A lot of people hit the same puzzle: you buy a magnetic pad rated at 25W, lay the phone on it, and the phone reports only about 15W while the pad gets clearly warm. The pad is not lying - it really can output 25W. The catch is that those 25W travel through several toll gates from the wall to the battery, and what actually lands there is often only around 70 percent.
The first loss is at coupling. The transmitter coil turns electricity into a magnetic field, and the receiver coil inside the phone turns that field back into current; the closer and better aligned the two coils are, the more energy is captured. Shift a few millimeters or add a thick case and efficiency drops a notch. Magnetic alignment fixes exactly this - it snaps the phone into the sweet spot, far steadier than hand placement.
The phone quietly takes its own cut
The second, and often overlooked, loss lives inside the phone's rectifier and power management. The AC the receiver coil induces first passes through a rectifier bridge to become DC, then the phone's PMIC steps it down to what the battery wants - and that round trip costs roughly another 8 to 12 percent. In other words, even if the pad cleanly delivers 25W, the phone's own circuitry scrapes off a layer before the battery sees it.
More decisively, the phone also applies its own brakes. Batteries follow a charge curve: fast gulps when low, slower as they fill, and the phone's internal thermal limit will proactively cut power to stay safe as the body warms up. So that only 15W you see is usually the conservative gear the phone picks after weighing charge level, temperature and its own circuit losses - the pad offers more, the phone declines to take it all.
Know this and shopping gets clearer
One line sums it up: what the pad states is how much I can output, not how much your battery receives. When choosing, do not let peak watts dazzle you - look for Qi2.2 certification, guaranteed alignment and a thermal design that holds temperature, which beats chasing the highest number. Wireless charging was never wired charging without the cable; its rules, from coil to battery, already bake in a loss ledger.
© SWPO. All rights reserved.