A car jump starter built into a solar Bluetooth speaker shell scored on Douyin. The cells were also cheap finds from the Douyin store, all around 2500; plus a Type-C 5V input charging board. He’d made a spot welder from a microwave transformer before — hard work and poor results; then he picked up an AWithZ spot welder, one charge lasts a long time, a hundred-odd yuan (bigger discount 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 + to −. 3P4S; the welder is on auto — it fires 0.5 s after contact; weld one end first and press down; welds are clean and very solid; the middle is not welded too tight, a little slack is left — ‘leave a line in the weld and a swollen cell will thank you’; a foolproof device, very handy. After all the positives were welded the charge was still full — the built-in power is way better than his big home-made unit. Series: join in pairs, then flip and join again, main + and main −; DC voltage reads 16.01V, just right for jump starting. A lead-acid battery tester made do: internal resistance 12.6mΩ, voltage 13.9V, discharge CCA 264 — should start a normal 2.0T car. The main + and − still need a few extra wires to raise discharge current. Tested stacking two and three layers of nickel strip — both weld through solid; conclusion: buying beats building, it’s in his shop window. Comments: someone points out that not using copper strip for a jump starter makes no sense and the cells can barely do 2C (creator agrees, ‘it’ll do’); someone says this kind of welder has a pouch cell that swells after lots of welds and loses performance over time until it can’t weld; a lead-acid tester can’t measure Li-ion accurately (creator: poor precision).