This has been on my todo list ever since I reviewed the Sunnko 709A spot welder machine in Hackspace Magazine issue number 48 and I finally got a bit of time to play this morning. Making battery packs from recycled 18650 cells, the cells commonly found in older end of life laptops and other devices, is nothing new and there are plenty of technical blog posts and youtube guides out there to explain the process. It appeals to me on a few levels, I like the recycling part but also it's handy to make some battery packs up that can be used to power FPVground equipment or even for use with my Pinecil soldering setup in the field. Of course it's also great practice perhaps for making 18650 packs with brand new cells which I may be tempted to do for long range drone or fixed wing projects as you can pack a lot more capacity into a smaller and lighter battery than a LiPo.
I recovered these three cells from an old Sony laptop battery someone gave me and whilst I recovered 5 cells, I only wanted to build a 3 cell battery. I spent some time charging the cells to find that they were all serviceable and then made sure to charge these three cells so that their voltages matched pretty closely. This is useful when making a battery which you wish to balance charge as then it doesn't take an age in the first instance getting the pack to balance. The cells as recovered retained the little paper disks on the positive end which afford protection from shorts, It is good practice also to add extra insulation between the 18650 cells to protect from accidents and you can see in the above image I arranged the cells with some barley gasket paper between them. Whilst many people use a jig to hold the cells and then the pack covering to reinforce them, I decided a few spots of hot glue make for a robust structure.
Next up I cut some nickel strips to make the connections and then spot welded them using the Sunnko machine. For the strips that connect the cells I simply made straight cuts but for the positive and negative terminals I wanted to make tabs with smoothed edges to solder to to avoid them snagging or cutting any insulation or wiring. The spot welder can create really nice joints and it's great to be able to avoid soldering directly to the cells as it puts a heap of heat into them and isn't massively safe.
All that remained was to solder up an XT60 battery connector and also a balance connector and associated wiring. I made up my own balance connector using a JST XH connector and it's relatively straightforward to wire up, soldering to the new spot welded tags in between the adjoined cells, again, with some decent flux and a hot iron it's much easier to avoid putting too much heat into the pack. Finally a good dose of sanity checking and triple checking with the multimeter that everything is connected correctly and it's happily charging correctly. I've just used some Kapton tape to insulate the pack and hold the wiring in place, a more common approach is to find some large heatshrink which is on the list to pick up at some point. As I'm not too worried about the weight or the ascetics I am tempted to just give it a good wrap of PVC tape.
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