Good find at garage sale!?  (Solar panel)

While checking out a garage sale over the weekend, I came across a set of solar powered christmas lights for a whopping $4. Figured it was worth it just for the solar panel alone. I have since got it home and discovered that inside the solar panel is a 3.2V 1200mAh Li-ion battery, a light detection sensor and a “smart” power control board. The power control board is a small PCB with an epoxied chip which presumably stops reverse current from the battery to the panel, and I would guess turns off the output whenever the light sensor sees light.

Now I need ideas/suggestions on how to hack this thing!

The fact that it’s a Li-ion battery gives the controller module at least two required functions. First in charge mode, cutoff charging current when battery reaches 4.2vdc as over charging can damage the battery. Second is a load disconnect to prevent the battey from discharging below around 3.5vdc or so as over discharging will also damage the battery. Other then that you should be free to wire any reasonable load to the modules output.


You could set it up to track the sun with the solar panel. You can do it with 2 detectors if you only want 1-axis of rotation, or use a quad-detector if you want it to track well over the seasons. This increases the efficiency a lot vs them just pointing straight up.


The slightly suprising conclusion on this page is that the advantage of active tracking is smaller than one might expect. If you just keep the panel pointed south, and adjust the tilt 4 times per year, you get about half the improvement of active tracking, and the remaining improvement is probably not worth the cost in money and energy for home users.


Well I’ll be! I would have thought the benefits of active pointing were a lot better than that. Good article Ran, though it might still be fun to have your own little solar farm always pointing towards the day star.


Yes, if I had the money to spare, I’d love to build a little “heliotropic solar farm” just for the amusement value, too.

I assume the reason that tracking has such a small advantage is that the panel doesn’t absorb much more energy if the sunlight is striking it dead-on perpendicular than it does when the light is coming in at, say, a 45-degree angle. So tracking only buys you a lot at the very beginning and end of the day, when some of the energy would be reflected off the surface because the angle of incidence is so acute.


I wonder, if you reduced the requirements of the hardware, if it might might improve the cost:benefit? Maybe instead of turning a heavy solar panel, turning a light reflector might make some sense? For larger, greater-than 10W solar panels, not the smaller project-type, of course.

Could use cheaper servos, anyway.

Just thinking out loud…

Anyway, great find!

Moving reflectors is a very interesting idea. I wonder if you could use it to increase the amount of light falling on the panel, kinda like the way they made a “morphing parabolic concentrator” for the Solar One plant in southern CA.

(Here’s an extremely dry engineering description of the mechanical details for the terminally geeky. For less-than-terminal nerds, it’s worth doing some googling to find good pictures of it: I’ve driven past it a few times, and will always regret that I didn’t take the time to do a good photo shoot of it. It’s really quite beautiful if you’re the sort who finds suspension bridges and GT race car designs “sexy”).

You might also be able to significantly improve efficiency by using reflectors with coatings that absorbed wavelengths that are turned into waste heat, instead of electricity, by the panel.

But, in many cases, you won’t really save anything mechanically: where I live, high winds would require the reflector movers to be almost as robust as ones to move panels. Maybe even more so, because the reflectors could wind up being pointed in directions that have higher wind loads than simple east-to-west tilting of fairly-horizontal panels.

But, on the other other hand :), you could make the reflectors somewhat porous, and trade off wind loading against some lost reflector efficiency. Hmmmm…

Maybe we should look into filing an R&D grant application ;D


While backpacking in Northern New Mexico, I ran across a solar panel that the Forestry service had up that tracked the sun without using any electric current at all. The frame had Freon or some low boiling point fluid in it and was arranged such that the fluid would boil and re-condense elsewhere in the hollow frame to control the tilt. Very cool. I had to sit and watch it for a couple of hours just to prove to myself that it worked.