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Solar powered watches have been around for nearly half a century, and the technology that powers them has improved dramatically in the past five decades. Modern designs are so sleek and fashionable that you wouldn’t even know they’re solar powered by looking at them. Just because something is new, however, doesn’t necessarily mean it’s better than previous versions.
Are solar powered watches reliable? Yes, solar powered watches charge whenever the watch face is exposed to either natural or artificial sunlight, and excess energy is stored in lithium-ion batteries so the watch can work in the dark as well. The lithium-ion batteries can store enough energy to run from 30 days to 3,175 days (over 8 years), depending on the model. Failing this, most solar powered watches also have a backup battery that can hold up to 80% of its charge for 10 years.
It’s arguable that solar powered watches are the most reliable. They don’t require daily winding or the replacement of batteries after a few months. When manually wound or battery powered watches operate on insufficient power, they can run at a reduced speed, resulting in the watch becoming more and more “late” relative to the actual time the longer the power is low. By charging while in use, solar powered watches can prevent this loss of accuracy, provided that the power source is reliable.
Do Solar Powered Watches Work?
There are many failsafe that make sure that solar powered watches work and keep working for a long time, even if they aren’t in direct sunlight at all times. To understand how solar powered watches work, we should first have an understanding of how solar power works.
How Solar Power Works
Because the transformation from sunlight into usable electricity takes place out of sight within the photovoltaic cells of the solar panel, it seems that the conversion is done by magic. The process by which photovoltaic cells create electricity from light, however, is actually fairly simple.
- Light activates the solar panel. Both natural sunlight and light from artificial sources like incandescent and even fluorescent light bulbs are able to activate the panels and initiate the production of electricity.
- The light is absorbed by a semiconducting material, usually silicon. As a result, electrons are knocked loose from the silicon (or other semiconducting material).
- The electrons begin to flow, forming an electrical current. The silicon in the photovoltaic cells is arranged into two layers. Each layer is treated, or “doped,” in such a way that one layer has a net positive, and the other has a net negative electrical charge, resulting in the loose electrons flowing in one direction, creating a direct current (DC) electrical current.
- The electrical current is captured by metal panels on the sides of each cell and transferred into wires. This electricity is used to power the movement of the watch, and any surplus energy is used to charge a lithium-ion battery.
Because the energy created in the photovoltaic cells is direct current (DC) power, the same type used by both the watch and the lithium-ion battery, there is no need to invert it into alternating current (AC) energy as is the case when solar panels are used to power homes.
Though the basic chemistry behind lithium-ion batteries was discovered by Gilbert Lewis (1875–1946), an American chemist, in 1912, they didn’t become widely used until the early 1990s when lithium-ion batteries started being used in cell phones and laptop computers.
Traditional batteries create an electrical current by making use of a chemical reaction that happens in only one direction, so when the chemical reaction is done, the battery is no longer useful and must be thrown away in a responsible manner. Lithium-ion batteries, on the other hand, can be recharged to provide mobile power over and over again. Typically lithium-ion batteries can be recharged hundreds of times and last up to 10 years.
Lithium-batteries store energy in the form of lithium ions traveling between a positive electrode made of lithium-cobalt oxide (LiCoO2) or, in newer batteries, from lithium iron phosphate (LiFePO4) and a negative electrode generally made from carbon (graphite). Electrons move through wires around the cells in the opposite direction as the lithium ions are traveling, creating an electrical current.
Not only can lithium-ion batteries be recharged hundreds of times, but unlike nickel-cadmium (NiCd) they do not suffer from a problem known as the “memory effect” in which NiCd batteries appear to become harder to charge unless they’re discharged fully first. Also, because lithium-ion batteries don’t contain the toxic, heavy metal cadmium, they are better for the environment.
Even though solar powered watches charge whenever the face is exposed to light (both natural and artificial), and lithium-ion batteries can hold a charge for anywhere from 30 days to 3,175 days (over 8 years) depending on the model, most solar powered watches also have an additional battery to act as a backup.
These disposable batteries are there to power the watch if all else fails, so of course, they need to work when needed. Studies have shown that these batteries can hold up to 80% of their charge for up to 10 years. This means that even if your watch were kept away from any source of light (such as stuffed in a drawer, perhaps) long enough to use up the lithium-ion’s power reserve, there would still be a backup battery to supply power to run the watch for another few months.
Power Saving Mode
Many models of solar powered watches have a power saving feature that allows the watch to conserve energy when light sources are not available. When in power saving mode, the watch ceases the constant supply of energy used to move the watches second hand so that energy can be conserved to maintain its high functions (such as keeping track of the date and day of the month and week) for a longer period of time.
Watches that are not run on solar power sometimes run at low power. When a traditional battery is nearly empty and needs to be replaced, or a manual watch needs to be wound, the watch motor may run at reduced capacity.
Because the watch’s motor must run at a specific rate in order to keep accurate time, the longer and more often the watch runs at reduced power, the further the watch will be from telling the accurate time because the motor will have spent more time running at a lower than accurate pace.
Solar powered watches do not have this problem because they constantly recharge any time their faces are exposed to natural or artificial light, so it’s only under extreme conditions that they ever run at a nearly empty battery capacity.
The Pros and Cons of Solar Powered Watches
Though solar powered watches have been available since the 1970s, they are still not as popular are traditional battery powered watches. Newer solar powered models, however, are becoming more fashionable. When considering purchasing a solar powered watch, it’s important to consider the pros and cons.
Pros of Solar Powered Watches
There are several positives unique to solar powered watches.
- They are easy to charge. There is no need to remember to change the battery every few months or plug them into a charger at night. As long as they are not stored in total darkness, solar powered watches are nearly always charging.
- They can be charged by any light source. It’s not only natural sunlight that can be used by photovoltaic cells to create electrical power. Solar powered watches can also charge via incandescent and even fluorescent light sources.
- They are easy to maintain. Because they are easy to charge and the batteries can last over a decade, solar powered watches rarely require any form of maintenance.
- They are more eco-friendly. Because of their rechargeable lithium-ion battery, solar powered watches result in the disposal of fewer traditional batteries, which can contain corrosive or toxic chemical remnants. nickel cadmium (NiCd) batteries, for example, contain the toxic heavy metal, cadmium.
Cons of Solar Powered Watches
The benefits of solar powered watches do come at a price, however, and this can mean that solar powered watches are more expensive than others.
Though the technology has and continues to improve and become more affordable, the solar panel technology they rely on makes solar powered watches more expensive than traditional quartz watches. This initial cost may be offset by the savings of not having to buy replacement batteries every few months.
A Brief History of Solar Powered Watches
The earliest solar powered watches, created in the 1970s, required relatively large, dedicated photovoltaic panels on the watch face. Because the solar power technology was still in its infancy, these cells had rather large to generate enough power to run the watch.
The watch designs had to accommodate these large panels, which sometimes took up more space of the watch face than the screen that displayed the time. Often solar powered watches were designed in a neo-futuristic fashion that was a 1970’s attempt to predict the fashion trends of the future.
As photovoltaic cell technology improved, the cells became smaller and smaller, and were able to be incorporated more subtly into more contemporary designs. Today the solar panel is typically beneath the watch’s crystal, with the dial either directly on or a layer above the solar panel itself. This allows the entire face to the watch to be both solar panel and display. In this way modern solar watches can be made in any design.
Types of Watches
Just as cars are powered by different types of engines (e.g. gasoline, diesel, electric), watches are powered by different types of movement. Solar powered watches are a type of quartz watch. An understanding of the various watch movements will make it clear why solar powered watches are arguably the most reliable type of watch.
A quartz watch uses an electronic oscillator that is regulated by a quartz crystal to keep time. The crystal oscillator converts direct current (DC) energy from a power source into an alternating current (AC) signal with a very precise frequency.
In traditional quartz watches, this power source is a battery that will last a few years, then need to be replaced. In a solar powered watch the direct current (DC) energy comes from the photovoltaic cells directly or from the lithium-ion battery storage.
The frequency of the crystal oscillator is extremely precise, and its cycles are counted by some form of digital logic in order to determine and display the time. Quartz watches are more accurate than mechanical watches by at least an order of magnitude.
They are also much less expensive to produce. Because they have electrical components, however, they will eventually fail at which point it is more economical to replace than repair.
While quartz watches function electronically, mechanical watches use a clockwork mechanism to tell time. Mechanical watches are driven by a mainspring that must be wound periodically. Most mechanical watches require manual winding, but automatic watches use the natural movement of the wearer to wind the mainspring.
As the mainspring unwinds, its force is transmitted through a series of gears to power a balance wheel, which is a weighted wheel that oscillates back and forth at a constant rate. An escapement releases the watch’s wheels a consistent amount with each swing of the balance wheel. This makes the watches hands move at a constant rate and produces the tell-tale ticking sound of a mechanical watch.
Mechanical watches are not as accurate as quartz watches. Even a simple mechanical watch contains about 130 components and periodically needs to be cleaned by a skilled watchmaker. This makes them significantly more expensive. They are now mostly high-end products typically bought as a status symbol.
Seiko Solar Versus Citizen Eco Drive
The two most popular brands for solar powered watches are Seiko and Citizen. Both are giant Japanese watch brands. These companies were a big part of the reason the center of the timekeeping universe shifted from Switzerland to Asia as quartz movement overtook mechanical movement beginning in the 1970s.
That being said, Citizen is more of a quartz brand, while Seiko is more of a mechanical brand. Citizen has been producing solar powered watches longer, and it’s made more advancements in the technology.
Citizen’s Eco Drive technology used in their solar powered watches won the Gold Prize at the Eco mark Awards in 2014. The Eco Drive watches are also a little less expensive, more widely available, water-resistant to twice the depth, and have a secondary battery that lasts twice as long as watches that use Seiko’s Solar system.
Can You Overcharge a Solar Powered Watch?
Solar powered watches are equipped with overcharge protection functions that make sure that the lithium-ion battery is not damaged when the watch is exposed to light when the battery is already fully charged.
Similar features are common on any modern device that uses a lithium-ion battery, like laptops and cell phones, so they can remain plugged in without harm. Typically when the battery becomes fully charged, the display will show 100%, but power will be diverted away from charging battery any further. When the battery’s capacity falls to somewhere between 95 and 97 percent, charging will resume, but the display will show 100% full the entire time.
The watch can be damaged in situations where it is overheated, but it is the heat and not the charging property of the light that causes the problem in these instances.
Avoid charging solar powered watches in the following ways which can result in overheating and damage to the watch:
- On the dashboard of an automobile – Cars are notorious for becoming dangerously hot saunas when parked in direct sunlight all day. The dashboard gets even hotter than the rest of the car’s interior (and more quickly) because it is a confined space with a highly reflective surface on bottom and a heat trapping glass on top.
- Too close to an incandescent light bulb – Incandescent light bulbs create light by passing electricity through an extremely thin metal filament, causing the metal to become so hot it glows. Anyone who’s seen Ferris Bueller’s Day Off (or forgot to study for a test) knows that the bulbs can produce a lot of heat.
- In direct sunlight for an extended period – The sun, being a ball of burning gas, also produces a lot of heat as well as light. Though it’s significantly further away than a light bulb, anyone who’s gotten a bad sunburn can tell you how harmful it can be, especially over a long period of time.
Because solar powered watches charge while you wear them, there isn’t any need to leave them these dangerous situations to charge anyway. If your watch were locked in a dark drawer for enough months (or years) for the battery to fully deplete, you would only need to charge it for a little bit, just long enough for it to start running again, so you can reset the time (and date, if it has a date window) and begin wearing it again.
How Long Does it Take to Charge a Solar Powered Watch?
Though you don’t need to fully charge your watch in order for it to function, people often wonder how quickly solar powered watches charge. The photovoltaic cells of solar powered watches can produce energy from both natural and artificial light, but the former is a lot more efficient than the latter.
Charging Via Natural Sunlight
Putting a solar powered watch in direct sunlight for 3 minutes will produce enough energy to run the watch for 24 hours.
To fully charge the watches lithium-ion battery from empty would take about 20 hours or more of direct sunlight of a warm climate or season.
During cold seasons in parts of the world further from the equator, there are not only less hours of sunlight, but the sunlight there comes at such a steep angle that a significant percentage of its energy doesn’t penetrate the Earth’s atmosphere. In such situations, it may take more than 60 hours to fully charge the watch’s battery.
Charging Via Artificial Light
Though photovoltaic cells can produce electricity when exposed to artificial light, it is not nearly as effective. To create 24 hours’ worth of power, a watch must be in the sun for 3 and a half hours. To fully charge the lithium-ion battery from empty via artificial light may take up to 150 hours.
Solar powered watches typically are fully charged before packages for sale. They are commonly packaged with the watch face exposed, which not only displays the watch for sale, but also allows light to penetrate the plastic packaging.
Maintaining a charge only takes a few minutes of sunlight (or a few hours of artificial light) per day, and the lithium-ion batteries can hold enough charge to last several months or years, so it’s unlikely that you’ll ever need to fully charge a solar powered watch from empty.
If you ever do need to charge a solar powered watch’s battery from empty, it is suggested that you charge it to at least 85 percent full before halting exposure to light.
How Long Will a Solar Powered Watch Last?
The lithium-ion batteries in solar powered watches typically last from 10 to 20 years and do not need to be replaced in the life of the watch. The batteries can be recharged hundreds of times, but eventually will wear out.
Because solar powered watches are quartz watches, not mechanical watches, when the battery finally does die, it typically makes more sense to replace the watch as opposed to having it repaired or the battery replaced.
The Best Solar Powered Watches
Nearly every major watch brand has its own solar powered watches. The technology has advanced so much since they first became available that modern solar powered watches come in all types of designs. If you’re in the market for a solar powered watch, here are the best reviewed models.
- Citizen Eco-Drive Skyhawk A-T Watch. This watch uses a sapphire crystal stainless steel that protects the glass from scratches and reduces light reflection. It also has an atomic time keeping two-toned band to increase accuracy.
- Casio ProTrek PRW- 3000- 1CR Watch. This watch has a lot of features that make it great for travellers, including a compass, a barometer, and an altimeter. It’s water resistant to a depth of 100 meters and resistant to temperatures as low as -10 degrees Celsius. This is on top of everyday features like a calendar, a timer, and a stopwatch.
- Bering Time 14640-222 Solar Watch. With it’s simple, sleek design, this watch is both comfortable and durable. It has an elegant look that feels as appropriate peeking out from under a suit sleeve as it does on a swimmer’s wrist.
- Citizen Eco-Drive NightHawk BJ7000- 52E Pilot Watch. This watch face is packed with so much information readouts it looks like it could run its own flight simulator. One of the most useful features is a GMT function that enables users to locate the time to their locations anywhere in their travels.
- Seiko Silvertone Black Dial Solar Calendar Watch. A perfect example of Seiko’s classic elegance, this watch is basically the Platonic ideal of what a solar powered watch should be.
- Citizen Eco-Drive BM8180-03E Japanese Quartz Green Watch. Another example of the elegance of simplicity, this watch features a military-inspired Army green weave band that evokes the rugged individuality of camping.
- Casio Pathfinder PAG240-1CR. This watch is designed to work optimally in any location. And they really mean any location. It features a thermometer, a barometer, and an altimeter, as well as over 30 different time zones. Its triple sensor enables it to adjust to the various terrains with their different lightings, making it easily visible in all lighting conditions.
- Casio G-Shock GW – 9400 Rangeman. This is a rugged sports watch. Designed for diving, it can handle depths of up to 200 meters. It also has a working digital compass, an analog direction indicator, and a thermometer, and when fully charged, its battery can last without charging for 10 years.
- Seiko SSC017 Prospex-Solar Powered Chronograph Dive Watch. Though it is also a rugged diving watch, this watch’s stainless steel case and bevels help it retain its elegant look.
- Casio G-SHOCK G- STEEL. This watch is a perfect balance of sporty and stylish. It can dive to a depth of 200 meters and still looks good with any outfit. It has a tough leather band and a 3-fold stainless steel band that secures the watch on the owner’s hand and also a resin layer that acts as a shock-absorber.
Any of these solar powered watches would be a wise investment. There are many options available for many manufacturers, so you are sure to be able to find one with the look and the features to meet your specific needs.
Solar powered watches have been available since the 1970s. In the 1990s lithium-ion batteries greatly improved their battery life. Today solar powered watches are available in a wide array of stylish options. Because they charge whenever exposed to both natural and artificial light and can hold a charge for many months (or even years) solar powered watches are incredibly reliable.