT570).

Small coils are also less forgiving. TI writes that coupling is best when the coils are close and matched in diameter, and that a slight offset between very small coils can cut coupling sharply and give very poor transfer. Its advice is to build a physical alignment aid into the charging base or cradle (TI SLYT570). On a wearable, that usually means a shaped cradle or magnets.

Metal, battery and shielding near the coil

The battery, a metal frame or a sensor board next to the coil changes how the coil behaves. TI's bq51003 datasheet says to measure the receiver coil inductance as it will sit in the final system, with the back cover and battery in place, and to compute the resonant capacitors from that value (TI bq51003 datasheet, SLUSBC8C, revised July 2018). TI's article adds that a shield behind the coil keeps the AC field out of the battery and electronics and can improve coil performance (TI SLYT570).

How much heat and charging time should you expect?

Inductive charging loses power in the transmitter, both coils and the receiver. TI measured total system efficiency for its wearable design, including the transmitter, coils and receiver, using a small transmitter reference board with a 5 V input. Reading TI's efficiency plot, the result is about 50% near 1 W of output, a little higher up to about 1.8 W, and much lower at light loads (TI SLUA748A, Figure 14). At 50%, roughly as much power turns into heat as reaches the device, and part of that heat sits in the device on the wrist.

Read chip numbers with care. The bq51003 product page lists 93% overall peak AC-DC efficiency and calls the part optimized for 2.5 W applications, as of November 2026 (TI bq51003). That figure describes the receiver chip's conversion. The roughly 50% figure covers the whole chain. Ask any supplier which one they are quoting.

Heat limits come from the charger side too. The bq51003 has a thermal shutdown and an NTC input that can end power transfer and tell the pad why. TI recommends placing the NTC closest to the user, so it monitors the surface the user can touch (TI SLUSBC8C). On a wearable, that is the surface against skin. Test charging in a warm room with the device in its band.

Charging time follows the charger IC current and the cell capacity, plus the taper at the end of the charge. The coil only has to deliver enough power to keep that current going. Measure the full charge on the real device, in its band or cradle, and log the temperature along the way.

Qi, magnetic charging puck, pogo pins or USB-C: which fits your wearable?

The table compares the four options on what a device team has to design and test.

TI's article gives the main case for going wireless on a small device: even plug-and-jack and micro-USB connectors were too large for some ultra-thin wearables, and connector contamination is a problem outdoors. Inductive charging transfers power through a sealed case (TI SLYT570). A closed housing also makes a water resistance rating easier to reach. See what IP67 and IP68 tests cover.

Pogo pins are common on fitness trackers and smartwatches. Mill-Max, a pogo pin maker, notes they are used there for charging and data, and that gold plating lowers contact resistance and helps prevent corrosion (Mill-Max, as of November 2026). Sweat sits on those contacts every day, so ask for a corrosion test that matches wear. Exposed contacts are also a direct path for static discharge into the charge circuit. Our guide to ESD testing covers how labs test them. For the protection parts at those contacts, see ESD protection design with TVS diodes.

Foreign object detection (FOD) matters for open pads. The bq51003 reports its received power so the transmitter can watch power loss, which is one way to limit heating of a stray coin or key. TI says FOD is enforced in WPC v1.2 (TI SLUSBC8C). For a custom low-power pair, TI's 2014 article lists FOD as optional. It argues that a low-power system over a small charging area greatly reduces the chance of a stray object heating up, and that how critical FOD is may depend on the cradle design (TI SLYT570).

What certification does a wireless charging wearable need?

Qi certification, only when you want the logo

To certify, you join the WPC, register the product, pass compliance testing at an Authorized Test Lab, then pass interoperability testing against previously certified products. The product owner must be a WPC member in good standing, and certified products may use the Qi logo on the product and packaging (WPC certification process, as of November 2026). The WPC says certification testing averaged 24 days in 2026 and that cost depends on the lab you choose (WPC knowledge base, as of November 2026). TI's article lists interoperability with other Qi devices as optional for a custom wearable pair (TI SLYT570). If you ship your own charger and skip certification, leave the Qi logo off.

FCC rules for the charger you ship

FCC guidance KDB 680106, published October 24, 2023 with attachment D01 v04, covers wireless power transfer devices. It says WPT devices operating above 9 kHz must be authorized under the FCC equipment authorization rules and follow Part 15 and/or Part 18. When a WPT device transmits information between the transmitter and the device being powered, it must be authorized under the Part 15 rules for intentional radiators and will generally require certification. When a device has charging and communication modes that are independent, the charging mode can go under Part 18 and the communication mode under Part 15. Both Part 15 and Part 18 devices must meet the FCC limits for human RF exposure (FCC KDB 680106, as of November 2026).

Qi uses in-band packets between receiver and pad, so ask your test lab early how this guidance applies to your charger. The wearable's own radio and digital circuits follow the usual FCC rules on their own; see how FCC certification works and Part 15B vs 15C for a BLE wearable.

What should you ask your hardware team?

Bring this list to the first enclosure and electronics review, before the stack height is fixed.

• Which charging option did we pick, and what did we rule out and why?
• What coil diameter fits, and how far is it from the charger coil once the housing wall is in place?
• Was the receiver coil tuned with the back cover, battery and any metal in their final positions?
• How much does output drop at the worst offset a user can produce, and how do we guide alignment?
• What is the measured efficiency from the pad input to the battery at our real charge current?
• Where is the temperature sensor, and at what reading does charging stop?
• Do we need the Qi logo, or is a custom pair with our own charger enough?
• For pogo pins: what plating, what sweat and corrosion test, and what ESD protection sits at the contacts?
• Which FCC rule part covers the charger we ship, and does it need certification?

Charging choices for wrist and ring devices are part of our health wearable development work.

If you are choosing between Qi, a magnetic puck and pogo pins for your device, tell us about your device.

Sources. All pages accessed November 6, 2026. Specifications and FCC guidance are revised, so confirm current versions before you decide.

• Wireless Power Consortium, Knowledge base (link)
• Wireless Power Consortium, Principle of inductive power (link)
• Wireless Power Consortium, Coupling factor (link)
• Wireless Power Consortium, Qi specification (link)
• Wireless Power Consortium, Qi certification process (link)
• Wireless Power Consortium, Qi wireless charging standard page (link)
• Wireless Power Consortium, Qi v2.0 Standard for Wireless Charging Is Now Ready, news release, November 15, 2023 (link)
• Texas Instruments, Adapting Qi-compliant wireless-power solutions to low-power wearable products, SLYT570, Analog Applications Journal 2Q 2014 (link)
• Texas Instruments, Power Management Reference Design for a Wearable Device with Wireless Charging Using the bq51003 and bq25120, SLUA748A (December 2015, revised April 2016) (link)
• Texas Instruments, bq51003 datasheet, SLUSBC8C (December 2013, revised July 2018) (link)
• Texas Instruments, bq51003 product page (link)
• FCC Office of Engineering and Technology, KDB 680106 Wireless Power Transfer (WPT), published October 24, 2023 (link)
• Mill-Max, Pogo Pins in Your Designs (link)