The design of a hearing-aid system involves three disciplines of applied science–electrochemistry, electrical engineering and audio engineering. This paper is concerned with the interface between the first two. Batteries are essentially non-linear components. Optimum performance is only achievable when the electrical requirements of the hearing aid are closely matched with the voltage, rate capability and impedance of the battery. After years of optimalization, the modern ‘675’ button cell has earned universal acceptability and is now used in most ‘behind-the-ear’ hearing aids. When more power is required, the larger and less specialized LR6 ‘penlight’ cell is typically specified. Higher voltage might lead to better circuit efficiency, and there is some pressure to introduce a 3 V lithium-based product. Lithium should give superior energy density, but there are problems which remain to be solved. In the end, it is quite possible that the market might settle for an ecologically acceptable long-life lower voltage metal-air cell. If so, the recent zinc-air system may well have a future and could conceivably succeed both the mercury ‘675’ and the alkaline ‘penlight’ cells.