Eco-Drive

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Citizen Promaster Eco-Drive AP0440-14F Diver's 200 m. Four solar cell segments are just visible under the dial.

This article is about wristwatches. For "eco-driving", see Fuel economy-maximizing behaviors.

Eco-Drive is the brandname of a line of light-powered wristwatches manufactured by the Citizen Watch Co., Ltd. The watches are equipped with a special titanium lithium-ion secondary battery that is charged by an amorphous silicon solar cell located behind the dial^[1]. The titanium lithium-ion composition precludes it from being affected from the regular charge/discharge cycle of other types of rechargeable batteries. Light passes through the covering crystal and dial before it reaches the solar cell.^[2]

Depending on the electronic movement model, a fully-charged secondary power cell could run with no further charging for 30 days to 5 years. If kept in the dark for too long, some movement models engage a hibernate mode, where the hands of the watch would stop running but the internal quartz movement would still keep track of time. If an ample supply of light is given, the hands would move to the proper positions and resume regular timekeeping.^[1]

According to Citizen Europe, laboratory test showed that after 20 years the secondary battery retains a power storage capacity of 80%.^[3] Citizen states that their Lubricants for Long-Lasting Precision Equipment when used in watches, timepiece movements remain smooth for a long time as the oil does not harden even after 20 years.^[4]

Some critics point out that both the low cost of quartz watches and the extreme longevity of the newest lithium-ion non-rechargeable batteries (said to be upwards of ten years in some watches) may make solar power, whilst an interesting technology, obsolete before it truly catches on. According to this view, battery-powered watches will become disposable items, so the purchaser will not care about the cost of replacing the battery. However, some purchasers may be persuaded by the ecological benefits of solar watches: less waste and lower energy input, not to mention the reduced hassle of not having to purchase a replacement watch every few years. In addition, the widespread use of watches as jewelry—worn as much for their aesthetic value as their timekeeping ability—makes the prospect of disposable watches less attractive.