In an effort to reinvigorate the economy, I recently bought a Presidian Solar Keychain Light from Radio Shack for the sum of $4.99, plus tax. As the picture shows, this is a simple flashlight (or torch for those of you brought up that way), with a single white light emitting diode (LED) and miniature solar panel to charge its battery.
Now, in principle, solar recharging is a fine idea. You probably won’t want a flashlight when the sun is shining, and if the system works, you won’t have to buy batteries, ever.
However, I am always somewhat suspicious of solar charging. Even with photovoltaic panels for houses, which are in constant use, it takes several years of operation to generate the energy which was expended just in the fabrication of the solar cells themselves. It takes a lot of energy to create a silicon crystal based product like a solar cell, and that energy is currently generated either by nuclear energy or burning fossil fuels.
So for an application like charging a simple flashlight, there is a strong possibility that buying such a device hurts the environment far more than buying a more primitive battery powered device.
Additionally, of course, one occasionally finds that electronic gadgets are the subject of cunning marketing ploys and hype.
So, I was curious to see how this flashlight worked in practice. The first thing I did after buying it was leave it switched on to run down the battery. In fact, this took many hours to accomplish. So, all the time the flashlight was sitting on a shelf in the store prior to purchase its batteries must have been well charged. The running down process took around 5 hours. I didn’t keep careful track of the timing, because I wanted to keep the flashlight with me so that I didn’t completely run the cell to zero volts, which I suspected it would not appreciate, and this necessitated monitoring of the light.
However, during the running down process, I was impressed by the light produced by the flashlight. The lens allows for some diffuse light to be emitted from its sides, and the main beam is produced by the lens of the LED itself. This is a nice arrangement; the diffuse light from the sides is useful for reading and similar activities and for more typical flashlight applications the main beam is moderately intense.
Eventually, though, the light died down to a very noticeably dimmed level. At this point, I switched off the flashlight and left it by a window for two days. This particular window does not get a great deal of direct sunlight, and these days were somewhat overcast.
However, after two days my patience was exhausted, and I decided to check how the flashlight was doing. The battery was indeed very nicely charged; I left it shining brightly for an hour to confirm that it was in a thoroughly recharged state.
At this point, I must admit that I was impressed by the performance of the device. I imagine that the LED takes something of the order of 50 milliamps, and perhaps the small solar cell produces something like 5 milliamps in full sunlight, the battery charging and storage cannot be 100 percent efficient, and so I thought that the time to recharge the battery might have been considerably longer. However, the recharging appears to work well.
Upon opening the flashlight to examine its internal workings, I saw that it had a cluster of three batteries. I presume these are 1.2 volt NiMH rechargeable batteries. There is a small circuit board which supports a diode and a resistor. The diode prevents charge from the battery flowing through the solar cells, and the resistor limits the current flow through the LED. Additionally, a robust sliding switch allows the flashlight to be left in the ‘on’ position (useful for testing the flashlight, but risky for accidentally leaving it permanently in the ‘on’ position).
The pictures show the inside and outside of the flashlight, including the circuit board and batteries.
All in all, I must admit that I am very impressed by this device. It performs well as a small flashlight and the recharging capability works as advertised. Its price is high but it is not outrageous, given that it contains a rechargeable battery, and a miniature solar panel.
The only real concern I have is that the somewhat costly components harm the environment more than the more mundane alkaline battery that might be found in a traditional flashlight. However, perhaps the perspective to take is that expressing a consumer preference for such devices may lead suppliers to increase future emphasis on solar technologies, potentially making these devices, and their production, even more efficient in the future.