When the Pad Stays Off

A wired charger delivers about 95 percent of its energy to the battery; a wireless pad reaches only around 80 percent. To stay safe, the pad also has to know what's sitting on it.

That job falls to foreign object detection. Before sending full power, the charger checks how much energy the field is absorbing. A coin, key, or loose SIM tray would soak up energy and heat up dangerously, so if the pad senses metal where a phone coil should be, it simply refuses to power up.