Plug a phone into a modern charger and the first 10 minutes are impressive. It’s the connected device protecting itself. As temperature rises during charging, a smartphone's battery management system reduces the current it will accept, because heat accelerates the chemical degradation that permanently reduces battery capacity.
The charger may be capable of delivering more, but the device simply stops taking it. For anyone building products in the portable power category, this creates an uncomfortable gap between specification and experience. A device rated at 25 watts is accurate in the sense that it can deliver 25 watts.
Whether it delivers 25 watts for the duration of a charge is a different question, and one the specification does not answer. The gap matters commercially because it is invisible at the point of purchase and obvious in use. Consumers compare wattage figures on packaging.
They don’t compare thermal curves, because thermal curves are not published publicly. The result is a category where products differentiate on a number that describes peak output rather than sustained output, and where the actual user experience of two products with identical specifications can diverge substantially. This is particularly acute in magnetic wireless charging.
Inductive power transfer generates heat at both the transmitting and receiving coils, and the magnetic attachment that makes these products convenient also places the heat source in direct contact with the device it is charging. Convenience and thermal performance are working against each other by design. The industry's response for the past several years has been materials science.
Graphite sheets, thermal interface materials, conductive housings, and heat-spreading layers have all improved how efficiently accumulated heat moves away from the source. Each generation has been incrementally better than the last. But passive dissipation has a structural limitation: it can only move heat that has already been generated, and only as fast as the surrounding air will accept it.
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