Heart rate and GPS take significant amounts of power, unlike MEMS accelerometers, Bluetooth low-energy, and watch displays. A thin, non-rechargeable battery is

Heart rate and GPS take significant amounts of power, unlike MEMS accelerometers, Bluetooth low-energy, and watch displays. A thin, non-rechargeable battery is a possibility for watches without these features. For watches with these features, it needs a rechargeable battery and a very thin water-proof connection to that battery.