A 6S 1300mAh LiPo gives you 3 minutes of aggressive freestyle or 6 minutes of cruising. Swap in a 6S Li-Ion pack built from 21700 cells, and you are looking at 15–25 minutes of flight on the same quad — same motors, same AUW, nothing else changed. The tradeoff is punch: Li-Ion sags more under load and cannot deliver the burst current that freestyle demands. But for long-range cruising, mountain surfing, and endurance flights, building your own Li-Ion packs is the single biggest upgrade you can make. Here is how to do it safely.
Li-Ion vs LiPo: The Tradeoffs
| Characteristic | LiPo | Li-Ion (21700) |
|---|---|---|
| Energy density | 140–180 Wh/kg | 230–270 Wh/kg |
| Continuous discharge | 50–100C (easy) | 8–45A per cell (cell-dependent) |
| Voltage sag under load | Low (stiff voltage) | Noticeable (softer voltage) |
| Cycle life | 100–300 cycles | 300–500 cycles |
| Thermal runaway risk | High (punctures = fire) | Moderate (venting, less violent) |
| Cost per Wh | Higher | Lower |
| Form factor | Pouch (custom shapes) | Cylindrical (fixed) |
The decision is simple: if you need high burst current for punchouts, flips, and hard racing turns, use LiPo. If you want to fly far and stay in the air, build Li-Ion.
Popular Cell Comparison
| Cell | Form Factor | Capacity (mAh) | Max Continuous (A) | Weight (g) | Best Use Case |
|---|---|---|---|---|---|
| Samsung 40T | 21700 | 4000 | 35–45 | 68 | Lightweight 4S / moderate 6S long-range |
| Samsung 50E | 21700 | 5000 | 9.8 | 69 | Ultra-endurance at low throttle |
| Molicel P42A | 21700 | 4200 | 45 | 68 | Best all-around — capacity and current |
| Molicel P45B | 21700 | 4500 | 45 | 68 | Higher capacity version of P42A |
| Sony/Murata VTC6 | 18650 | 3000 | 30 | 47 | Compact 4S builds, lightweight |
| Samsung 30Q | 18650 | 3000 | 15–20 | 46 | Budget lightweight packs |
The Molicel P42A and P45B are the standout choices for FPV in 2026. They deliver 45A continuous per cell while holding 4200–4500 mAh — enough current for a 4S pack to sustain a 7-inch cruiser at 40% throttle, and enough capacity to fly for 18–22 minutes. The Samsung 40T is a close second if you want to save a few grams. Avoid the Samsung 50E for anything above gentle cruising — its 9.8A continuous rating means a 4S pack can only deliver about 150W total before sag becomes dangerous.
18650 vs 21700 Form Factor
18650 cells (18mm diameter, 65mm length) are lighter and smaller — a 6S pack of VTC6 cells weighs roughly 280g. 21700 cells (21mm diameter, 70mm length) are heavier — a 6S pack of P42A cells weighs about 410g — but deliver far more current and capacity. For 4S builds on ultralight 4–5 inch quads, 18650 packs make sense. For 6S on 7-inch long-range rigs, the extra 130g of 21700 cells is completely worth it for the current headroom and flight time.
Pack Configurations: 4S and 6S
Li-Ion cells are nominally 3.6V (3.0V minimum, 4.2V maximum). A 4S pack gives 14.4V nominal / 16.8V full, suitable for smaller builds where weight is critical and voltage sag is less punishing. A 6S pack gives 21.6V nominal / 25.2V full — the standard for 5–7 inch long-range builds.
You arrange cells in series by connecting the positive of cell 1 to the negative of cell 2, and so on. The pack’s total capacity equals one cell’s capacity (series does not add capacity). The total continuous current rating equals one cell’s rating. A 6S P42A pack is 6 × 3.7V nominal = 22.2V, 4200mAh, capable of 45A continuous — roughly 1000W at full charge.
Spot Welding: Technique and Nickel Strip Sizing
You cannot solder directly to Li-Ion cells. The heat damages the internal separator and risks internal short circuits. Use a spot welder — a capacitive-discharge or battery-powered unit that fuses nickel strip to the cell terminals in milliseconds.
Required equipment:
- Spot welder: kWeld, Malectrics, or a budget Chinese unit (Sunkko, SeeSii). The kWeld is the gold standard — it measures energy delivered per weld and produces consistent results.
- Pure nickel strip: 0.15mm thick × 8mm wide for series connections. For the main discharge leads, stack two layers of 0.15mm or use a single 0.2mm strip. Do not use nickel-plated steel — it has higher resistance and heats up under load.
- Cell holders: 3D-printed or off-the-shelf 21700/18650 holders. They align cells for welding and provide physical protection.
- Fish paper or Kapton tape: Insulators for the positive terminal ring (the entire can except the positive button is negative — short circuits happen easily).
The welding process for a 6S pack:
- Arrange cells in a 3×2 or 6×1 layout in the cell holder. Mark polarity on every cell with a Sharpie.
- Apply fish paper rings or Kapton dots to the positive terminal of each cell, covering everything except the center button.
- Weld nickel strip between cell 1 negative and cell 2 positive, cell 2 negative and cell 3 positive, and so on. Two weld points per cell terminal — four points if using high current.
- Test each weld by gently pulling — a good weld tears the nickel strip before detaching from the cell.
- After all series connections are done, weld the main discharge leads: thick silicone wire (12–14 AWG) with a nickel strip “tab” soldered to the wire before welding the tab to the cell.
Balance Lead Wiring
The balance lead gives your charger access to each cell’s individual voltage. For a 6S pack, you need a JST-XH 7-pin connector (6 cells + 1 ground). Wire it as follows:
- Pin 1 (black/ground): Cell 1 negative (pack negative)
- Pin 2: Cell 1 positive / Cell 2 negative junction
- Pin 3: Cell 2 positive / Cell 3 negative junction
- …continue for all cells…
- Pin 7 (red): Cell 6 positive (pack positive)
Solder a short pigtail of 22 AWG wire to each nickel strip junction point, then solder the other ends to the JST-XH connector. Double-check continuity with a multimeter before plugging anything into a charger — reversed balance leads destroy chargers and can start fires.
Main Connector: XT60 vs XT30
XT60 is the standard for 4S–6S packs drawing 10–60A. If your quad pulls under 30A continuous (lightweight 4S cruiser), XT30 saves weight and space. Solder the connector directly to 12 AWG silicone wire from the pack terminals. Use a generous amount of solder and heat the connector cup — not the wire — to get a clean joint. A cold solder joint on the main discharge lead will cause voltage sag and erratic behavior in flight.
Shrink Wrap and Insulation
After all connections are verified, wrap the pack in a layer of fish paper or Kapton tape, then cover with PVC heat shrink (available in diameters from 50mm to 80mm for common pack sizes). Use a heat gun — not a lighter — and keep it moving to avoid melting the nickel strips. Leave the balance lead and main discharge lead accessible. Add a second layer of shrink if the pack will be mounted in a location exposed to prop strikes.
Safety Precautions
- Work on a non-conductive surface. No metal workbenches. A ceramic tile, glass, or silicone mat is ideal.
- Remove all jewelry. Rings and watches are perfect short-circuit bridges across cell terminals.
- Never short a cell. A shorted 21700 can dump 100A+ instantly — enough to weld itself to whatever it is touching and start a thermal runaway cascade.
- Use a cell checker. Before building, measure every cell’s voltage. All cells should be within 0.05V of each other. Building a pack with mismatched cells guarantees balance issues and reduced capacity.
- Charge outdoors. First charge of a new pack should happen in a LiPo bag, on concrete, away from flammables. Once the pack has 3–5 cycles without issue, it is considered stable.
- Inspect after crashes. A dented Li-Ion cell may look fine but have internal damage. Replace any cell with visible deformation.
Pack Testing and Internal Resistance
After building, measure internal resistance (IR) of each cell using a charger with IR capability (ISDT, HOTA, ToolkitRC). A new P42A should show 12–18 milliohms per cell. Higher IR (25+ mΩ) indicates a poor weld, a used cell, or a counterfeit. Check every cell individually — a single high-IR cell drags down the entire pack under load.
For the first flight, set a conservative current limit in your OSD and land at 3.3V per cell under load (not resting). Li-Ion voltage sags significantly under throttle — a cell reading 3.3V at 50% throttle may recover to 3.7V at idle. If you land at 3.0V under load, the resting voltage may be below 2.8V, which permanently damages the cell.
When Li-Ion Makes Sense — and When It Does Not
Build a Li-Ion pack if you fly long-range, mountain surf, chase endurance records, or want to cruise a 7-inch for 20 minutes without landing. Stick with LiPo if you fly freestyle, race, or need punch above all else. The two chemistries are not competing — they solve different problems. Having both types of packs for the same quad is the ideal setup: LiPo for the afternoon rip session, Li-Ion for the sunset cruise up the ridge.