A car jump-start pack built from a solar Bluetooth-speaker shell grabbed cheap on Douyin. The cells were also grabbed from the Douyin store, all around 2500mAh; paired with a Type-C 5V input charging board. He had built a spot welder from a microwave transformer before — hard work and poor results; then grabbed an AWithZ spot welder that runs a long time per charge, a hundred-odd yuan (an even better deal when it was ¥168), and it comes with small nickel strips. Before welding, stick insulating paper on the negative-can side so overheating can't short positive to negative. 3P4S; welder in auto mode, fires 0.5s after contact, tack one end first and press firmly; welds are clean and very solid; don't weld the middle too tight, leave a little slack — 'leave a line when welding and swelling stays friendly'; a foolproof device, very convenient. After all the positives were welded the charge was still full; the built-in battery beats his big homemade rig by far. Series: connect pairs in series, flip and connect again, main positive and main negative; measured DC voltage 16.01V, just right for a jump starter. Measured with a lead-acid battery tester as a makeshift: internal resistance 12.6mΩ, voltage 13.9V, discharge CCA 264 — should start a 2.0T car normally. Main positive and negative still need a few extra wires to boost discharge current. Test: two and three layers of nickel strip stacked both weld through solidly; conclusion, buying beats building, it's in the shop window. Comments: someone pointed out that a jump starter without copper strip makes no sense and the cells can barely do 2C (creator agrees, 'it'll do'); someone says this kind of welder uses a pouch cell that swells after heavy use and loses performance until it won't fire; a lead-acid tester can't measure Li-ion accurately (creator replies the accuracy is poor).