Feel free to use this invention/design for any non-commercial application. For commercial applications contact me and we can work something out. I completed a micro pack assembly to test out my process and also test how well the silicone heat transfer pad will cool the cells in the micropack before building a full size leaf module. The U shaped micro pack will show the ½ thickness buss bar that will be in the full size module between – and +, and the remote cooling blocks will allow the buss bars to also act as the cooling heat conductors.
The hard part about assembly is the devilish details in the process of attaching all the parts, I wanted to make it so that with modest cash the process could be reproduced, so a multi-hundred dollar spot welder will be needed, so long as it can provide ~1200amps to a parallel electrode weld handle. I made a few handles before settling on my own design. The electrode have to be independently supported by springs to get good welds, once the power hits the metal turns to jelly, and if the electrode comes off the surface it arcs and sprays metal everywhere, its nasty.
The file can be downloaded here:
It uses 2x ¼” diameter 4” long RWMA class 11 (75% tungsten 25% copper) electrodes threaded about ¾”up each end with ¼-20 threads, the points were formed on a drill press with a lathe cutter in the vise and the threaded the same way, with the die in the vise. That way it threads strait, tungsten copper is hard. Also this way the points and threads can be easily redressed as they wear out. The weld process is direct energy AC with a phase-fired SCR as the control mechanism, also known as a spot welder. The spot welder I used was a current feedback controlled, 480v/30amp input, but only 5.3v output, but with up to ~2500amps though. The .2mm pure nickel strap takes more power than the thinner, plated steel straps. There are several 3kva/kw spot welders on eBay (SUNKKO 709 or similar) that might have enough power for .2mm Ni, or maybe two layers of .1mm strap could be used. The straps need to conduct 17 amps peak when connecting two cells. And that puts the current density at 6.5amp/mm which is the maximum, 4.5amp/mm is the recommendation for best performance. .1mmx12mm should be enough for single cell connections. hmm..maybe I should only do single cell connections since the staggered cells are not suited for doubled up cells anyway.... To make sure the welds are good, I do a 90 degree pull test, if the welds pull out the strap, and I can see though the hole, it’s a good weld. If the strap peels off in the middle of the weld, it’s weak. The negative side of the battery is thinner than the positive side and so it needs less power, I used a longer 10 a/c cycle, cooler 1000 amp weld for the negative side so the electrode points did not press into the can. Then on the thicker positive side I turned the power up to 1250 amps, but cut the time down to 5 A/C cycles and that was also used for welding two layers of .2mm strap. I used 1 a/c cycle for upslope to cut down on sparking, you tube videos of welding batteries always have spark flying everywhere, that does not make for good welds.
Connecting the strap to the 2mm thick copper buss bar was not as easy as I had thought it would be, it won’t spot weld on. I think I will try blind copper pop rivets for the module assembly, which is cheap and easy for the home hobbyist. However, for the micro pack I laser welded it. I know this is not a practical solution, since multi-kw lasers are expensive, but for the curious, it worked really well and here are the weld details.
My next steps will be to test the micro pack with various charge and discharge currents that represent the important conditions the module will see, just at 2/27ths of the current. Then I’ll wrap it in insulation and measure the heating effect with no cooling, and then with cooling, like I did with the pouch cells, and document what I find.
Here is the assembly steps of the micro pack and how the silicon heat transfer pad is cut, installed, and folded into the module. The picture I posted awhile ago shows this from an end view but in real life the details are apparent. The silicon pad is sticky like chewing gum so it conforms to the surfaces for good heat transfer, I hope.