# FPV Long Range Li-Ion Battery Guide: 18650 vs 21700 Pack Building
LiPo batteries dominate FPV, but if you want to fly for 15, 20, or even 30 minutes instead of the usual 3-7 minutes, you need lithium-ion (Li-Ion) packs. Li-Ion cells pack nearly double the energy density of LiPos, making them the undisputed champion for long-range cruising, mountain surfing, and endurance flights. Here’s everything you need to know about building and using Li-Ion packs for FPV.
## Li-Ion vs LiPo: The Energy Density Difference
| Parameter | LiPo (Typical 6S 1300mAh) | Li-Ion 18650 6S 3000mAh | Li-Ion 21700 6S 4000-5000mAh |
|—|—|—|—|
| Energy Density (Wh/kg) | 140-180 | 220-250 | 250-270 |
| Continuous Discharge (C) | 75-120C | 8-15C | 10-20C |
| Voltage Sag Under Load | Low (0.2-0.5V) | Moderate (1-2V) | Low-Moderate (0.5-1.5V) |
| Weight (6S pack) | 200-250g | 300-350g | 400-550g |
| Cycle Life | 150-300 cycles | 300-500 cycles | 300-500 cycles |
| Flight Time (7″ cruiser) | 5-8 minutes | 15-25 minutes | 20-35 minutes |
| Burst Capability | Excellent | Limited | Good |
| Cost per Pack | $25-45 | $25-40 (DIY) | $35-60 (DIY) |
## Cell Selection: 18650 vs 21700 vs 26650
### Top Li-Ion Cells for FPV
| Cell | Format | Capacity (mAh) | Max Continuous (A) | Weight (g) | Best Use Case |
|—|—|—|—|—|—|
| Sony/Murata VTC6 | 18650 | 3000 | 15-20A | 46.6 | Lightweight cruising |
| Samsung 30Q | 18650 | 3000 | 15A | 45.6 | Budget long range |
| Molicel P28A | 18650 | 2800 | 25-35A | 45.0 | High-draw 18650 builds |
| Molicel P42A | 21700 | 4200 | 30-45A | 66.5 | Best all-around 21700 |
| Samsung 40T | 21700 | 4000 | 25-35A | 68.0 | Long range 7″ builds |
| Samsung 50S | 21700 | 5000 | 25A | 68.0 | Maximum endurance |
| A123 ANR26650M1B | 26650 | 2500 | 50-70A | 76.0 | High power Li-Ion (LiFePO4) |
### 18650: Lightweight Champion
– **Best for**: 4-5″ micro long range, lightweight 7″ builds under 250g AUW (sub-250g category), twin-pack parallel setups
– **Pros**: Lighter, cheaper, easier to fit in slim frames
– **Cons**: Lower capacity per cell, higher voltage sag under load
– **Sweet spot**: 6S 3000mAh for a 250-400g quad
### 21700: The Goldilocks Format
– **Best for**: 7″ long range cruisers, mountain surfing rigs, endurance builds
– **Pros**: 40% more capacity for only 45% more weight vs 18650, much better current handling
– **Cons**: Slightly larger diameter (21mm vs 18mm) — may not fit all frames
– **Sweet spot**: 6S 4000-5000mAh for a 400-700g quad
## Pack Building: Spot Welding vs Soldering
### Spot Welding (Recommended)
Professional Li-Ion packs use nickel strip spot welding:
– **Strips**: 0.15mm x 8mm pure nickel (not nickel-plated steel — test with a magnet!)
– **Welder**: kWeld, Malectrics, or similar capacitor-discharge welder
– **Pattern**: Dual parallel strips for current-carrying series connections
– **Pros**: Safe, fast, no heat damage to cells, professional finish
– **Cons**: Requires spot welder ($80-200 investment)
### Direct Soldering (Riskier)
If you must solder directly to cells:
– **Temperature**: 370-400°C maximum
– **Dwell time**: Under 2 seconds per terminal
– **Flux**: Essential — use generous amounts
– **Surface prep**: Sand the cell terminal lightly with 400-grit for adhesion
– **⚠️ Warning**: Soldering heat damages the internal separator and PTC safety device. Spot welding is always safer.
## Pack Configuration Guide
| Configuration | Cell Count | Voltage Range | Capacity (P42A) | Use Case |
|—|—|—|—|—|
| 4S1P | 4 | 12.0-16.8V | 4200mAh / 62Wh | 4″ micro LR, sub-250g |
| 4S2P | 8 | 12.0-16.8V | 8400mAh / 124Wh | 5-6″ ultra-endurance |
| 6S1P | 6 | 18.0-25.2V | 4200mAh / 106Wh | 7″ standard LR cruiser |
| 6S2P | 12 | 18.0-25.2V | 8400mAh / 212Wh | 7-10″ extreme endurance |
## Critical Safety: BMS and Balance Leads
Li-Ion cells are NOT inherently safer than LiPos. They need:
1. **Balance leads**: JST-XH connector for per-cell voltage monitoring
2. **Main discharge leads**: XT60 or XT30 with 14-16AWG wire
3. **Optional BMS**: A Battery Management System PCB protects against over-discharge, over-charge, and short circuits. Recommended for DIY packs.
4. **Heat shrink**: Heavy-duty PVC or PET wrap to protect the pack mechanically
## Flying Li-Ion: Throttle Management
Li-Ion packs sag more than LiPos — manage your throttle:
– **Cruise throttle**: 20-30% on 7″ with efficient props
– **Full throttle limit**: 5-8 seconds maximum to prevent excessive sag
– **Land at 3.0V per cell** (resting): Under load, the pack may show 2.7-2.9V — this is normal. Land when resting voltage recovers to 3.0V.
– **Current sensor calibration**: Essential! Your OSD mAh reading must be accurate to avoid over-discharge.
– **GPS rescue voltage**: Set GPS rescue minimum voltage to 3.0V/cell to auto-return before the pack is dead
## Motor and Prop Pairing
Li-Ion packs work best with efficient powertrains:
| Frame Size | Motor | Prop | Pack | Expected Flight Time |
|—|—|—|—|—|
| 4″ Micro LR | 1404 3800KV | Gemfan 4024 | 4S 3000mAh 18650 | 15-22 min |
| 5″ LR | 2004 3000KV | HQ 5.1×2.5×3 | 4S 4200mAh 21700 | 18-28 min |
| 7″ LR | 2806.5 1300KV | Gemfan 7040 | 6S 4200mAh 21700 | 20-35 min |
| 7″ LR | 2806.5 1300KV | Gemfan 7040 | 6S2P 8400mAh 21700 | 35-50 min |
| 10″ X-Class | 3115 900KV | HQ 10x5x3 | 6S2P 8400mAh 21700 | 25-40 min |
## Build or Buy?
Building your own Li-Ion packs saves money but requires equipment and skill. For pilots who want reliable, professionally-built long-range packs without investing in a spot welder, check the [FPV Battery collection at UAVModel](https://uavmodel.com) for pre-built Li-Ion packs using genuine Molicel and Samsung cells.