Class is in session, everyone — the ‘Whampoa Academy’ of the tool world

Fix Tools Blindfolded ‘Li-ion battery class’, 8 minutes: 18650 basics, choosing storage 3C vs power 5–15C cells, internal-resistance spread ≤10–15%, voltage spread ≤20mV, why fish paper goes on the positive end, assembling a Dayi A3 kit into 5S3P 21V 6000mAh, spot welding at gear 25 with two welds per cell, soldering the BMS with an iron, reading 18.6V and fitting the case

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.

Outcome

Steps11 steps · click a frame to jump

1. 
   1. Intro; terms; 18650; buying advice ‘not recommended’

   Hand-built lithium battery class; it’s ‘lithium battery’ not ‘aluminum battery’; 18650 is the most common, 21700/26650 basically never needed; the only channel is the ‘seafood market’ (Xianyu), couriers won’t ship cells to individuals, quality is a minefield and there are no returns, asks for channel recommendations in the comments.

   00:00 – 00:57
2. 
   2. Choosing storage 3C vs power 5–15C

   Storage cells 3C (levels / power banks / flashlights / toys), big capacity and cheap; power cells for cordless tools 5C/8C/10C/15C; 5C is enough for drills and impact wrenches, angle grinders / rotary hammers / saws want 10C or more or they cut out under load.

   00:57 – 01:40
3. 
   3. Capacity and internal resistance; barrel effect; IR spread 10–15%

   Bigger capacity is better, lower IR is better; 2000mAh at 10mΩ discharges fully, at 50mΩ it may only charge to 10% and get seriously hot; the shortest stave of the barrel = the high-IR cell; measure IR on arrival, spread 10–15% (if the max is 15, the whole group sits 12.75–15); an IR meter costs two or three hundred.

   01:40 – 02:42
4. 
   4. Voltage 3.6/4.2V; spread ≤20mV; measure before grouping

   An 18650 is typically 3.6V, 4.2V full; within a group keep the spread ≤20mV, otherwise the high cell charges the low one, overcharging some and starving others; comments keep saying nobody measures — the first thing on receiving cells is measure voltage, measure IR, then group.

   02:42 – 03:15
5. 
   5. Tools and kits: Dayi A3 holds 90%

   Needs a spot welder, a soldering iron and a kit (power bank / power tool); the Dayi A3 style holds 90% of power tools, 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.

   03:15 – 03:53
6. 
   6. Why fish paper goes on the positive end; kit contents

   Before assembly ring the positive end with fish paper: on a real cell only the top bump is positive, everything else is negative, and a nick in the wrap while welding strip easily shorts and catches fire; Dayi A3 kit: BMS, holders, fish paper, label, nickel strip, screws, shell.

   03:53 – 04:31
7. 
   7. Aligning holders and BMS; three rows alternate direction

   Line the holders up, BMS notch against the holder bump; B-/B+ are main negative/positive; cell negatives face the B- side; three per row, second row flipped positive-down, third row flipped again; snap the holder on; B- to negative, B+ to positive.

   04:31 – 05:14
8. 
   8. Voltage/capacity math: 5S3P = 21V 6000mAh; don’t trust xxVF

   15 cells of 2000mAh is not 30,000; series adds voltage, 3.6×5=18, 4.2×5=21; 18V is the EU rating, 21V the US rating, there is also 42V; 48VF/188VF/99999VF can get lost, none of them tops 1500mAh; parallel adds capacity: 3P 6000mAh = 6Ah.

   05:14 – 06:24
9. 
   9. Spot welding: three-point contact against cold welds; gear 25; two welds per cell

   Power the spot welder on; pen, strip and cell in full contact at all three points to avoid a cold weld; set current by strip thickness and machine brand, usually gear 25, no fixed value, beginners practice first; two welds per cell, four welds per face; flip and weld the other side.

   06:24 – 06:57
10. 
    10. Fish paper, strips into the BMS, six solder joints with the iron

    More fish paper for insulation; feed the strips through the BMS holes, fix the screws; six solder joints with the iron, don’t let the solder bridge two pads; clean the tip.

    06:57 – 07:26
11. 
    11. Read 18.6V; case and label; closing

    Check the voltage to be safe: DC on a multimeter doesn’t care about polarity, 18.6V, 18–21 is reasonable; casing: spring clip onto the holder bump; label on, OK; won’t mention what some brands charge for the same quality — making your own is satisfying.

    07:26 – 08:09

Bill of materials

Measured on camera

Creator’s tips

• Measure voltage and IR on arrival, then group: IR spread ≤10–15%, voltage spread ≤20mV 02:22
• 5C is enough for drills / impact wrenches, angle grinders / rotary hammers / saws ≥10C 01:28
• Ring the positive end with fish paper: everything on a cell except the top is negative, so a nicked wrap shorts 03:54
• Pen, strip and cell in full contact at all three points to avoid cold welds; two welds per cell 06:29
• BMS notch against the holder bump, B-/B+ set the cell direction, three rows alternate 04:37
• When soldering the BMS don’t let the solder bridge two pads; clean the tip afterwards 07:15
• Cells labeled 48VF/188VF can’t be trusted 05:51

Pitfalls

• The only channel for cells is the ‘seafood market’, couriers won’t ship to individuals, quality is a minefield and there are no returns 00:44
• One high-IR cell drags the whole group down (barrel effect) 02:16
• Comments: fish paper absorbs moisture and cells rust in damp conditions — plastic insulation paper suggested

Li-ion safety

• A nicked cell wrap lets the negative can short against the strip and catch fire — the positive end must be insulated 04:13
• A large voltage spread leads to overcharged cells 02:54

Quotes

Cost & time