How to Solder Solar Cells Together to Make a String Ready to Wire into a Solar Panel
If you are thinking about doing a DIY solar project, there are heaps of technical and practical details that one really needs to come to grips with. I really do recommend getting a comprehensive DIY solar guide on how to build a solar panel, without it, you will be wasting your time and money. If nothing else, it will get a great overview and can decide whether DIY is right for your situation.
Just one of the aspects of a DIY solar panel project is joining all your solar cells together, typically about 36 per panel. Each cell is joined to the next in a series or “string” by soldering a specially coated “tabbing wire” to each cell, in other words, they need to be wired together. You may already know, but before we begin, it is good to remember that there are both “tabbed” and “un-tabbed” solar cells on the market. Tabbed cells are more expensive, but “trust me”, it is worth paying more and it will save a heap of time, broken cells and frustration. The tabbed cells come with connector strips already attached to the front bus (on the sunny side) and long enough to attach to its neighbouring cell. Most cells are negative on the front and positive on the back. So, basically what we are doing is connecting the negative of one cell to the positive back of the next cell and so on so that it can form an electrical circuit.
Why use tabbed cells? Good question, simply, if you don’t there will have twice the soldering work, and soldering to the front bus is the trickiest, fiddliest, frustrating job and if there is not get a good connection, that cell may not operate properly. Basically, with un-tabbed cells, you need to solder tabbing strips to each cell before soldering your string together, so – buy tabbed cells. Also, get a good quality adjustable 65 to 75Watt soldering iron, use it set at about 700F.
Aw Gee – I know, you already bought un-tabbed cells. Well, I’ll tell you what to do quickly. There are two choices, both fiddly. Now, solar tabbing ribbon typical consists of 10-15 micrometers of solder alloy coated on copper strip, and is commonly SN60 (60% tin and 40% lead), note this contains lead. Each strip needs to extend across two cells and the gap in between. You can solder the strip directly to the cell with flux or, pre-solder each strip and then solder it to the cell. If careful with the soldering iron, you can get a good bond between the cell bus and the tabbing strip. Remember, if the bond is no good, no current will flow.
Since I’m lazy, I would go without pre-soldering the tab strip. Either way, apply flux to the bus bar (the big shiny strip on the front). Lay the tab strip over the fluxed bus bar and with a hot flat tipped soldering iron, run slowly down the strip. The back of the tinned strip should melt and bond to the cell. If you did a rough job, the tab will simply pull straight off. Its a really good idea to either waste a cell or use a broken cell to get a feel of soldering to the front bus. Now maybe you can see why it is better to buy tabbed cells.
Now we all have all the cells tabbed and are ready to rock and roll. We will now start to solder a “string” of cells together. So, lay your cells out in your panel format, (e.g. 3 strings of 8 cells), or whatever your design happens to be. It is a good idea to draw a template on some cardboard to make it neat and match your array box, allow a small space between them.
Now here’s the trick, flip them all over (sunny side down), but, place the 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” so all the tails are laying over the back of the next cell. Basically we will be soldering the free end of the tabbing strip to the contact points on the backs of the next cell and repeating the process down your string. If your cells are like mine, there will be 6 small white patches that are the contact points we will solder to.
Line up the free tab strip so it passes over the contact spots then apply flux to the spots. Use something to hold the tab still and place the hot tip of your soldering iron on the tab strip over the contact point, touch the solder wire to the tab & let it flow. Don’t use too much and don’t overheat the cell or you may cause damage. I’ve take to using solder paste rather than fluxed wire and this works well. Once all contact points are soldered, you have the beginning of a string. Just repeat the process for all the cells in that chain.
I like to check that the connections are good as I go by exposing the cells to light and checking with a voltmeter that there is current. 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. Once that’s done, you have a string of cells connected.
Now there is one more thing to do. On the last cell of your string, you will have the “top” sunny-side tabs(negative side),free and unconnected. Later these will connect to your next string via a connecting bus or wire. But at the other end (the beginning), there are no free connections. We need to connect some short connecting tabs onto the BACK of the first cell (positive side) so that we can form an electrical circuit. So, on the “first” cell, solder tabbing strips across the back contacts to they extend out to where you will connect them, now you will have connecting tabs at both ends of your string. With these connecting tabs on both ends of the string, you have a working “string” of cells. Actually, now that you have read this far, I usually solder these on first, and then continue down the string.
So, now you have a completed string ready to place in your panel box after you have properly tested it. Now you can see that there is quite a bit of work involved in building a DIY solar panel. Make no mistake, DIY is fun and can save money, but there is work involved. After reading this or getting a DIY solar guide, you may opt for an off the shelf solution, but – you will still be miles ahead and better informed about what’s involved in a solar system. However, if you have the patience, you can save a fair bit of money by making your own panels.
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