Eco-Drive watches use a battery recharged by a solar panel hidden under the watch face. In the rare and discontinued Eco-Drive Duo series, the solar power was supplemented by an automatic quartz power source. One early model, called the Citizen Vitality, used the watch hands to drive a small electric generator, but was discontinued following complaints that the device could explode and cause wrist injuries. There was also an Eco-Drive Thermo model that exploited temperature differentials between the wearer’s skin temperature and ambient temperature to recharge the battery.