In our first article, I explained my needs/wants for the alarm clock and ended up selecting the Elgin 3350E shown above. It is a battery powered alarm clock with an automatic back lighting feature. If the room is lit up by the sun or by lights, the clock turns the back lighting off to save on battery life. However, once it gets dark in the room, it turns the back light on so you can see what time it is. It also has some pretty good reviews on amazon and it is also pretty inexpensive.
The clock requires 3 AA batteries. The batteries I will be using are what you see above. They are some nimh rechargeable batteries I nabbed from the local harbor freight. Any nimh battery will work, but you may have to change what solar panel you use according to what batteries you have to use in the clock.
Now that we know what batteries we’re going to use, we can figure out how to charge them. I did a bit of surfing to find some good info and came across this site:
“The cheapest way to charge a nickel metal hydride battery is to charge at C/10 or below (10% of the rated capacity per hour). So a 100 mAH battery would be charged at 10 mA for 15 hours. This method does not require an end-of-charge sensor and ensures a full charge. Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over C/10. The minimum voltage you need to get a full charge varies with temperature–at least 1.41 volts per cell at 20 degrees C. Even though continued charging at C/10 does not cause venting, it does warm the battery slightly.”
This is a great bit of info for us. It contains basically everything we need to figure out our solar setup. Prepare yourself for a little math and electronics speak as we delve into the world of charging batteries.
The batteries I am using are rated at 2200 mAh. The quote above says I can charge them at C/10 safely. C is the battery capacity which is 2200 mAh. This simply means that I can charge them at a rate of 2200/10 = 220 mA. So, I want to find a solar panel that puts out 220 mA or less to avoid doing any damage to my batteries. This is great news because it means I don’t need a charge controller or any additional circuitry to complicate things.
The next step is figuring out the solar panel voltage. Again, we’ll look at the quoted paragraph that says we need at least 1.41V per cell. Well, the clock is using 3 batteries, so we need to provide 3 x 1.41V = 4.23V. Unfortunately, you can’t just go out and buy a 4.23V solar panel. However, 5V and 6V panels are readily available. Either of these will work, but we have to figure out a way to get them to put out 4.23V.
I ended up purchasing the solar panel you see above from ebay. It is rated at 5V output and 200 ma. A 6V panel that is rated for 220 ma or less would work just fine too.
But, we still have to get the panel’s voltage down to 4.23V. To get this voltage drop is actually quite easy, and it is going to be done by another component we need for the solar setup anyway. That component is a diode. It is essentially a one way / check valve for electronics. The diode is not only going to provide a voltage drop down to 4.23V, but it also stops the solar panel from draining the batteries when the sun isn’t shining on them. All solar panel setups use these, and you can get them with different voltage drops.
To get the diode, I went to digikey.com. They have an insane amount of electrical components and shipping is pretty cheap too. This is the diode I selected if you want to use a 5V panel like me. It will provide an approximate voltage drop of .75V giving an output of 4.25V which is close enough for this application. Alternativly, if you wanted to use a 6V panel, this diode should work fine for you. It has a 1.75V drop and will give you a similar 4.25V output.
Once I get all these things in the mail we’ll start putting them together!
For the latest info and a bit more detail, see the forum thread.