A step-by-step guide to designing and building an 18650 lithium-ion pack: series/parallel math, cell choice, BMS, nickel strip, spot welding and the safety rules you can't skip.

A homemade 18650 pack can power an e-bike, a portable power station, a robot or a solar backup system for a fraction of the cost of a pre-built one. Done carelessly, it can also start a fire. This guide covers the parts you need, how to put them together, and the safety steps you shouldn't skip.

Shop: Molicel P28A INR-18650-P28A 2800mAh 35a Flat Top Battery

1. Design the pack before buying anything

Every pack is described by its series (S) and parallel (P) count:

• Series sets voltage. Multiply the cell count by 3.6V nominal (4.2V full). 10S = 36V nominal, 13S = 48V, 14S = 52V.
• Parallel sets capacity and current. Multiply the cell count by cell capacity and by the cell's continuous current rating. 4P of 3,000mAh/15A cells = 12Ah and 60A.
• Energy (Wh) = nominal voltage × amp-hours. A 13S4P pack of 3Ah cells is 48V × 12Ah ≈ 562Wh.

Size the pack so the cells run well below their maximum current at your peak load. Running cells at 50–70% of their rating keeps them cooler and extends cycle life.

2. Choose the right cells

Cell	Capacity	Strength	Good for
Samsung 30Q	3,000mAh	15A continuous (Samsung spec)	E-bikes, tools, general packs
Molicel P28A	2,800mAh	High drain (35A class)	High-power packs, scooters
Samsung 25R	2,500mAh	High drain	Power tools, solar storage
LG MJ1	3,500mAh	High capacity, 10A class	Long-range e-bikes, power banks, storage

Rules for cells: use one model, buy from one batch, and buy from a supplier who can show manufacturer documentation. Counterfeit and rewrapped cells are common, and they often fall far short of their label. The LG MJ1 Safety Data Sheet and other documents are in our 18650 Data Center.

Shop: Samsung 30Q 18650 15a High‑Capacity 3000 mAh Bulk

3. Parts list

• Cells, matched by voltage (within 0.01–0.02V) before assembly
• Cell holders or spacers: 18650 plastic pack brackets (100 pack) keep cells evenly spaced, stop them rubbing through their wraps, and let heat escape
• Nickel strip: nickel-plated strip tape, sized to the current each connection carries
• Insulator rings: fish-paper rings on every positive terminal
• Battery management system (BMS) rated for your series count and continuous current. It provides over-charge, over-discharge, over-current and short-circuit protection and balances the cell groups.
• Wire and connectors sized for your current (e.g., XT60/XT90 or Anderson), plus a fuse on the main positive lead
• Fish paper or fiberglass board for the ends, Kapton tape, and heat-shrink or a hard case
• Optional: a voltmeter / capacity indicator module to show state of charge
• For small builds and prototypes: 3-slot 18650 holder with leads and positive tip caps (converts flat tops to button tops)

4. Tools

• Spot welder: mini spot welder DIY kit. Spot welding joins strip to cell in milliseconds without heat soaking into the cell. Don't solder directly to cells.
• Digital multimeter, insulated tools, safety glasses
• A balance-capable or BMS-compatible charger at the right pack voltage

Shop: Mini Spot Welder Machine DIY Kit 18650 Battery Spot Welding Pen Tools

5. Assembly steps

1. Test every cell. Check voltage and, if you can, internal resistance. Reject dented cells, torn wraps, or anything below about 3.0V on arrival.
2. Arrange the cells in holders. Place parallel groups side by side with the same orientation, and alternate group orientation for the series connections.
3. Insulate the positive ends with fish-paper rings.
4. Weld the parallel connections, then the series connections. Two to four welds per tab. Pull-test each weld.
5. Connect the BMS balance wires to each series junction in order, starting from B− and moving up. Double-check with a meter before plugging the balance harness into the BMS.
6. Connect the main leads through the BMS, with an inline fuse on the positive lead.
7. Insulate and enclose. Pad sharp edges, cover welds with Kapton, then heat-shrink or case the pack so it can't be crushed or punctured.
8. First charge at a low rate while watching for heat. Confirm the BMS balances groups and cuts off at the right voltage.

6. Safety rules

• Work on a non-conductive bench, away from flammables. Keep sand or a lithium-rated extinguisher on hand.
• Remove rings and watches. A metal band across a pack's terminals can cause severe burns.
• Never mix old and new cells, different models or different capacities.
• Never charge a pack unattended, on a bed or couch, or below 0°C (32°F).
• Retire any cell that gets hot, swells, leaks or smells sweet or solvent-like.
• Unprotected cells must always be used inside a pack with a BMS.

Read our full 18650 battery safety guidelines and charger safety page before your first build.

7. Shipping and certification

Lithium-ion packs ship as dangerous goods under UN3480 (batteries alone) or UN3481 (packed with or in equipment). Most carriers require UN38.3 test documentation for the battery design. If you're building packs to sell, budget for pack-level UN38.3 testing, and start with cells whose manufacturers publish UN38.3, IEC 62133 and SDS documents.

Need cells in volume? See our bulk supply program for pack builders or request a quote.

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18650 Guides Pack Building Safety

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