A friend brought over a faulty pesticide sprayer; a check showed the battery had been drained dead, so a new Li-ion pack is made. Parts: battery box, connecting nickel strip, BMS, 18650 cells — HIGHSTAR ISR18650-2500 cells, 2500mAh; measured 4.2V, internal resistance 15.7/16, consistency is decent. The welder is an AWithZ U3 (nameplate: spot-weld output voltage 4.2V MAX, output current 650A MAX, Type-C input 5V/2.1A, USB output 5V/2.4A, battery 11000mAh 3.7V 40.7Wh, manufacturer Shenzhen Juequi Technology), only palm-sized, 11000mAh battery, 2-inch HD screen, 80 adjustable gears, manual trigger button; not just a spot welder but also a power bank. Weld strength test: the nickel strip tore and the weld still didn’t let go. The pack is 12V, three in series, three in parallel; the BMS is soldered to the cells in order; 4.2V, 8.4V and 12.6V all check out, so the welds are fine; the pack’s positive and negative leads are soldered to the terminals on the housing; the housing is sealed with glue; fitted, switch on, water comes out — perfect repair. Comments: it’s a sprayer, not a spray (liked by the creator); a 3S2P with a BMS gave no output even after swapping several boards; 9 cells isn’t enough; swap the original lead-acid for Li-ion and one charge on the lead-acid charger and “uh-oh”.