Dec
11
2009

How well do solar powered watches work?

Solar powered watch showing the H/M/L battery gauge (photo by Roger Browne - CC-BY)

Solar powered watch showing the battery gauge on the lower right (photo by Roger Browne - CC-BY)

The promise made by a solar powered digital watch is a big one—an end to the need to periodically replace the watch battery. And if the solar powered watch also has radio controlled timesetting then the watch should “just work”. It should always show the correct time and should never run down. But how well do solar powered watches work?

A watch has a very small area on which the solar cell can be placed, so the amount of electricity generated is also very small. And no power is generated during the night when there is no light, or when the watch is placed in a drawer or a pocket.

Because the watch will not be constantly exposed to sunlight, it has an internal rechargeable battery. Provided there is enough light in the long term, the watch will keep going through short-term darkness. The rechargeable battery will power the watch for several months at a time, even in complete darkness. However, it will then take many days of sunshine to bring the battery back to full charge.

The timekeeping function of the watch uses very little power. Producing the sound of the alarm signal consumes more power, but by far the most power is used to operate the light. Displaying the light for just one second can use up as much charge as is produced from many minutes of exposure to bright sunshine.

As a “rule of thumb”, a solar powered watch will keep operating provided you spend some time outdoors in short sleeves every week, for most of the year. This allows for a period during winter when your wrist sees very little daylight. The watch should have a way to indicate to whether the battery holds a low, medium or full charge, so you can see how the charge is holding up.

What if you can’t manage enough sunshine? Suppose you always wear a jacket when you’re outdoors, with long sleeves that cover the watch and therefore keep its solar cell in darkness? In that case you will need to take the watch off during the day and leave it on a sunny windowsill.

If your watch is not maintaining enough charge, there may be some optional features that you can turn off. For example, my Casio WaveCeptor watch can automatically light up the display when I hold my wrist in a certain position. This is convenient at night, but results in many accidental activations of the light so I have switched off this feature.

Rechargeable batteries have a limited lifetime, up to a thousand recharge cycles but possibly much less. During the lifetime of your watch the capacity of its rechargeable battery will gradually decrease, and at some point you may wish to have the battery replaced. Unfortunately, this rather defeats the goal of an everlasting always-correct solar powered watch, but it’s as close as the technology can get today.

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1 Comment »

  • Robert says:

    Your article is largely true for most solar powered watches. The exception is the Citizen Eco-Drive. Their watches will probably last 20 years with NO maintenance. Their technology incorporates:
    1. Transparent (or translucent) dial which allows the underlying solar cell(s) to be nearly as large as the watch face.
    2. The use of amorphous silicon photo cells.
    3. The use of a titanium – lithium ion battery. This battery will probably retain 80% of its capacity after 20 years.
    4. In some models a “sleep” feature which shuts off the motor after a long period of darkness. The clock continues to run and on exposure to light, the motor moves the hands to show the correct time.
    5. Special long life (20 year) lubricants.
    These features preclude the need for a battery charge indicator which in itself is an energy loss.

    An atomic movement would be silly. My watch (30 mo old) maintains time within one minute between dst settings. The analog movement is more accurate than the dial resolution.

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