Archive for August 25th, 2009
Much as I expected, there’s just not enough energy in a 5 V / 200 A resistance soldering unit to weld 8-mil nickel strips to AA cells. But the gadgetry needed to contact the cells works fine for resistance soldering.
I took a pair of sacrificial cells, grabbed the positive terminal of the blue one in the pliers, applied a flattened snippet of good old rosin-core tin-lead solder (see below), laid the strip atop it, and held things together with pressure from the tungsten electrode.
About 800 ms of current did the trick; the electrode heated to middling orange by the time the current shut off, which indicates it was the highest-resistance part of the circuit. Eyeballing an ammeter clamped around a secondary lead says the peak current was 250 A, a bit over the nominal 200 A, but close enough.
The obvious dent in the strip over the positive terminal shows that the center of the solder strip melted first; I could feel the tungsten electrode sinking into the strip as it heated.
For the negative terminal, I grabbed both cells in a small vise (resting on insulation below the bottom terminals!), tucked another solder strip under the nickel tab, pressed one jaw of the pliers against the cell, and hit the tab with the tungsten electrode. Lovely fillet, isn’t it?
The joints look good inside, too. I cut the strip, then peeled the joints apart: they’re both fully wetted. You can see some tiny bubbles from the rosin, but I doubt that’s a problem.
Now, you don’t want to solder to AA cells by hand with a soldering iron, because it’s entirely too easy to cook the piss out of the plastic insulators, pressure relief valves, and other internal gadgetry. Yeah, I’ve done that too, and it works most of the time, but it’s not recommended.
A controlled pulse is all over and done with before the rest of the metal case has time to get more than warm. In fact, by the time I put down the electrodes, the nickel strip was cool enough to touch! The copper jaws act as a heat sink for the positive button and the negative terminal is the entire can around the cell, so I think this will be OK.
I’ll do a bit of testing on some sacrificial cells to figure out the minimum time required for a good joint; I think 600 ms will do. I might use a carbon electrode for the positive terminal to get a somewhat larger contact over the whole button and eliminate that unsightly dent.
Solder prep: I flattened about 3 mm of ordinary solder wire by whacking it with a polished brass hammer on a chunk of PCB stock. Flat solder works better than round solder for resistance soldering, as everything stacks up neatly with lots of contact area. The pix there should give you the general idea.
I’m mildly unhappy with the pliers, which must open a bit too far for my paws. A fixture that fits in the bench vise might be in order…