A while ago I bought 40pcs poly-crystaline solar cells (5 x 2 cm2) with the hope that I will find some spare time to build a small 5W solar battery charger.
Each cell provides an average of 0.28A @ 0.5V ~ 0.14W, just enough for my project. I payed only 4.4 USD which means ~0.8 USD/Watt. That's OK with regards to the manufacturing cost which is around 50 cents/watt nowadays. An off the shelf 5W solar battery charger starts from 18$ on eBay and can go as far as 100$.
If you just wonder how a solar cell works and most important why it works then maybe you should read more about the photoelectric effect, something that brought the Nobel price to guess who? If you answered Albert Einstein you are right. I wrote an article about this, if you can Swedish then be my guest. If not then remember, .
What you need for this project:
- solar cells (it would cost between 4-5$)
- tabbing wire, bus wire, flux pen, one diode (it would cost around 11$); I was lucky enough to have these from a previous project so they costed me nothing this time
- tools (multimeter, soldering iron, etc)
Few things you should know before you start:
- these solar cells are in particular very-very thin (like ~0.4 mm, see the picture below) and thus they are very-very fragile. So handle them carefully. If it's happening that they crack don't despair; usually with a bit of care you can solder them in such a way that they can be used at their full potential. It happened to me many times and I always managed to recover them successfully.
Solar cell thickness
- It would be cumbersome to use the bus wire as it is (5 mm wide) so my suggestion is to cut/split the wire in two stripes of 2.5 mm each. The 5mm bus wire is excellent for those >60W solar panels but not for this one, although the only rule about this is that there is no rule so it is entirely up to you 🙂
The bus and tabbing wire (from left to right)
Below you can see how I split the 5mm wide bus wire (forget the typo error in the picture) in two 2.5 mm wide stripes:
Bus wire splitted in two stripes
And here it goes:
The result: 4 x 2.5 mm bus wire
Before soldering remember this:
- cut 40 pieces (i.e. one for each solar cell) of 3-4 cm long tabbing wire stripes
- solder each tabbing wire on the top (blue/cathode) side of your solar cell;
- once you're done place the un-soldered part of the tabbing wire under the back of the next cell; on the back of that cell there is a white stripe/spot (anode); you should solder the tabbing wire end to that spot. Repeat this process for each cell until you get a soldered string (of a desired length) like in the picture below.
I would also recommend you to use some paper tape to hold together these solar cells because they are very fragile and very hard to handle otherwise. This trick will save your time and money. I am not going to teach you the technique of soldering these cells (it's not a big deal!). Anyway, this video should be a good point of starting. Just search on the YouTube about this topic, there are thousands videos about this ("soldering solar cells"):
A stripe of soldered solar cells, front and rear
If you have soldered all these cells (i.e. you've got 4 stripes of soldered cells) then it's the time to hook them together. Depending on what you want to achieve you could solder these stripes in series or in parallel. In the first case you'll get more volts but less amperage while in the later you will get less voltage but more amperes. Anyway, don't worry, your solar panel will still deliver the same power in both cases since P=UI (power is voltage multiplied by amperage).
Four stripes of soldered cells ready to hook together
If you are going to solder them in series you will probably get around 0.28A @ 20V. Anyway see the picture above and the one below if you want to understand how the current flows through these cells and thus how those bus wires should be soldered together with the tabbing wire ends of each set (of 4).
Take a look at the picture below. Do you see the bus wire that is soldered on one end of the stripe and to the other end of the next stripe (that silverish wide wire positioned just under those red arrows on the top)? That's how you solder the tabbing wire wire (the vertical wires which now keeps together those two stripes of solder cells) with the bus wire (the horizontal wide wire on the top and even on middle-bottom). We want the current to flow through each wire, from the left to the right so solder them always like this: the end of the tabbing wire from the top of a solar cell (cathode) to the end of the tabbing wire from the bottom (anode). If you will solder them anode to anode and cathode to cathode then you'll get a parallel connection which might be OK if that is what you want.
The four stripes soldered together with the bus wire
If everything went OK then your multimeter should shows something like 20V or so (depending on the quantity of solar radiation that falls hopefully perpendicular on the solar cells):
Not bad for an amateur, right?
Assuming that your toy already works you can hook a DC fan or 5W DC bulb to the two ends of tabbing wire (bottom-left and bottom right respectively in the picture above).
Yes, it works!
Note that the final dimension of this panel is something like 20cm x 20 cm ±1.5 cm depending on the spaces you let between the cells, for the frame, etc. At this size and weight it should be regarded as portable!
This project is not 100% finished, it still needs (TBD):
- a 2mm sheet of glass (like 21.5 x 21.5 cm2) to protect the cells and also a wooden frame (why not a modeling clay frame?)
- two wires (preferably red and black/blue) soldered on those two tabbing wire ends; additionally a diode soldered on one of these two wire so that the current will flow always only from the solar cells to battery (i.e. from - to +) and never inversely
- a micro USB and/or standard USB socket so that we can hook any USB device to it (to charge it or to feed it with real power 🙂
- a solar charger controller that handle the current and the potential delivered to the battery/device
Now, if you think that this article was interesting don't forget to rate it. It shows me that you care and thus I will continue write about these things.
Eugen Mihailescu
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