LiPo packs deliver power. Li-Ion packs deliver endurance. A 4S 3000mAh LiPo weighs 340g and flies for 8 minutes. A 4S 3000mAh Li-Ion 18650 pack weighs 200g and flies for 25 minutes at cruise. The catch: Li-Ion current delivery is much lower, so you fly differently — smooth, efficient, and planned. Here’s how to build packs that won’t sag into the ground on the first throttle blip, and how to fly them for maximum range.
Li-Ion Cell Selection: 18650 vs 21700
The cell choice determines everything — weight, current capability, and flight time. For FPV, you need high-drain cells rated for at least 10A continuous, preferably 15A+.
Top Cell Options for FPV (2026)
Molicel P28A (18650): 2800mAh, 35A continuous. The gold standard for FPV Li-Ion. Each cell weighs 45g. A 4S pack = 180g. At 35A, you can punch out of a bad situation without sagging below 3.0V per cell. The downside: only 2800mAh — your flight time tops out around 20-22 minutes on a 5-inch.
Samsung 40T (21700): 4000mAh, 35A continuous. Each cell weighs 67g. A 4S pack = 268g. 4000mAh gives you 30-35 minute flights, but the weight penalty is noticeable. On a sub-250g build, the 40T’s weight pushes you over the limit. Use for 7-inch builds where the extra capacity offsets the weight.
Sony/Murata VTC6 (18650): 3000mAh, 15A continuous. Lighter-duty but lighter weight (46g). Good for fixed-wing FPV where current draw is steady and low. Not recommended for quads — 15A is marginal for a 5-inch at cruise throttle.
Molicel P42A (21700): 4200mAh, 30A continuous. Each cell weighs 67g. The best capacity-to-current ratio in a 21700 form factor. 30A continuous is enough for a 7-inch at 60% throttle.
How to Build a 4S Li-Ion FPV Pack (Step by Step)
Required Tools
- Spot welder (kWeld, Malectrics, or similar — soldering directly to cells damages the internal separator and risks thermal runaway)
- Pure nickel strip (0.15mm × 8mm — don’t use nickel-plated steel, it has higher resistance)
- Kapton tape and PVC heat shrink (fishpaper optional but recommended for cell isolation)
- XT60 connector and 14AWG silicone wire
- Balance lead (JST-XH 5-pin for 4S)
- Digital multimeter for voltage verification
Step 1: Match Your Cells
Before assembling, all four cells must be at the same voltage within 0.05V. Use cells from the same batch if possible. Charge or discharge individual cells to 3.60V ± 0.02V. A pack built with mismatched cells will have one cell hit 3.0V while the others are at 3.4V — that cell becomes a fire risk because the others drive current through it during discharge.
Step 2: Configure the Physical Layout
For a 4S pack, arrange four cells in a 2×2 grid: two cells side by side, then the other two stacked on top (but electrically in series, not parallel). This configuration means:
– Top layer: cell 1 (+) connects to cell 2 (-) via nickel strip on the side
– Bottom layer: cell 3 (+) connects to cell 4 (-) via nickel strip on the side
– Series connection between layers: cell 2 (+) to cell 3 (-) via a wire or strip running between layers
If this sounds confusing, trace it out on paper first. One wrong connection shorts a cell, and Li-Ion shorts release energy fast enough to melt nickel strip instantly.
Step 3: Spot Weld the Series Connections
Set your spot welder to the appropriate energy level (typically 20-25 joules for 0.15mm pure nickel on 18650s). Each series connection needs a minimum of 4 spot welds — two per cell terminal. The welds should be evenly spaced and show a slight indentation in the nickel strip without burning through.
Critical: The nickel strip carries the pack’s full current. For a 4S pack pulling 15A, 0.15mm × 8mm nickel can handle it with acceptable voltage drop. For 30A+, double up the nickel strip (two layers welded on top of each other) on the main discharge path.
Step 4: Attach the Main Discharge Leads
Weld nickel strip tabs to the main positive and negative terminals of the pack (the + of cell 1 and the – of cell 4). Solder your 14AWG silicone wire to these tabs — do NOT solder directly to the cell terminals. The heat from soldering transfers through the cell can and damages the internal separator, creating micro-shorts that grow over time.
Step 5: Attach the Balance Lead
Solder the balance lead wires to the nickel strips at each series junction:
– Wire 1 (black): Pack negative (cell 4 -)
– Wire 2: Junction of cell 4 (+) and cell 3 (-)
– Wire 3: Junction of cell 3 (+) and cell 2 (-)
– Wire 4: Junction of cell 2 (+) and cell 1 (-)
– Wire 5 (red): Pack positive (cell 1 +)
Verify voltages before proceeding: each adjacent pair of balance wires should read ~3.60V.
Step 6: Insulate and Wrap
Layer Kapton tape over all exposed nickel strips and solder joints. Fishpaper between cells (optional but recommended) prevents cell cans from rubbing against each other — a wear-through shorts the pack internally. Slide the pack into PVC heat shrink and shrink with a heat gun at moderate temperature. Too hot and you can trigger the cell’s CID (current interrupt device), permanently disabling the cell.
Step 7: Final Verification
Measure total pack voltage. It should equal the sum of individual cell voltages. Connect to a charger, verify all four cells read correctly on the balance port. Charge to 4.20V per cell (not 4.35V — Li-Ion cells are NOT high-voltage chemistry unless explicitly labeled HV).
Li-Ion Cell Comparison for FPV
| Cell | Capacity | Continuous Current | Weight | Best Use | Voltage Drop at 15A |
|---|---|---|---|---|---|
| Molicel P28A (18650) | 2800mAh | 35A | 45g | 4-5 inch long range, light builds | ~0.15V per cell |
| Samsung 40T (21700) | 4000mAh | 35A | 67g | 7-inch long range, heavy payload | ~0.12V per cell |
| Molicel P42A (21700) | 4200mAh | 30A | 67g | 7-inch max endurance | ~0.14V per cell |
| Sony VTC6 (18650) | 3000mAh | 15A | 46g | Fixed-wing FPV only | ~0.25V per cell |
| Samsung 30Q (18650) | 3000mAh | 15A | 46g | Fixed-wing, ultra-light quads | ~0.25V per cell |
| Molicel P26A (18650) | 2600mAh | 25A | 45g | Budget alternative to P28A | ~0.18V per cell |
Flying Strategy for Maximum Li-Ion Endurance
Li-Ion packs can’t deliver the 80A bursts that LiPos laugh at. You fly differently:
- Cruise at 40-50% throttle. Above 60%, current draw doubles for marginal speed increase. Your amp-hours burn twice as fast.
- Plan your route for prevailing wind. Fly into the wind on the outbound leg when the pack is full. Return with the tailwind when voltage is lower and current capability is reduced.
- Land at 3.0V per cell resting (not under load). Li-Ion voltage sag on a 15A cruise is roughly 0.3V per cell, so if your OSD shows 2.7V at cruise, you’re at 3.0V resting — time to land. Going below 2.5V under load permanently reduces capacity.
- Cool the pack between flights. Li-Ion cells run hotter than LiPos at equivalent current draw because their internal resistance is higher. Let the pack cool to ambient temperature before charging.
What Most Long-Range Pilots Get Wrong About Li-Ion
Mistake 1: Soldering directly to cells
A 60W soldering iron on a cell terminal for 3 seconds raises the internal temperature enough to damage the separator. The cell works fine for 5-10 cycles, then develops an internal short that manifests as rapid self-discharge — and eventually thermal runaway. The fix: Spot weld only. If you don’t have a spot welder, buy pre-built packs from a reputable builder.
Mistake 2: Using laptop cells recovered from old battery packs
Laptop 18650s (typically LG, Panasonic, or Samsung ICR chemistry) are rated for 5-7A continuous, not 15-35A. At FPV cruising current, their voltage sags to 2.5V within seconds and they heat to 60°C+. Capacity is irrelevant if voltage collapses under load. The fix: Use INR or high-drain NCA chemistry cells specifically rated for your current draw.
Mistake 3: Flying Li-Ion packs to the same low-voltage cutoff as LiPo
A LiPo at 3.5V per cell has 20% capacity remaining. A Li-Ion at 3.5V has 5% remaining. The discharge curves are completely different. The fix: Set your OSD voltage warning to 3.2V per cell and land when you see 3.0V per cell at cruise throttle. Anything below that and the voltage cliff is seconds away.
Mistake 4: Thinking Li-Ion is a drop-in replacement for LiPo
You can’t fly Li-Ion like LiPo. If you arm, punch out, and immediately pull a power loop, the voltage sags to 2.8V per cell and the quad falls out of the sky. The fix: Li-Ion flying is about efficiency. Smooth, gradual throttle changes. Plan your maneuvers. If you want to fly freestyle, use a LiPo.
⚠️ Regulatory Notice: Self-built Li-Ion battery packs involve spot welding, soldering, and handling of high-energy-density cells that can vent with flame if shorted or damaged. Always build, charge, and store packs in a fireproof container (LiPo safe bag or ammo can with vent holes). Follow the latest 2026 drone regulations in your country or region regarding long-range and beyond visual line of sight (BVLOS) flight. Most jurisdictions require visual observers or special waivers for BVLOS operations. Regulations vary significantly between the FAA (US), EASA (EU), CAA (UK), CAAC (China), and other authorities. A 30-minute flight at 60 km/h covers 30 km — well beyond typical VLOS limits.
Long-range builds demand careful component selection beyond just the battery. Our long-range FPV build guide covers GPS, antenna, and redundancy planning. For LiPo pilots transitioning to Li-Ion, understanding LiPo storage and maintenance helps you appreciate the key differences in discharge curves and care requirements.
For pilots who want a proven long-range motor to pair with Li-Ion efficiency, the T-Motor F series in 2207 or 2806 sizes delivers the cruise efficiency that Li-Ion packs demand — check the motor lineup at uavmodel.com.