Building a 48V e-bike battery from 78 × 18650 cells

Sized to the customer’s old case: 78 × EVE 2.5Ah 18650 cells in 13S6P for a 48V 15Ah e-bike pack

A customer sent in a 48V e-bike battery case whose cells were dead. The creator measured the inside (130 × 370), worked out that 13S6P would fit, and chose EVE 2.5Ah 18650s — 78 cells for 48V 15Ah. Cells go into the holder alternating positive-up / negative-up (parallel across, series down), get spot-welded into one block with 0.2mm nickel-plated steel strip on both faces, with the main positive and negative brought out separately. The pack is wrapped in strapping tape, the BMS is fitted with its balance leads soldered to each series group in order, the input/output leads are connected, and a charge/discharge test (49.14V / 6.50A on screen) is run before delivery.

Outcome

Steps9 steps · click a frame to jump

1. 
   1. Receive the empty case, confirm the cells are dead

   The customer’s 48V e-bike battery arrived as a bare case with dead cells inside; the brief was to rebuild a pack to the largest size the case allows.

   00:01 – 00:19
2. 
   2. Measure, settle on 13S6P

   The case measures 130 × 370; that fits 13 series × 6 parallel, giving 48V 15Ah.

   00:20 – 00:34
3. 
   3. Pick cells: EVE 2.5Ah 18650

   2.5Ah per cell, so 6 in parallel = 2.5 × 6 = 15Ah.

   00:35 – 00:47
4. 
   4. Load the holder alternating + / −

   First group positive-up, second group negative-up, alternating for all 13 groups; across is parallel (adds capacity), down is series (adds voltage); 78 cells in total.

   00:48 – 01:23
5. 
   5. Spot-weld into one block with 0.2 nickel-plated strip

   Plain nickel-plated steel strip (0.2 thick) welds every cell into a single block; check each weld is solid or it will crack loose under road vibration; weld one face, then the other the same way; bring the main + and − out separately.

   01:29 – 02:03
6. 
   6. Wrap with strapping tape

   After welding, wrap the whole pack in strapping tape for extra strength.

   02:06 – 02:17
7. 
   7. Fit the BMS, solder balance leads in group order

   Choose the BMS by the pack’s type, voltage and current; tidy the balance leads, then solder them to each series group in order so the BMS can read every group during charge and discharge; the leads must not be swapped or the BMS won’t work.

   02:18 – 02:48
8. 
   8. Connect the input/output leads

   Connect the pack’s input and output leads so the battery can work.

   02:55 – 03:00
9. 
   9. Charge/discharge test, then deliver

   On the tester run a charge and a discharge test — the screen reads 49.14V / 6.50A; if all is fine, hand it to the customer.

   03:07 – 03:16

Bill of materials

Measured on camera

Creator’s tips

• Across is parallel (adds capacity), down is series (adds voltage); alternate + / − when loading — first group positive, second negative 00:57
• Choose the BMS to match the pack’s type, voltage and current 02:21
• Measure the inside of the case before deciding S/P; capacity = cell capacity × parallel count 00:21

Pitfalls

• A weak weld will crack loose under road vibration — check every weld is solid 01:46
• Swap the BMS balance leads and the BMS won’t work 02:44

Li-ion safety

• Balance leads must never be swapped (a mis-wired BMS fails and you lose protection) 02:44

Quotes