How to build a 12V outdoor-power Li-ion pack from 18650 cells

The pesticide sprayer’s battery was drained dead, so a 12V 3S3P Li-ion pack is built from 9 HIGHSTAR 18650 2500mAh cells (4.2V, internal resistance 15.7–16): AWithZ U3 (nameplate 4.2V/650A, 11000mAh, 80 gears, manual trigger, doubles as a power bank) — the nickel strip tears before the weld lets go; BMS soldered in order, 4.2/8.4/12.6V check out, leads soldered to the terminals, sealed with glue, the sprayer pumps water again

A friend brought over a faulty pesticide sprayer; a check showed the battery had been drained dead, so a new Li-ion pack is made. Parts: battery box, connecting nickel strip, BMS, 18650 cells — HIGHSTAR ISR18650-2500 cells, 2500mAh; measured 4.2V, internal resistance 15.7/16, consistency is decent. The welder is an AWithZ U3 (nameplate: spot-weld output voltage 4.2V MAX, output current 650A MAX, Type-C input 5V/2.1A, USB output 5V/2.4A, battery 11000mAh 3.7V 40.7Wh, manufacturer Shenzhen Juequi Technology), only palm-sized, 11000mAh battery, 2-inch HD screen, 80 adjustable gears, manual trigger button; not just a spot welder but also a power bank. Weld strength test: the nickel strip tore and the weld still didn’t let go. The pack is 12V, three in series, three in parallel; the BMS is soldered to the cells in order; 4.2V, 8.4V and 12.6V all check out, so the welds are fine; the pack’s positive and negative leads are soldered to the terminals on the housing; the housing is sealed with glue; fitted, switch on, water comes out — perfect repair. Comments: it’s a sprayer, not a spray (liked by the creator); a 3S2P with a BMS gave no output even after swapping several boards; 9 cells isn’t enough; swap the original lead-acid for Li-ion and one charge on the lead-acid charger and “uh-oh”.

Outcome

Steps6 steps · click a frame to jump

1. 
   1. Diagnosis: the sprayer battery was drained dead

   A faulty pesticide sprayer; the check shows the battery was drained dead, so a new Li-ion pack is made.

   00:04 – 00:15
2. 
   2. Parts and cells: HIGHSTAR 2500mAh, 4.2V, internal resistance 15.7–16

   Battery box, connecting nickel strip, BMS, 18650s; HIGHSTAR cells 2500mAh; measured 4.2V, internal resistance 15.7/16, consistency is decent.

   00:19 – 00:46
3. 
   3. AWithZ U3: nameplate, 11000mAh, 2-inch screen, 80 gears, manual trigger, power bank

   The welder is an AWithZ U3; nameplate 4.2V/650A MAX, Type-C 5V2.1A in, USB 5V2.4A out, 11000mAh; palm-sized; 2-inch HD screen; 80 gears; manual trigger button; also a power bank.

   00:46 – 01:14
4. 
   4. Strength test: nickel strip tears, weld holds; 12V 3S3P

   Weld strength — the nickel strip tore and the weld still didn’t let go; this pack is 12V, three in series, three in parallel.

   01:14 – 01:28
5. 
   5. Solder the BMS; verify group voltages 4.2/8.4/12.6V

   The BMS is soldered to the cells in order; 4.2V, 8.4V and 12.6V all check out, so the welds are fine.

   01:33 – 02:19
6. 
   6. Solder the terminals, seal with glue, fit and pump water

   Solder the pack’s positive and negative leads to the housing terminals; seal the housing with glue; fit it, switch on, water comes out — perfect repair.

   02:19 – 02:47

Bill of materials

Measured on camera

Creator’s tips

• Measure cell voltage and internal resistance first to check consistency 00:32
• Solder the BMS in order; afterwards check 4.2/8.4/12.6V group by group 01:44
• Tear the nickel strip and see whether the weld lets go to judge strength 01:18

Pitfalls

• Comment: a 3S2P with a BMS gave no output, same after swapping several boards
• Comment: after swapping lead-acid for Li-ion, users may keep using the lead-acid charger

Li-ion safety

• A lead-acid to Li-ion conversion needs a matching Li-ion charger, otherwise the lead-acid charger overcharges (comment)

Quotes