like modify SMPS output, add registor to the voltage feedback to trick the welder high voltage protection. When you plug it into 20v battery pack the welder will alarm and not work. And with the registor trick, the welder will recognize the power source as 12v and can weld normal. There's a reason that why the welder annoying for the high voltage, so be careful, do it at your own risk.

@@TuanPham-eg4cp Hi. I see you are an expert. I'm new to it and mine, exactly like this one, is arriving from AliExpress next week. Do you think I could power it with a 3S2P 12.6V 18650 Li-ion pack with... say... around 4~5A, made with good quality and very low internal resistance salvaged laptop cells? (with BMS balance). One more question. As my pack is 12.6V, this 100K resistor trick isn't necessary, right? Thank you.

yes, you can use 3s2p 18650, it's ok. the registor trick is not necessary. But it will not work, the principle of spot welding is that the high ampe from shorting circuit will melting the metal materials. 18650 cells from laptop usually have large capacities and low discharge rate, which is not suitable for spot welding work. 4-5A is too low discharge rate. According to the manufacturer's recommendations, you need at least 100A peak current to work with 0.1 zinc.

@@TuanPham-eg4cp Understood. Thank you for answering.