offered over 320 Eco-Drive watch models in various types, styles and price ranges. Eco-Drive technology Light as power source Most Eco-Drive type watches are equipped with a special titanium lithium ion secondary battery charged by an amorphous silicon photocell located behind the dial. Light passes through the crystal and dial before reaching the photocell. Depending on the electronic movement, a fully charged secondary power cell could run with no further charging from 30 days to 3,175 days (8.7 years), though most Eco-Drive men's watch models offer a six-month power reserve. If kept from light for an extended period, some Eco-Drive movement models can hibernate, where the hands of the watch stop and the internal quartz movement continues to track the correct time. When subsequently exposed to sufficient light, the hands move automatically (without human intervention) to the proper positions and resume regular timekeeping. Temperature difference as power source Citizen Eco-Drive Thermo watches were introduced in 1999 and use the temperature difference between the wearer's arm and the surrounding environment as a power source. The rare Eco-Drive Thermo watches use the Seebeck effect to generate thermo electricity that powers the electronic movement and charges the secondary power cell. In the sun or in the tropics the ambient temperature can come close to or exceed the temperature of the wearer's wrist causing the watch to stop generating thermo electricity. In case no power is generated, an Eco-Drive Thermo movement will save power by moving the second hand in ten second increments until the production of thermo electricity is resumed. Citizen has stopped making Eco-Drive Thermo watches. Hybrid Eco-Drive movements Citizen also built an automatic quartz powered watch, the Citizen Promaster Eco-Duo Drive (released in December 1998). Novel to this watch was the use of both mechanical power as well as a solar cell to power the electronic movement and charge the secondary power cell. This model was an attempt to enter higher-priced markets (at a cost of around US$1,000). The Eco-Duo Drive technology failed to attract consumer interest and Citizen has since stopped making use of the unique movement. Solar cell and secondary battery life expectancy According to Citizen, experimental data showed the solar cell and secondary battery will last for more than 10 years. According to Citizen Europe, laboratory tests showed that after 20 years the secondary battery retains a power storage capacity of 80% of its initial capacity. Newer Citizen