I too have this Red inexpensive Spot welder.
There are actually several versions of these red welders available, and apparently the first one offered 3 years ago, was the best, as it used an actual mosfet driver, and not an optocoupler like the later versions.
There are plenty of videos on these cheal mini red ed spot welders on Youtube, some are nearly useless, but the ones by ‘Luca’, go into extreme detail. He talks about what modifications are required to get decent life out of the mosfets. different value resistor here, add capacitor there, check to see if this diode is blown. My red BSW was slightly different than the 3 different red ones he details and I was unsure what i should do, and when.my Lipo arrived, the only thing i did when testing was cut off the 0.8mm thick fork terminals and properly crimp on 1.12mm thick ring terminals.
From what i gather running these on too weak a battery will fry the mosfets. The capacitor mod is to Prevent the gate driver voltage falling too low during the weld and the diode resistor mod is basically in place of an XT90S Antispark. that huge capacitor inrush blows a diode which goes unnoticed until the mosfets blow.
I did not perform the mods Luca recommended, but I do use a separate 12v battery with xt90 antispark to power the circuit board, and a Zee 3s5200mah 80c lipo, with xt90s antispark as the weld battery. I also added more solder to the brass buss bars and mosfet legs.
Before the dual battery trick, I was getting solid welds with 2 layers of 0.15 pure nickel with a 95 millisecond pulse. It goes upto 99ms.
I also ordered a black BSW for about 20$, and it needed a 85ms pulse on its 3rd power level to do one layer of 0.15 nickel. I was disappointed, and hooked it to a pair of AGM golf cart batteries, and it went machine gun on me, itloudly tried to blow several holes in the battery can. Mosfets shorted.
Then the Purple BSW arrived. Luca reviewed this one as well, recommended a few mods, gate decoupling mod, and buzzer disabling. Said it was 3 times faster than the red spot welder. It also has room on the backside of circuitboard for 5 more infineon mosfets.
Another guy on youtube, added 5 more mosfets and beefed up the solder traces,.input and output leads, cranks it upto 99, and solidly welds 0.2 copper 0.2 nickel sandwiches, but no second driver baTtery, and i dont know what the weld battery was.
My purple BSW came with nothing , so I transferred the leads from the Red, used two 12V batteries, one for driving mosfets, one for welding.
I can solidly weld 0.15 copper with 0.10 nickel steel cap with a 50ms pulse, using the infinite slot method and the purple spot welder. No slot between electrodes, and the weld is not acceptable
With the copper nickel sandwich method, nickel plated steel works better than pure nickel. I am cutting 12mm wide 0.15 copper in half lengthwise, and 10mm nickel steel.in half lengthwise, then lightly welding those together, off battery, with a 8ms pulse, then laying two pre-sandwiched strips with a small gap across cells tops and 45 to 50ms pulse leads on opposite side of gap solidly welds both to battery.
Takes forever cutting the Nickle and copper strips welding them together, positioning sandwich on battery, then welding them. If 0.15 copper equals 0.6 of pure nickel, it seems worth it, to me
I can do the same with the RED BSW, at 85ms. Same battery same leads. The purple one has more gusto. The red has a better interface with the programmable delay and optional foot switch ability.
There is definitely a Goldilocks zone on the weld battery size/ health/ resistance with these.
How hard one pushes the welding pens has huge effect on the weld. Too hard yields a poor weld. I have no experience with malectrics or Kweld, but i imagine they yield better consistency of weld.
Please note the welds which appear to be in wrong spot near shoulders were the pre welds joining copper to nickel off battery, before welding that sandwich to battery.
Aligning all 4 strips on cell tops with gap is near impossible, so welding them together first and positioning just 2 is much easier.
otherwise id forget to drink my coffee. Really though i want a bench top ac welder but those are beyond hobby prices and a kweld is just a middle ground compromise imho.