Plan, build and safely charge a 6S Li-ion drone pack from Samsung 40T 21700 cells: configurations, parts list, spot welding, balance leads and airline Wh limits.

Long-range FPV pilots, mapping crews and cinelifter builders have moved to Li-ion packs made from 21700 cells. A LiPo still wins on bursts of punch. But for flight time per gram at steady cruise, a well-built 21700 pack is hard to beat. This guide walks through planning and building a 6S drone pack from Samsung 40T cells.

Shop: Samsung 40T 21700 – 4000mAh Battery (Bulk)

Safety first: building lithium packs means handling high-energy cells that can short, overheat or catch fire. Work on a non-conductive surface, keep a Class D or lithium-rated extinguisher (or a bucket of sand) nearby, and never leave a charging pack unattended. If you aren't comfortable soldering high-current leads and spot welding, buy a pre-built pack instead.

Why the Samsung 40T?

According to Samsung SDI's product specification for the INR21700-40T:

• Nominal capacity: 4,000mAh
• Nominal voltage: 3.6V (4.20V full charge, 2.5V discharge cutoff)
• Max continuous discharge: 35A
• Weight: 70.0g max
• Rated charge: 6A CCCV to 4.20V

35A continuous from a 4Ah cell is rare. Most high-capacity 21700s are rated 10–25A. That headroom covers the current spikes of climb-out and punch-outs. The Samsung 40T is IEC 62133 certified. You can download its IECEE CB test certificate and Safety Data Sheet.

Step 1: Plan the configuration

Drone packs are described as S (cells in series, which sets voltage) and P (cells in parallel, which adds capacity and current).

Config	Cells	Nominal / full voltage	Capacity	Max continuous	Energy	Cell weight
4S1P	4	14.4V / 16.8V	4,000mAh	35A	57.6Wh	~280g
6S1P	6	21.6V / 25.2V	4,000mAh	35A	86.4Wh	~420g
6S2P	12	21.6V / 25.2V	8,000mAh	70A	172.8Wh	~840g

Start with your motor and ESC voltage rating, then size capacity to your frame's all-up weight. Watch the Wh number if you fly commercially. Under IATA airline rules, spare lithium batteries over 160Wh can't go on passenger aircraft, and packs from 100–160Wh need airline approval. A 6S2P 40T pack is 172.8Wh and would have to ship as freight.

Shop: Samsung 40T 21700 – 4000mAh Battery (Bulk)

Step 2: Gather parts and tools

• Cells: Samsung 40T 21700 4000mAh. Buy all cells from one batch and match them by voltage (within 0.01V) before assembly.
• Nickel strip: nickel-plated strip for welding. Size it to your current. High-current drone packs often need wider or stacked strip, or copper-nickel sandwich busbars.
• Spot welder: mini spot welder DIY kit. Never solder directly onto cells, because heat damages the internal seals and separator.
• Cell holders or spacers: plastic pack brackets keep cells aligned and leave airflow gaps. (Check the fit against 21mm cells before ordering.)
• Insulation: fish-paper rings on each positive terminal, Kapton tape, and heat-shrink sized for the finished pack.
• Leads and connectors: silicone wire sized for your current (often 10–12 AWG on 6S builds), an XT60 or XT90 main connector, and a JST-XH balance lead.
• Meter and charger: a multimeter, plus a balance charger that supports Li-ion (4.20V/cell) profiles.

Step 3: Assemble the pack

1. Check and match cells. Measure every cell. Set aside any that are dented, have torn wraps or read noticeably lower than the rest.
2. Add insulators. Put a fish-paper ring on each positive end, where the can (negative) sits millimeters from the positive cap.
3. Arrange the layout. For a flat 6S1P, alternate cells +/−. For 6S2P, pair cells side by side (same orientation) to make each parallel group, then alternate the groups.
4. Spot weld the parallel groups first, then the series links. Do two to four welds per tab and test each weld with a gentle pull.
5. Wire the balance lead. Connect one wire to each series junction in order, from main negative up to main positive. Check every pin with your meter before plugging it into a charger.
6. Attach the main leads to the end tabs, solder to the nickel (never to the cell), and strain-relieve the wires with tape or a fiberglass board.
7. Insulate and wrap. Pad sharp edges, add Kapton over the welds, then heat-shrink the pack.

Step 4: First charge and flight checks

• Balance charge at a gentle rate the first time (1A or less per parallel group) and check that every cell group ends at the same voltage.
• In flight, set your low-voltage warning around 3.0–3.2V per cell under load. Landing earlier extends cycle life. The 2.5V spec cutoff is an absolute floor, not a target.
• Store packs at about 3.6–3.8V per cell in a fireproof bag or container.

Other 21700 cells to consider

• Molicel P42A 21700 4200mAh: a popular long-range FPV cell
• Molicel P50B 21700 5000mAh: more capacity for heavier, efficient frames
• Molicel P60B 21700 6000mAh
• Samsung 50S 21700 5000mAh 25A

Shop: Molicel P42A 21700 Li-Ion Battery 42A 4200mAh

Building packs for a fleet or a product line? We supply matched-batch cells in bulk with the certifications your shipping and compliance teams need. Request a quote or browse our Data Center.

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21700 Drones Guides Pack Building Samsung 40T

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