|Many people have asked me how to solder cells end-to-end, so I have prepared a series of photographs to show how I do it - this is not the only way, but it works for me. I use a Weller 80W soldering iron. You need a hot iron for soldering cells so that you can work quickly, and 80W seems to be fine. I also find this iron to be ideal, because it comes with a turned down soldering tip that is not too bulky - just right for our applications.
I also buy a spare, fat soldering tip which is 10mm in diameter. This will fit in to the Weller 80A, and it has enough meat for me to grind a hammer head shape in to the end of the tip. See the narrow tip fitted to the iron in the photograph, and the fat, modified tip beside it.
First, grind the ends of the cells ready for soldering. This provides a key for the solder, and also removes any grease and grime that would cause a poor joint. Then, tin both ends of the cells. While you are at it, tin the straps that you will use to join the two sticks of cells. I use 1/4" brass strips from the K&S stand at the hobby shop - thickest variety available.
You will need a jig like the one in the photograph. This is made from two pieces of wood joined at right angles, and standing vertically. I fixed my jig to a wooden base, so that I can clamp it to my work bench, and remove it when I do not need it. The jig must have some way of holding the cells in place so that you can have both hands free. I drilled holes in the jig, and threaded rubber bands through the holes, and attached them to small nails on the back of the jig. The rubber bands then hold the cells in place in a stack.
Place one cell in the jig, standing upright. Hold another one above it in the jig. Insert the soldering iron with the hammer head, and bring the top cell down on to it, pushing it down on to the bottom cell as well. Please understand that one face of the hammer head bit is touching the top cell where it was tinned, and the other face is touching the bottom cell where it was tinned. The V-grooves that you ground in the bit are there so you will not touch the heat shrink on the cells while you are doing this. It is important now to work quickly and smoothly. Count to five - do not leave the hot iron in place for too long or you will overheat the cells. On the count of five, lift the top cell, lift the iron out with a slight upwards movement (this is to prevent dragging solder on to the shoulder of the cell in the jig), hold for a split second, and then push the top cell down on to the bottom cell while the solder is still fluid. Do not be too hasty or you will splash the solder, and possibly cause a short-circuit. If you wait too long, the solder will cool too much and you will have a poor joint.
Test the joint. If the cells break apart, let them cool and then repeat the soldering job. Also test the voltage of the stick of cells with a multi-meter. You must see the voltage increase by at least one volt for each cell you add. If the voltage does not go up, you have a short circuit - you must then break the joint, tidy up the cells, and do it again. This rarely happens, but every now and then I have had to repeat a joint.
When you have the sticks assembled, tape them together in the correct configuration, and add the solder tabs which will either join the sticks together (in the case of an even number of cells), or add an extra cell in the case of an odd number of cells. If you are adding an extra cell, bend the brass strip (copper is even better if you can get it) so that there will be no short circuiting. Try to keep the strap off the shoulders of the cell at the positive end. It is possible for the strap to get quite hot during use (especially if you are drawing lots of amps), and it could melt the heat shrink casing on the cell and cause a short. I usually add a piece of balsa or ply between the cell and the rest of the pack to avoid any problems in that area as well.
Next, add the cables and check with a multi-meter that you have the correct voltage. If the voltage is not correct, check your assembly to see where you might have placed a cell the wrong way around. It is infuriating when it happens, but it is better to find out now. Does it sound like I have done it? Yes, you would be right. I have made blunders before, and that's why I keep checking all along the way.
The last job is to add the heat shrink. I prefer clear heat shrink because you can see what is going on with your pack. If you overdo it and vent some cells and leak electrolyte, you can see it. If you have corrosion starting, you can see it.
You can see what type of cells that you have in your pack. There is nothing more frustrating than trying to work out which pack you have because you cannot see the cells.
You can also see all your work, some of it which may not be as perfect as you would like - but it is better to be able to see what is going on (in my opinion).
I usually add a pieceof ply to the front of the pack where the cable exits - tape this on before you add the heat shrink, and then shrink the covering down on to the end plate.
Add your connectors, and trickle charge your pack for the first time - then go flying, and enjoy it! The performance will be down on the first flight, but the pack will improve with age as the cells "wake up". Happy landings!