#BlindFix

A systematic lesson in hand-building a lithium battery pack. Terms: it’s ‘lithium battery’, not ‘aluminum battery’; 18650 is by far the most common, 21700/26650 you basically never need. Buying advice: not recommended — the only real channel is the ‘seafood market’ (Xianyu second-hand app), couriers won’t ship cells to individuals, and quality is a minefield; channel recommendations welcome in the comments. Choosing: storage cells discharge at about 3C (levels, power banks, flashlights, toys), big capacity and cheap; power cells are for cordless tools, 5C/8C/10C/15C; on a budget, 5C is enough for a drill or impact wrench, angle grinders / rotary hammers / saws want 10C or more or they cut out the moment you load them. Bigger capacity is better, lower internal resistance is better: a 2000mAh cell at 10mΩ can charge full and deliver the full 2000, at 50mΩ it may stop taking charge at 10% and get seriously hot (a loose example); barrel effect — the shortest stave is the high-resistance cell; measure IR as soon as cells arrive and keep the spread within 10–15% (if the max is 15mΩ the whole group sits at 12.75–15); an IR meter costs two or three hundred yuan. Voltage 3.6V, 4.2V full; within a group keep the spread ≤20mV, otherwise the high cell charges the low one and some overcharge while others starve; the first thing on receiving cells is to measure voltage and IR and group them. Assembly needs a spot welder, a soldering iron and a kit (power bank or power tool); the Dayi A3 style holds 90% of the power-tool market — cheap brushless tools are basically Dayi A3, a small share Makita, twenty-to-thirty-yuan brushed tools are basically the Worx small-foot style, the rest by brand (Dongcheng, Devon, Worx, Dongke). Ring the positive end of each cell with fish paper first: in reality only the top bump is positive, everything else is negative, and if the wrap gets nicked while welding strip it easily shorts and catches fire. Kit contents: BMS, holders, fish paper, label, nickel strip, screws, shell. Line the holders up, the BMS notch against the holder bump; B-/B+ are main negative/positive, cell negatives face the B- side; three cells per row, the second row flipped, the third flipped again, snap the holder on. Voltage/capacity math: 15 cells of 2000mAh is not 30,000; series adds voltage, 3.6×5=18, 4.2×5=21, 18V is the EU rating and 21V the US rating, there is also 42V; anything labeled 48VF/188VF/99999VF ‘can get lost’, not one of them tops 1500mAh; parallel adds capacity: 5S3P = 21V 6000mAh (6Ah). Welding: pen, strip and cell must be in full contact at all three points to avoid a cold weld; set current by strip thickness and machine brand, usually gear 25, beginners practice on strip first; two welds per cell, four welds per face, weld then flip. More fish paper for insulation, feed the strips through the BMS holes, screw it down; six solder joints with the iron, don’t let the solder bridge two pads; clean the tip. Check the voltage: DC on a multimeter doesn’t care about polarity, 18.6V, 18–21 is reasonable. Casing: the spring clip onto the holder bump, label on, OK. Making your own is satisfying. Comments (8174): ‘I bought a cordless rotary hammer and it’s no good — wrong brand?’ 4273 likes; ‘the original is too expensive, can I hand-build one?’ 1739; ‘the main cost is the spot welder’ 229; asking for a spot welder under ¥100; fish paper absorbs moisture, suggest plastic insulation; can a self-built power bank go on high-speed rail / planes.

Fix this for me

Boss, can you give a link for the spot welder

No offence, I'm a girl, don't roast me, I genuinely don't know — once the pack is assembled and runs flat, can it be recharged?

Xiao Zhao takes an order: make me a Dongcheng battery, 20V with 2000 cells, deliver to the 23rd floor when done. Grab purple-wrapped cells and a yellow holder, spot weld (DA DA DA), case it; labelling: no label looks unprofessional, ‘let’s get professional’, stick on the 20V (20Ah) label, now it looks the part; 130 (yuan), little sister takes the battery up to the 23rd floor; the square impact socket set goes to one side; her e-scooter dies one block out and she pushes it back, call a courier? how much can this order make and we’d hire a courier; take my car, petrol costs more than a courier; ours is electric, no petrol, just bought from Guazi Used Cars for only 99k (placement: official inspection, OEM warranty on the three electric systems, 100-day battery-degradation guarantee); a scatterbrained day, this family would fall apart without me. Another battery ‘this one’s no good now’, put back as it was, 80; done, 50. Sister comes back, the car’s in nice shape. Imported STIHL 381 chainsaw: an old sawyer managed to fit the chain backwards; won’t start, swap one; too much oil too, not turning so look at the brushes, rotor isn’t great; 10 yuan; screws all gone makes it easy, pull the battery first; 5 yuan; the all-rounder, 10 yuan, take it slow. 1812 comments: fix this for me (150), Bosch drill burned drilling steel, can I mail it in (99), you dare let sister deliver, not afraid it goes dark (37), they asked for 20V and what’s yours (2, about the 20V20Ah label); how big can a Metabo 18V go.

A customer wants a battery for his newly bought bait launcher; the original battery is dead. The shop owner digs out a similar cell — “not sure it’ll work” — and builds a pack on the spot: insulate as well as possible, wrap another layer of heat-shrink, then one size bigger, safety first; the heat-shrink is a bit large but works; plugged back into the launcher, switch on, works, “that’s it”, call it ¥50. After that: the customer’s drill isn’t working because the bit is too worn to grip; an angle grinder opened up with not a single bearing ball left and the grease dried solid; a pile of tools for repair, one by one: gears to one side, seals to one side, no power to one side, screws all gone to one side, impact jammed to one side, nothing wrong to one side; when fixing an angle grinder the wheel can shatter all over you, wear goggles; quotes ¥25/20/10/15, “jackpot”; eat the fan’s food first; swap the customer’s ratchet chuck for ¥15, the drill is a write-off. 33k comments: top comment “the boss as customers see him”, 480k; four 18650s plus a board and labor for only ¥50, make me two; is ¥15 for a ratchet chuck even enough for the part; the factory battery may not be as good as the one you made.

I’ve figured the internet out — whatever you want to watch I’ll make it for you, ‘but not that one’. First the power bank: make a 9600 (mAh) one, charge it up; I’ll use my own nickel strip; basically done; they don’t even give you fish paper, use this instead; do the cushioning first; real materials; power on, try charging, try discharging, done, and not bad looking, sis, you get a discount. Starts fine but the chain isn’t cutting fast — chain off, really hard to get off, way too much material removed, mind the chain tension, ¥5; a drill that won’t turn, oil coming out (singing); find a screw, charging for the screw and the gloves isn’t unreasonable, ¥5; another one won’t turn, the wire is spliced, snip a section, wire is bad; brute-force it in; a special tool for taking the blade off, ¥15; done. Comments: list of Douyin’s 6 great logistics organizations (4421 likes); petition of ten thousand to see the tree at the door (3161, ‘I said no’); can this be fixed, it keeps saying wrong password (249); which store sells genuine 18650s; welder link / brand; how much for the power bank.

A customer’s hot-glue gun lights up but won’t heat; open it, a wire has come off, stick it back, nothing wrong. Another wants a battery ‘delivered to Hanlin Huafu’ and asks whether to use an original or have me build one — ‘whichever works’; take 15 cells, open the cover, beautiful; measure a single cell at 395 (3.95V); pack assembled, done, ¥130. Then: a stripped screw inside a drill, apply some tricks (scorching hot; I have a way to fix it, done in the open, no secrets); an angle grinder won’t turn, the carbon brushes are worn, and the breaker tripped again (protecting the motor); another one also brushes and a broken wire, brushes are aftermarket not original; bearings are all gone, housing ¥40, bearings ¥10, swap in 607 and 608 bearings; ¥10 each. 9339 comments: top comment ‘original battery or one I make’ at 240k; ‘original isn’t as good as hand-built’; experienced viewers advise not to stick paper on the cell sides (absorbs water), use PET; someone asks whether the assembled pack can be recharged.