Welding: Sunkko 787A Hack

This page is a link from Conversion to eBike.

The Sunkko spot welder has a bad reputation because of its poor quality. It is also popular because of its price. And last but not least: Expect fuses to fly around your ears, it sucks full 16 Amps (and maybe even a bit more when switching on).

This link will probably die in time, but I bought it at this shop: https://www.aliexpress.com/store/615881 for USD 80 + USD 50 transport. Recommended!

Design is Quick and Dirty, good enough for just assembling a few batteries for laptops for example. A battery pack with 78 cells is a real challenge, if not impossible. The knobs on the front take away space that is needed so desperately, not smart to place them there.

I have build two battery packs 13s6p. In other words a row of 13 in series and 6 rows parallel. Now if I can weld three rows, I can rotate he pack and do the other three. To achieve this, an extension of the levers is needed. Below you’ll find the pictures. But first a picture of the hack in action:

It all worked great. I used ordinary 16A slow melting fuses. Welding was set to “double pulse”, max current (98A). I used the pedal, don’t forget to order that one! Just place it on the table. Each weld was four times executed and each cell pole received three weld spots (so pressing the pedal 24 times per cell, and I had 2*78 cells …). It took time but I am happy with the result. Removing nickel from a test cell means tearing the nickel apart and that is a good thing. And before I forget, after every 80 pedal presses I had to take a break to cool down the welder, helping it with cooling the copper tubes first with a dampen cloth. Now you understand it took a while to finish the pack

This is the end result. There is enough space now from the needles to the case, or more particular, the greenish knobs.

These are the extra parts from left to right:

  • Panhead M5 * 60 mm:
    • 7 mm shortened, threadlength 53 mm
    • Head grinded, so it just fits in the needle holders
  • Needles and holders: not modified. Square is 12mm, diameter is 10 mm.
  • Windings: Made of aluminium welding rod 2.5 mm because it is very easy to bend. The function is important, it positions the tubes.
  • Tubes: Copper from the plummer, outside 12, inside 10mm, lengths 35 mm.

Hope it inspires!

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