#Lithium battery repair

Opens with the customer: the battery hasn’t lasted well since 2024, used to charge for three hours, now goes green the moment it’s plugged in. Label says 48V 15Ah, made 2024-09; the problem is it won’t charge or discharge. Opened up: cylindrical 18650s; the case says 15Ah, but the cells are 2.5 each in 4P, really 10Ah; clearly salvaged old cells used a second time, many already leaking; very crude workmanship; he decides to scrap the cells and rebuild with brand-new ones; the only usable part of the old pack is the BMS, removed for reuse. Rebuild: 18650 at 2.5Ah each, 48V10Ah needs 13S4P, 52 cells, brand-new EVE; laid into the holder one group positive-up, next negative-up; series raises voltage, then 4P adds capacity. Welding uses 0.2 nickel-plated steel strip on a capacitor spot welder, settings on the screen (preheat auto 0.05ms, interval 20ms, gear 29, trigger 0.6s, continuous); the process is simple as long as nothing is missed; check for missed or wrong welds afterwards. Fit the BMS, usually on the negative side; pre-cut insulating paper where the sense leads run; tidy the ribbon and weld it to each group’s tab in order — the order must not be wrong; the sense leads monitor each group’s voltage and cut the BMS circuit if it goes too high or too low; to avoid mistakes, after each weld the lead order is checked on a test board before connecting the BMS. Finally the + and − power leads go on, a proper charge/discharge test (49.53V 4.92A), then insulation, back into the case, done. 16 comments: are the EVE 25Ps genuine (print and barcode misaligned); how much for a pack (1); how much for a 48V50Ah for a ride-share bike; how much per cell; are you in Shenyang; the original didn’t even have a holder; what BMS is that small.

36V 4.4Ah 144Wh, can you build a small battery like this

I've got one just like this, is it worth repairing

A 48V Li-ion battery; the customer says it only goes one or two km. No need to check the capacity: pick it up and you can tell the case isn't small but the cell capacity certainly is; no-name e-bikes off the internet all come with this kind; open it up and one section inside is empty; 18650 cells in 3P13S, ordinary low-capacity cells that won't last long even with light home use; test cell by cell with the resistance tester: readings 3.5–3.6V, resistance 16–24, screen says 'battery condition extremely poor, please service', no repair value at all. Replace everything: EVE 2.5 cells in 4P13S = 48V 10Ah, honest capacity with no inflation, an ordinary small bike will do 30-odd km no problem; spot-weld the cells in parallel and series with nickel strip; fit the BMS, charge/discharge test, battery done. 25 comments: many asking price (48V 10A / 30A / 5P13S, how much); 21700 5000mAh in 4P makes 48V 20Ah; lots of fake EVE cells, ¥2-odd each, zero volts after three months in storage; BAK 2900 without holders in 6P rides 60 km; a designated-driver bike's 48V 8Ah drops out as soon as the throttle turns, fixable?; Wuyang buyers got scammed with fake capacity.