Conventional hearing aids need batteries of size 13 (7.8 mm in diameter and 5.35 mm in height). By replacing the battery with one of a smaller size, a volume of 7.8 mm in diameter and 1.9 mm in height was gained to integrate a secondary coil and charging electronics.
Different coil types and positions were investigated to minimize eddy current in the battery and maximize the efficiency of the electromagnetic transmission. To ensure safe battery charging and discharging, the whole electronics is integrated into the given space and controlled by a tiny microcontroller. As the battery is based on a new chemistry (silver-zinc based), different charging algorithms were implemented and evaluated.