Making your own Portable Solar Panel

I want to take my laptop out into the mountains and use a portable solar panel to power it. Simple enough. And you can buy portable solar panels precisely for this purpose... for $600-$800!!! Yikes! I found, instead, a semi-rigid flat panel from Uni-Solar for more like $120 that supplied sufficient power. (I bought mine from Online Solar.) This panel worked great with my HP Pavilion. So, of course, I started to modify it! (Very much in the spirit of Ray Jardine: if you can't or are afraid to modify it, then it isn't really yours, is it?)

Before I go on, let me briefly discuss power and voltage requirements of laptops (or any appliances, I suppose). The rating on the AC adapter doesn't tell you very much. In fact I was unable to find a good answer on the web or by asking sales or technical reps (no surprise there). I was really hesitant to do the following, but it's really the only way to get the information you need: chop open your AC adapter cord, and attach a volt-meter and an ammeter. (Be careful!! Even a laptop draws well over an amp -- enough to fry some meters... and you.) Now play around with your laptop and make note of variations in voltage, power, and current consumption. My HP Pavilion uses 16.5V and draws anywhere between 4 Watts to 40 Watts depending on usage. It draws the minimum while charging when it's off, and it averages around 6-8 Watts while running, but will spike up to 40 Watts when running speakers, bright screen, DVD drive, hard disk, etc. That corresponds to .25A to 2.4A. Fortunately laptop batteries are really sophisticated, so even if your panel delivers less than the max power requirement, so long as it delivers more than the average, the battery will make up the difference during spikes in usage. So strictly speaking I could get away with an 8 Watt panel, but I chose a 20 Watt panel so it would work on days with suboptimal lighting, and it would be able to deteriorate quite a bit with time before dying. It is important that you get a panel that has the right voltage at max power (see the discussion on photocell ratings later on) -- you will get significantly less power if you try to run it at a different voltage. Mine are rated for something like 20V open circuit, which is more than the 19.5V the AC adapter provides. I was a little concerned about this, but it seems the laptop can handle it -- after all, the voltage drops to the 16.5V max power voltage almost immediately upon loading it.

Uni-solar makes a fantastic photocell. It's all laminated together into a super-light flexible sheet. The problem is you can't just buy the cell. (Maybe you would have better luck -- I called and harassed some poor sales person and technical rep and got absolutely nowhere. Maybe I should've lied and said I was building a high school science fair project and made up some suitable story...) They sell both flexible and rigid panels, but even the flexible panel (much more expensive, by the way) is laminated to something to prevent you from flexing it too much. The rigid one I got isn't really rigid -- it's just a flexible one with a rigid aluminum frame around it. Tear that shit off immediately!

But now the damned thing is still laminated to a heavy semi-rigid piece of sheet metal. I was willing to let it go at this. But the problem is that it's just too damned big -- about 3 feet by 1.5 feet. This stuck out the top of my backpack and whacked the back of my head with every step (and drew lots of unwelcome attention from passers-by!) By time I got to my destination -- a secluded little niche a few miles up Eaton Canyon north of Pasadena -- I'd broken some connection inside so that it would only work if I curled the panel up just so. And even that stopped working soon. Okay, so I've clearly got to take it apart now and figure out how to rebuild it so that it will fold up more compactly.

Maybe you will have better luck stripping the solar cells from the stupid backing plate. I used a thin metal yard stick (sometimes I lack a certain finesse), pried up a corner with a knife and screwdriver, then jabbed the ruler in there repeatedly until I'd stripped it apart. In the process I scored a number of the cells, but none seemed fatally damaged. I utterly failed to do the same thing on a smaller panel later, so I ended up just cutting through the backing board and everything to disassemble it.

Now I took tin shears, cut the 11 cells apart, and cut away the insulation on the two ends. The basic wiring is really simple: (side view)

    _          _ (top)   _
(+)  \        / \       / \
     ======= / ======= / =======
          \_/       \_/       \__ (-)
                (bottom)

Anyway, now I was able to rewire it together. It may seem really straightforward at first glance -- just a bunch of batteries in series, right? Not quite. The voltage does add like you expect, but the current is limited by the weakest link. So if any cell is in the dark, for example, the entire chain goes dead. This is why you typically put diodes across each cell -- this lets current from the other cells flow even if some cells go dead. Likewise, if you want to add a couple low-power cells to your panel to give it a small boost be sure you wire them right. If your existing cells are rated for 1A, and your new cells are rated for 0.25A, then add them on in groups of four wired in parallel. (Here is a slightly more technical discussion of the physics of photodiodes.)

The rating of photocells is confusing. There are three points of interest. One is the short-circuit current (Isc). This is the current that the cell puts out if you short-circuit it. This can also be thought of as the theoretical maximum current the cell will ever put out no matter how much of a load you put on it. The next point is the open-circuit voltage (Voc). This is the voltage the cell puts out with no load on it. It can be thought of as the maximum voltage the cell can produce. The most important point is the maximum power point (Vmp and Imp). This is the point at which the product of the current and voltage is maximized -- ie. gives the maximum power. (The power is zero at both Isc and Voc, because Vsc = 0 and Ioc = 0, respectively.) (This is all so long as you're on Earth. :) All the ratings assume some nebulous average "full sun illumination". The max voltage isn't going to change much with increased illumination, but current is directly proportional to the intensity of light.)

Another thing to keep in mind is resistance in your wires and connections. For example my 20W panel usually runs 1 - 1.25A. Even 0.1 Ohm in each connection will cause a voltage drop of over a volt. Just do a good job soldering, and don't use super-thin wire.

The rest is mechanical work. I glued all my cells to a big piece of 5 oz cordura. The cloth was twice as wide as the panel and a little bit longer on each end. I can fold the sides in to protect the "business side" of the photocells, then the whole thing accordions, and the ends wrap it up into a nice compact bundle. I tried lots of different glues. I settled on barge cement (see the bottom of the moccasin page for places to find barge cement). I sanded down the back surface of the photocells with a hand drill fitted with a coarse grit disc sander, then cleaned both the cloth and the back of the photocells with acetone, dried them, painted them with cement, scraped off the excess with a putty knife, let that dry for a little while, then stuck them together (be careful -- once it touches it's stuck for good!), and set a bunch of tool boxes and heavy books on top of it. After this cured for a few days the bond became pretty strong.

The other thing I did out of paranoia is epoxy over all the solder joints. The top joints, especially, are attached to super-thin copper ribbon wire. I could totally see these tearing to pieces in my backpack. I used JB Weld, although many other glues might have worked equally well for all I know.

Another point to make is it might be hard to find a connector that fits your laptop's AC power port. (By the way, it's not really AC power when it reaches your laptop -- the AC adapter has already converted it to 16.5V DC!) I found some cool plugs at Radio Shack. They all fit a kind of standard two-prong male plug. So I attached the two-prong plug to my solar panel's cord, then bought the plug adapters I needed for my two laptops (I have my moms' old Dell laptop as well). I just switch plug adapters whenever I switch computers.