Re-celling a 48V Li-ion battery

Li-ion tech Zhang re-cells: a customer’s 48V “15Ah” battery (made 24-09) goes green as soon as it’s plugged in and won’t charge or discharge; inside are salvaged old 18650s in 13S4P, really 10Ah, many leaking, crude workmanship; cells scrapped, BMS kept, rebuilt as 48V10Ah with 52 brand-new EVE 2.5Ah 18650s (13S4P), 0.2 nickel-plated steel strip on a capacitor welder (preheat 0.05ms / interval 20ms / gear 29), pre-cut insulating paper where the sense leads run, tabs welded in order and checked before connecting the board, 49.5V/4.92A charge/discharge test

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.

Outcome

Steps8 steps · click a frame to jump

1. 
   1. Customer’s account: green light as soon as it’s plugged in; label 48V15Ah made 24-09, won’t charge or discharge

   Range has been poor since 2024, used to charge three hours and now goes green as soon as it’s plugged in; label says 48V charging voltage, 15Ah, made 2024-09; won’t charge or discharge; open it up to find out why.

   00:00 – 00:40
2. 
   2. Cells out: 18650, 2.5 each in 4P, really 10Ah not 15Ah

   Cells taken out of the box; cylindrical 18650; case says 15Ah, but 2.5 per cell in 4P is really 10Ah.

   00:40 – 01:15
3. 
   3. Salvaged old cells, many leaking, crude work; scrap the cells, keep only the BMS

   Clearly salvaged old cells used a second time; many already leaking; very crude workmanship; he decides to scrap them and rebuild with new cells; the only usable part is the BMS, removed for reuse.

   01:15 – 01:50
4. 
   4. 52 brand-new EVE 2.5Ah 18650s, 13S4P; one group positive-up, next negative-up in the holder

   18650 at 2.5Ah each, 48V10Ah needs 13S4P, 52 cells; brand-new EVE; laid into the holder one group positive-up, the next negative-up; series raises the voltage, then 4P adds capacity; all laid out in order.

   01:51 – 02:32
5. 
   5. 0.2 nickel-plated steel strip, capacitor welder (UF20B): preheat 0.05ms / interval 20ms / gear 29; miss nothing; check

   Welding uses 0.2 nickel-plated steel strip; capacitor spot welder; settings on the screen: preheat auto 0.05ms, interval 20ms, gear 29, trigger time 0.6s, continuous; simple as long as no weld is missed; check for missed or wrong welds at the end.

   02:32 – 03:18
6. 
   6. Fit the BMS (negative side); pre-cut insulating paper where the sense leads run; weld the ribbon to the tabs in order, no mistakes

   The BMS usually goes 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.

   03:18 – 03:51
7. 
   7. Sense leads watch each group’s voltage and cut out on faults; check the lead order on the test board before connecting

   The sense leads connect to every group and monitor each group’s voltage while charging and discharging; too high or too low and the BMS circuit is cut for protection; to avoid mistakes, after each weld the order is checked on the test board before the BMS is connected.

   03:51 – 04:17
8. 
   8. Connect the + and − power leads; charge/discharge test 49.53V/4.92A; insulate and back into the case

   Finally the + and − power leads go on and it charges and discharges normally; a finished pack must go through a charge/discharge test (49.53V 4.92A); after testing, insulate, back into the case, done.

   04:17 – 04:51

Bill of materials

Measured on camera

Creator’s tips

• Check real capacity as cell capacity × parallel count: 2.5Ah × 4P = 10Ah, not the 15Ah on the label 01:05
• Fit the BMS on the negative side; put pre-cut insulating paper down where the sense leads run 03:27
• After welding the sense leads, check the order on the test board before connecting the BMS 04:10
• UF20B on 0.2 nickel-plated strip to 18650 for reference: gear 29, preheat 0.05ms, interval 20ms 02:59

Pitfalls

• Salvaged old cells, many leaking, overstated capacity, crude workmanship 01:18
• Comments: EVE print and barcode misaligned, may not be genuine

Li-ion safety

• Sense leads in the wrong order will damage the BMS / battery 03:49

Quotes