#Boston 5300

Bought a complete Boston Swing 5300 Li-ion pack at a scrapyard for ¥30 (a single cell normally goes for ¥8–9); 5 of the series groups are fine, each parallel group about 3mΩ, one group has two dead cells; bought two replacements on Douyin and there is a complete 24V pack; it is no good without a BMS, and he happened to pick up another pack with a BMS to fit to this one. Repair: weld the nickel strip on with the welder (a bit crooked; the welder is very easy — press it down and one click, bang); push the cells into the holder in order, insert from the front, flip it over and weld the underside too, leave some room to move; very solid; the portable welder is just like a power bank; not a neat job but it works; the welding pen tips need regular cleaning, they oxidise and cause poor welds (the pen got hot); ugly after welding but very solid. This parallel group measures 3.5, the others are all 2.9–3.5; three are welded, keep welding the rest into one strip, rough but solid. Total voltage 24V, internal resistance 19.8mΩ, nearly 20. Fit the BMS: the salvaged one, opened up, was cracked and water-damaged, no good, buy another. Discharge test: multi-function tester clipped on at 25V, two H4 bulbs (yellow + green together is 24V, black-yellow / black-green is 12V, don’t mix them up or they burn out), sparks flying, discharge current 10.8A (about 261W), no problem; the voltage drops fast because one group isn’t full — charge it up and test total capacity when the BMS arrives. Comments: with hands like that, thumbs-down for not building your own welder; AC or DC resistance meter (seems to be AC); it’s got some years on it (five or six); what resistance and what C rate (single cell 22mΩ, new 13mΩ, 3–6C discharge, mine I reckon 1C is fine); what is it for (storage / backup power); this kind of welder won’t hold on these cells, goes intermittent after a while; chicken-poop welds.