My first diy battery pack, need helps!

I am going to build my first diy battery pack for my ebike. It will be 14s4p. Normal current needed is 10-15A. Maximum current is 20A. I will install the cells inside a battery case bought from taobao.com.

I am planning to buy the Sunkko 737G+ for spot welding. Any comments on this spot welder?

I am planning to buy the cells from tumich (https://endless-sphere.com/forums/viewtopic.php?f=31&t=61608). Will those cells with signs of removing welds difficult to spot weld? What should I pay attention to when doing the spot welding on those cells with signs of removing welds?

The battery case is with a cell holder. The cell holder is designed to use strip with width of 6mm maximum for cells connection. Will a single 0.15mm x 6mm pure nickel strip be good enough for my purpose?

I have heard from the seller of the 737G+ that spot welding 2 pure nickel strip is very difficult. Is this correct? If yes, then how can I make the connection between each group of cells. Will nickel plated strip be okay for my purpose and good for welding two strips together?

The connectors of the battery case for connecting to the cells are something like stainless steel or plated metal. Any suggestion in making good soldering to those connectors?



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I've heard of many problems with the 120V version of the Sunkko, but apparently the 240V version seems to be reasonably reliable...

Your planned nickel strip is 0.9mm2, good 4a optimal ampacity or 5 .95a acceptable ampacity for the 20a max you may see. Using 7.mm x .15mm H strip nickel will be 1.125mm2 and have a little more ampacity in it, 5a optimal and 7.44a acceptable.

Those welders are kind of worthless, and with pure nickel you'll have to use the electrodes directly on the body of the welder or make your own extensions. The extension cables on mine were copper plated aluminum. I ended up throwing that welder away and had better luck with a lipo powered one made by a korean hobbyist (aulakiria) which says a lot.

In the end it is very difficult to weld pure nickel, especially to itself, without good settings, good cleanliness, and a decent machine with enough power for hot quick pulses.

Your power requirements are minimal so I'd look into using nickel plated steel. It will spot weld much easier, even with the welder you already own.

https://endless-sphere.com/forums/viewtopic.php?f=31&t=68865
Read up on this model it might be up your alley. There is also a support thread for that machine on E.S. I have read of many problems with that sanko welder. Use a high power cell less stress.

If you do get the electric welder, replace the probe wires with 6 gauge pure copper welding cables (you can buy pure copper cables for less then 20 on ebay). I also recommend the battery powered welders, I use the malectric tab welder (about 100 dollars). The electric welder sunko I read reviews where some say its the best, but also lot of bad reviews, you need strong circuit breakers in your house to run it.

The malectric welder (available at malectrics website) runs on either 12 volt lead acid battery or even a 3s lipo battery, even on the low settings easily welded .15 tabs, but I use those 6 gauge pure copper probes that I made myself, they pass all the current to the probe tips. The thin copper clad welding probes (shown in your picture on the electric welder) get too hot during use, you might do 10 welds and have to take a 10 minute break so the cables cool down. Also those thin probe cables jump around everytime you do a weld, it surprise me when I was using them, but its normal.

If you got some old batteries, do test welds on those before you start on your good batteries. The power setting you use will depend on the type of wire you use on your probe cables. 6 gauge pure copper, need less power when welding nickel strip.

Thank you for all your comments.
Sorry for my late response because I am out-of-town now and had limit access of the internet in the past few days.

spinningmagnets said:

I've heard of many problems with the 120V version of the Sunkko, but apparently the 240V version seems to be reasonably reliable...

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Yes, I saw this advice from this forum. Therefore, I am planning to buy the 240V version. I plan to get 240V from the range. There is a power socket at my range at home, but it is just 120V. I have opened up my range and found that it is possible to modify that socket to supply 240V. Sorry, I am now out-of-town, I cannot take photos on that. Since the spot welder just needs high current for a very short time, I guess I can do the modification using the same socket (which is not designed for 240V). If I am wrong, please advise.

If I was purchasing a spot welder this is what I would buy..but you will need a power source like lipos or a decent lead acid battery

https://malectrics.eu/product/diy-arduino-battery-spot-welder-prebuilt-kit-v3/

I would do more reading up on the sunkko. Good looks aren't everything.