Eco-Drive

Eco-Drive is a model range of watches manufactured and marketed worldwide by Citizen Watch Co., Ltd. — and powered primarily by light. As of 2007, the company estimated the drive system had eliminated disposal of 10 million batteries in North America.

Citizen introduced the Eco-Drive line to Asia, Latin America and Europe in 1995 and to the United States in April, 1996.

The Eco-Drive concept introduced several technical refinements over previous solar powered watches, including light-capturing cells that could be made virtually invisible behind the dial instead of highly conspicuous--enhancing the appearance of the watch.

The technical platform that made the Eco-Drive concept possible was the Eco-Drive caliber 7878 movement. This movement was the first light powered movement where the solar cells could be mounted under the dial. Previous light powered watches from Citizen and other manufacturers had the solar cell(s) mounted directly on the dial. This innovation was enabled by marked improvements in thin film amorphous silicon solar cells, which, by the early 1990s had become significantly more efficient. By locating a sufficiently translucent dial material over the now more efficient solar cells, enough light could pass through the dial face to power the movement. Though the Eco-Drive caliber 7878 movement solar cells remained slightly visible through the dial, the physical styling of the light-powered watch was no longer constrained by visible solar cells.

To store electrical energy the first Eco-Drive movements employed titanium lithium-ion rechargeable or secondary batteries. This battery type became available in the early 1990s, enabling an Eco-Drive 7878 movement to run 180 days on secondary power before requiring recharging via light exposure — a marked improvement in energy storage over previous light-powered watches. The movement also featured an "insufficient recharging" indicator.

In addition to the first three Eco-Drive models introduced in 1995, Citizen marketed numerous other Eco-Drive models during the 1990s, including the 6.05 mm (0.238 in) thick Eco-Drive Slim of 1996.

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