NiCad & NiMH Cell Soldering Help
Oh, I've read it a hundred times. "You can't solder cells together without ruining them." Bunk! If you are decent at soldering wires together you can solder cells together. Can you ruin them? YES! But, you won't once you understand the problems. First, you don't want to get the cell hot, just the surface where you are soldering. The big mistake people make is they rig up the cells, lay the tab across them and then set off to soldering. This can work, but is too slow and can easily overheat the cell.
First things first. You need a good hot iron, in most cases, 50 watts or hotter. This way, the cell does not wick all the heat out of the iron before the surface is hot enough to melt the solder. Remember, the cell is like a big heat sink and it serves to cool the iron. The bigger the cell, the bigger the problem. The hotter the iron, the faster you can work, and the "cooler" the cell will be, as you work.
You need THIN ROSIN CORE solder suitable for electronics, like that available at Radio Shack. Don't try to do it with large solder (takes too much heat out of the iron and it is clumsy to work with) and don't try plumbing solder. You must use the right type of solder.
Glue the cells together in the proper configuration with CA glue so you don't have to struggle trying to hold the pack shape. Set them against a straight block of wood while doing this so they all go together evenly. This way you won't have one cell higher than the other making your tabs not lay flat across the surfaces.
Sand the end of each cell where the solder is to go. 100-150 grit works well, just take the shine off the cell where the tab, not the plating. Just the area where the tab will contact has to be sanded.
When iron is good and hot, tin the cell. This should flow out thinly and be shiny if you have the iron hot enough, and it should happen quickly. Just a little bit of solder is all that is required, way less than a drop. The first moment of solder contact should be right at the joint of the iron and cell, this will melt a bit of rosin on the cell, a bit of solder on the iron which will jump to the cell, now you have a good thermal contact with the cell, and the solder will melt on the cell. This step takes only about 1 or 2 seconds, if it is taking 5 or 6 seconds, then you don't have hot enough iron, or you did not sand the cell, or you have crappy solder. If it is right, it is quick!
Now, tin the tab where it will contact cell. You'll have to hold the tab down with a stick so when you lift the iron the tab does not cling to it. This tinning is easier than the cell, as the tab is usually designed to take solder and it has a much lower mass; meaning the soldering iron can heat it up easily.
Now, lay the tab across your tinned cells, solder against solder.
Heat the tab from the back side with the iron while holding the tab down with a balsa or hardwood stick (wood does not take heat out of the joint). When the solder melts you can push the tab down flush with the cell, the tab has less resistance to current flow than the solder, so in a perfect world you want to get the tab right down against the cell. This should happen quickly. If it does not work after a short bit, stop, let iron reheat and let the cell cool down, try again. It is helpful to tin the back of the tab so you can wick the heat through the tab quickly. Once the tab squishes down to the cell, hold it down with your wood stick and lift the iron. Give a few seconds to cool and you are done!
The idea here is to solder each joint in 3 steps (tin the cell, tin the tab, solder both together) so that you never overheat the cell. ANY cell can be soldered without damage with this method and some practice.
Several types of solder tabs, pack shrink, wire, cells and wire heat shrink are all available from Radical RC in our battery parts link.