I’m trying to find out if I can use my hulking resistance soldering setup to weld nickel strips on AA cells, with the intent of making some decent 8-cell packs that don’t have crappy stainless-steel springs. Having slit the copper sheet for the jaws, I just now kludged together some electrodes…
The positive terminal on an AA cell is almost exactly 3/16 inch in diameter, call it 0.188 inches. That’s the hole in the middle of the copper sheet, which is neatly split so it clamps the terminal button from all sides with nearly equal griptitude.
The pliers are snap-ring pliers, with the original weird metric screws (neither 3 nor 4 mm, which is all I have) replaced with stainless steel 8-32 screws. Drill-and-tap the pliers jaws, clearance drill the not-quite-rectangular clamping plates, bend the jaws so the copper sheet aligns properly. It’s all good.
I plan to add a jumper connecting the two copper sheets; obviously, you don’t get good current transfer without a solid connection. The darker gold-copper color in the center section is Kapton tape insulating the top of the jaw sheets.
The cable goes off to one terminal of the resistance soldering transformer, which is a rewound kilowatt-class microwave oven transformer. The basics are 5 V RMS at about 200 A, with a foot switch into a microcontroller that drives a triac on the transformer primary. I can set the timing in multiples of 100 ms (6 AC line cycles) and the duty cycle from 1 to 6 of the cycles in each 100 ms. More on that later; the triac triggering is nightmarishly complex because I was doing a Circuit Cellar column and wanted to show how a triac gets all confused driving an inductive load. It really needn’t be that fancy in real life.
Anyhow, 200 A is at least an order of magnitude less than the current from a capacitive-discharge welding setup, but I’m hoping that with some tweaking I can get enough heat to make it all work out. If not, it’ll still be a king-hell resistance soldering setup.
The center electrode started life as an oil-burner ignition electrode. It’s a steel shaft joined to a (most likely) tungsten probe within the ceramic insulating tube. The cable goes off to the other transformer terminal.
Tungsten is a fairly crappy conductor, so I forged a copper clamp around the end of the electrode. It started as a section of the same copper pipe that went into the pliers, hammered around the wire. That took many annealing cycles, which basically consists of heating the copper red-hot with a propane torch and letting it cool for a bit.
The two smaller screws apply clamping pressure to the copper around the electrode, which ought to improve the contact area. I plan to anneal the clamping area one more time, scrubulate the inside of the clamp, then screw everything together nice & tight with maybe a bit of anti-oxidation compound in there for good measure.
The general idea is to apply the current as close to the AA cell’s terminal as I can. I think I must file / grind down the end of the probe so that it’s applying the juice exactly to the center of the nickel strip at the middle of the terminal.
The first test was 500 ms at 100% duty cycle, which produced a nice spatter of sparks from underneath the strip, the tungsten glowed orange, but the 8 mil nickel strip didn’t weld itself to the cell top. No weld nugget. Bupkis.
I think it’s got potential, though.
The Smell of Molten Projects in the Morning 
I’d just like to take this opportunity to groan at the terrible, terrible pun in the last sentence.
Gotcha!
“The first test was 500 ms at 100% duty cycle, which produced a nice spatter of sparks from underneath the strip…”
Inverter spot welders (and I assume capacitive discharges ones as well) can be programmed to first apply a “cleaning pulse” to burn through any finger prints, oxidation, etc before the big weld pulse happens. I wonder if this would help?
– Steve
Having written the firmware, it’d be easy enough to add a pattern with a cleaning blip before the main pulse train.
Do you think a few cycles would do the trick? Or is it more complicated than that?
What would really help, I think, is having a pointy electrode under significant pressure, with a large return path from the end of the cell. That’d concentrate the heat on the spot marked X, which didn’t really happen here.
What are you using for nickel strips? Can you recommend a good source for inexpensive raw material?
No battery pack goes to its grave around here without donating its conductors to the collection and I have a sheet of nickel from some unknown source in the shimstock folder.
Every now & again Electronic Goldmine offers a stock of useful metal; I have several sheets of mu-metal just waiting for something to shield. I have some phosphor bronze sheets with K&S on the label that I think came from Micro-Mark.
Although eBay is my parts locker, I’m not convinced they’re the low-cost commodity metal supplier, even though they generally win for single-piece purchases of something I probably won’t buy again.