Don’t count on driving your phone with it, but Panasonic is introducing a tiny battery that produces 13mAh at 3.75 volts. The LiIon CG-320 battery is 20 mm long and 3.5 mm in diameter — about 1/20 of the size of a AAA cell.
An iPhone 6 battery holds 1800 mAh; a Jawbone Up24 requires 32mAh. Panasonic says they’re developing other, slightly larger, batteries with a capacity of up to 50mAh, which would do quite nicely for a display-less wearable. The CG-320 could about power a Bluetooth and NFC radio.
Panasonic expects to ship about 100,000 of these starting in February.