A home-made jump starter from 18650 cells — only 2C cells, picked up for ¥20-odd on the Douyin store; DIY car emergency power, built into a bought Bluetooth-speaker shell

Jump starter, part 2: three layers of nickel tabs welded with almost no charge used; 4S BMS + balance board wired (main +, +1, +2, +4, main −), charges at 14.5V, panel shows full, USB is a slow charge, Type-C charging works; main + / − leads swapped for thicker wire, bundled and insulated with electrical tape, back into the shell

The whole pack is welded up, with three layers of tabs on top — very solid; the charge level has barely dropped, so it should weld a lot of packs — very handy. Wiring the BMS: main + goes to +1, +2, +4 and main −; the leads are short, so extend them; red sleeve on the main +, yellow on the rest, foam pads for a first fix; trim any sharp protrusions; fix the pack first, then wire it, or it’s hard to secure afterwards. With the BMS and balance board connected it charges at 14.5V from a low-voltage supply; press the switch and it shows full, the lamp is quite bright; USB charges a phone at 29% — works, but a very slow trickle; Type-C charging shows normal, the last lamp blinking means not quite full. The main + and − ‘thick’ leads aren’t actually thick, so they’re swapped for wire a good size up from stock. With all the main leads connected and tidied, no heat-shrink to hand, so it’s bundled with electrical tape, two turns over each contact point; it fits back into the original shell exactly, with extra insulation on the main + to prevent shorts. Comments: a car draws 200–600A to start, 3P 18650 gives 60–90A at most, and with enough current the nickel-plated steel strip burns through in seconds (creator: cells picked up from a live stream, 2C is as good as it gets); does drawing power straight off the pack without going through the BMS matter (creator: discharge doesn’t go through the board); copper strip thickness and cell C-rate are both too low, too dangerous; where to buy the welder (shop window); leave a watermark to stop reposts; part 3 tests capacity.

Outcome

Steps7 steps · click a frame to jump

1. 
   1. Three layers of tabs welded, almost no charge used

   Welded rock-solid; the whole pack is done, with three layers of tabs, very solid; the charge has barely dropped, should be good for a lot of packs.

   00:01 – 00:29
2. 
   2. Wire the BMS: main + / +1 / +2 / +4 / main −, extend the leads

   Lead positions for main +, +1, +2, +4 and main −; the leads are a bit short, so extend them.

   00:30 – 00:56
3. 
   3. Red / yellow sleeves, foam pads, trim sharp edges, fix before wiring

   Red sleeve on the main +, yellow on the others; foam pads for a first fix; trim sharp protrusions; get it basically fixed before wiring, otherwise it’s hard to secure afterwards.

   01:07 – 01:37
4. 
   4. BMS + balance board wired, charging at 14.5V

   Main +, +1... and finally main − connected; BMS and balance board done; 14.5V, can charge from a low-voltage supply.

   01:38 – 02:03
5. 
   5. Panel test: full, lamp on, USB slow charge, Type-C fine

   Press the switch and it shows full, long-press and the lamp is quite bright; USB charges a phone at 29%, works but a very slow trickle; Type-C charging normal, the last lamp blinking means not quite full.

   02:03 – 02:31
6. 
   6. Thicker wire for main + and −

   The main + / − ‘thick’ leads aren’t really thick; swap for wire a good size up from stock.

   02:31 – 02:54
7. 
   7. Bundle and insulate with electrical tape, into the shell, extra insulation on the main +

   Main leads connected and tidied; no heat-shrink, so bundle with electrical tape, two turns over each contact; bundled tight; fits back in the original shell exactly; extra insulation on the main + against shorts; next step is the mod.

   03:01 – 03:36

Bill of materials

Measured on camera

Creator’s tips

• Fix the pack first, then wire it, otherwise it’s hard to secure afterwards 01:27
• Red sleeve on the main +, yellow on the rest, for easy identification 01:07
• Trim sharp protrusions so they can’t pierce the insulation 01:20
• Extra insulation at the main + to prevent shorts 03:23

Pitfalls

• BMS leads are short and need extending; the stock main leads aren’t thick enough 00:41
• The USB output is a very slow trickle charge 02:03
• Comments: 3P 18650 gives 60–90A at most, starting needs 200–600A, nickel-plated steel strip will burn through; discharge doesn’t go through the BMS

Li-ion safety

• Discharge is taken straight off the pack, bypassing the BMS; at starting currents the nickel strip / cell C-rate is insufficient and may burn out (comments; the creator admits the cells are only 2C)
• Comments: beginners, remember to wear gloves

Quotes