This is just a small introduction to soldering solar cells together. You may already know, but before we begin, it is good to remind you that there are both “tabbed” and “un-tabbed” solar cells on the market. Tabbed cells are a little more expensive, but if you are intend to build more that one panel, they will save you a heap of time, broken cells and frustration. Solar cells are very fragile, and the more you handle or mis-handle them, the greater are the chances of breaking them. Soldering solar cells requires care, and because untabbed cells effectively requires not only soldering right across a cell, there is twice the work and – well, chances are you will break some.
Tabbed cells come with the metal connector strips already attached to the front “sunny side face” of the cells, this strip has a free end long enough to lay across the following cell. These tab strips will connect the front of one cell to the contact points on the back of the next, and so on. Most cells are negative on the front and positive on the back and so allow the current to flow between the cells in your panel.
Why are tabbed cells good? Well, Like I said, there is a lot of work involved in soldering untabbed cells. Basically you have to solder a new metal tab strip to the front of each cell BEFORE you even look at connecting your cells together. In other words, you do a double job and that adds up to hours of fiddly work with fragile solar cells. So, my advice is, buy tabbed cells. By the way, the soldering iron should be a good quality 65 to 75Watt adjustable unit set at about 700F. If you run the soldering iron too cold, the solder will not run properly, too hot and you risk damaging the cells.
But, just in case you went and bought untabbed cells to save money, let’s explain what has to be done. Firstly, you need to solder a tabbing strip to the “sunny-face” of each of your cells. There are two ways to do this, both fiddly. You can solder the strip directly to the cell with flux or, pre-solder or “tin” each strip and then solder it to the cell. The idea is to get a solid connection to the solar cell or it will not transfer current. Solar tabbing ribbon typically consists of 10-15 micrometers of solder alloy, commonly SN60 (60% tin and 40% lead) coated on copper strip, note this contains lead. I would go without pre-soldering the tabbing strip as I’m a bit lazy. Either way, apply flux to the the big shiny strips(the bus bars) on the front “sunny-face” of the cell. Lay the tabbing strip along the fluxed bus bar and with a flat tipped soldering iron, run slowly down the strip. Extra fingers are good here. If you did a bad job, the tab strip will peal straight off. If done carefully, the tabbing strip will bond to the bus bar. I suggest practicing on a broken cell with some spare strip to get a feel for the soldering. Since there are usually two of these on each solar cell, there is a lot of fiddly soldering to do. Now do you see why it is better to buy tabbed cells?
Now to the next step, connecting your tabbed cell to its mate, the next cell. Basically you will be soldering the free end of the top “sunny-face” tabbing strip to the bottom of the next cell and repeating the process down your string. So, if you did not lay your cells out according to your panel layout, (e.g. 3 strings of 8 cells), or whatever your design will be, do that now. Leave a small space of about 1centimeter between each of them. It is a good idea to draw a template on some cardboard or Masonite to keep the strings neat and match the size of your array box.
Now that the tops are tabbed, flip them all over (sunny side down), but, slip out the tabbing strip from the first one so it lays over the back of the next. Do the same all the way down your “string”. You will be soldering the strip to the contact points on the back of the cells. If your cells are like mine, there will be 6 small whitish contact squares. Make sure your tab strips line up so they will sit over the contact spots then apply flux to the spots. Hold your soldering iron to the strip on the spot and when its hot, touch the solder wire to the point and let the solder flow, don’t apply too much and don’t over heat the contact as you may damage the cell. An alternative is to use a solder paste on the contact, lay the tab on and hold the iron on the point till it flows, for me, this was a better solution. Do the same for all six contact points, now have two cells connected. Repeat this process for all the solar cells along the string.
It is a pretty good idea to check that you have good solder connections, by exposing the cells to light and testing the output voltage. At the very least, check each string, it is too late when you have connected them all up to find you have a “dry solder” somewhere and have to trace it back. With the string of cells all soldered together, there is one more thing to do. At this stage you will have at one end of the string, the last one, with its “top tabs” free. But at the beginning of the string, nothing to connect to. So you need to solder a short length of tabbing strip to the back of the “top” (first) cell to have something to connect to and complete a circuit. So, on the “first” cell, solder tabbing strips across the back contacts with enough of a free end to allow you something to connect to. Now you will have connecting tabs at both ends of your string ready to connect to its neighboring string or a connector bus. Actually, now that you have read this far, its a good idea to do solder this “first” set of back tabs before soldering up a string, it saves some double handling.
So, there you have it, a completed string of solar cells you can use. How many strings of cells you make up per panel depends on what voltage you are aiming for. Building a DIY solar panel is fun and can save money, but make no mistake, there is work involved. You may opt for simply buying your solar panel off the shelf, but if cost is a factor and you have patience, a DIY solar panel is not a difficult project.