Teaching you the 8-cell car jump starter

Hand-build Bro makes a car jump starter from 8 tabless 21700s (¥18 each): IR meter reads 2.67mΩ, all matched → 2P4S, fish paper on the positives, AWithZ P60F (nameplate 3250A, voice-over ‘3000-odd amps / nearly 30kW’) at gear 41 welding copper stacked on nickel strip, anyone who knows can tell from the spot it’s welded through; pack IR 6.71mΩ, lower than lead-acid; B1/B2/B3 soldered in order to each group’s positive, EC5 soldered onto the copper-nickel tab, leads soldered to the copper and on to a PD module; the result is both a power bank and a jump starter

Weld these 8 cells and you can start any car on the market, whatever the engine size. Build a car jump-start battery from 8 tabless 21700s (brand new, about ¥18 each): the IR meter reads around 2.67, normal, and if they all match, assemble; two parallel, four series, fish paper on every positive, layout: the bottom two as main positive, the top as main negative, fix it loosely and spot weld; copper stacked on nickel strip, welded with the AWithZ P60F supercapacitor spot welder — with a low-power welder, stacking copper is very hard; nameplate 9V MAX, 3250A MAX, input 9V/15A, 3000-odd amps instantaneous, nearly 30kW peak; straight to gear 41 for the copper stack, anyone who knows can tell from the spot that the nickel strip and copper are fully welded onto the cell; weld all the rest. Extend the copper tabs out from main + and −; pack IR 6.71mΩ, lower than a starter lead-acid battery. Wiring the BMS: pads B1/B2/B3 go in order to each cell positive, the B1 lead is soldered to the bottom positive, flip it over for B2, feed it through and solder B2, then B3; the EC5 connector’s + and − are soldered straight onto the copper-nickel tab, very handy; two leads soldered to the copper and extended to a PD module; the result is a high-capacity power bank with an EC5 on the side to jump-start a car, lots of fun. 892 comments: ‘can’t afford this welder at three thousand-odd’ (creator: three thousand what, where did you hear that — 3000A mistaken for the price); 6 supercapacitors strapped straight on start it in a minute and don’t mind heat in the car (18); 4 BYD blade cells cost 100-odd (8); never buy a 3D-printed case (4); only 3000A, a DIY capacitor welder does 20,000 amps; Doubao says yours will explode.

Outcome

Steps6 steps · click a frame to jump

1. 
   1. 8 tabless 21700s (¥18 each), IR meter 2.67mΩ, all matched

   Starts any engine size; buy 8 brand new at about ¥18 each; IR meter reads around 2.67, normal; matched, so assemble.

   00:00 – 00:22
2. 
   2. 2P4S layout, fish paper on positives, fix; copper on nickel strip, P60F

   Two parallel, four series; fish paper on every positive; bottom two as main positive, top as main negative; fix loosely; copper stacked on nickel strip; AWithZ P60F supercapacitor spot welder.

   00:22 – 00:41
3. 
   3. Nameplate 3250A; low power makes copper stacking hard; gear 41 copper stack, welded through

   Low power makes copper stacking very hard; nameplate 9V / 3250A / input 9V 15A; 3000-odd amps instantaneous, nearly 30kW; straight to gear 41 for the copper stack; anyone who knows can tell from the spot the nickel and copper are fully welded onto the cell; weld the rest.

   00:41 – 01:06
4. 
   4. Extend main + / − copper tabs; pack IR 6.71mΩ, lower than lead-acid

   Extend the copper tabs out from main + and −; how scary is the pack IR: 6.71mΩ, lower than a starter lead-acid battery.

   01:20 – 01:34
5. 
   5. BMS B1/B2/B3 soldered in order to each group’s positive; EC5 on the copper-nickel tab; leads to PD module

   Pads B1/B2/B3 go in order to each cell positive: B1 lead to the bottom positive, flip over, feed B2 through and solder, then B3; EC5 + and − soldered straight onto the copper-nickel tab; two leads soldered to the copper and extended to the PD module.

   01:34 – 02:13
6. 
   6. Result: power bank + EC5 jump start

   Final result; a high-capacity power bank with an EC5 on the side to jump-start a car; lots of fun.

   02:17 – 02:30

Bill of materials

Measured on camera

Creator’s tips

• Measure IR first; assemble only if they match 00:14
• Stacking copper needs a high-power supercapacitor welder, low power makes it very hard; P60F at gear 41 does it 00:41
• Extend the main + / − copper tabs for easier connections 01:20
• Solder the B leads in order to each group’s positive; solder the EC5 straight onto the copper-nickel tab 01:34

Pitfalls

• Commenters mistook 3000A for ¥3000
• Comment: never buy a 3D-printed case (unless you own a printer)

Li-ion safety

• Fish paper on the positives; P-series pen tips may only touch the same terminal plate (nameplate warning) 00:14
• Commenters worry about explosion (per Doubao)

Quotes

Cost & time