You’ve got a pack that charges fine, balances to 4.20V per cell, and still sags to 13.2V the moment you punch out — while a newer pack holds 14.5V on the same quad. That sag is internal resistance (IR) climbing as the pack ages. A pack with 25mΩ per cell is a different animal from one with 5mΩ, and pretending they’re the same is how you ruin flight controllers and lose quads in the tall grass. Here’s how to measure IR, what the numbers mean, and when to retire a pack.
What Internal Resistance Actually Tells You
Every LiPo cell is a chemical battery with non-zero internal resistance. When you pull 100A from a pack with 5mΩ per cell, Ohm’s law says you lose 0.5V per cell to IR alone — that’s 3V across a 6S pack. If IR has climbed to 20mΩ per cell, that same 100A pull drops 12V. Your quad browns out, the ESC resets, and you’re walking.
IR rises because the electrolyte decomposes, the SEI layer thickens, and physical degradation separates electrode material from the current collector. Every charge cycle adds a tiny increment. High storage temperatures accelerate it dramatically — a pack stored at 40°C degrades 3-4x faster than one stored at 20°C.
How to Measure IR — Three Methods, One Correct Answer
Method 1: Dedicated IR Meter (Best)
A standalone IR meter (like the ISDT BG-8S or Wayne Giles ESR Meter) applies a known AC current at 1kHz and measures the voltage drop across each cell. This is the gold standard — accurate to ±0.5mΩ.
Procedure:
- Bring the pack to 22-25°C (room temperature) — cold packs read artificially high
- Connect the balance lead to the meter
- Read per-cell values — write them down
- Compare to the pack’s baseline (if you tracked it from new) or to known-good thresholds
Method 2: Charger IR Reading (Decent)
Most modern chargers (ISDT Q6, HOTA D6, ToolkitRC M6) have a built-in IR measurement mode. These are good enough for trend tracking but are typically ±2mΩ less accurate than a dedicated meter.
The right way: Always measure at the same state of charge (storage voltage, 3.80-3.85V/cell) and at the same temperature. A pack at 25°C and 3.80V will read a different IR from the same pack at 40°C and 4.20V after a flight.
Method 3: Voltage Sag Calculation (Field Check)
If you don’t have an IR meter, fly the pack and note the voltage sag on punch-out in the OSD. Divide the sag by the current (if your current sensor is calibrated) to estimate total pack IR. This is crude (±5mΩ) but better than nothing.
IR Thresholds: When to Retire a Pack
| Cell IR (mΩ) at 22°C | Pack Condition | Action |
|---|---|---|
| ———————- | —————- | ——– |
| < 5mΩ | New / Excellent | Full power, no restrictions |
| 5–10mΩ | Good / Normal | Normal use, monitor trend |
| 10–15mΩ | Aging | OK for cruising, sag noticeable on punch-outs |
| 15–20mΩ | Worn | Reduce max throttle, retire from high-draw builds |
| 20–30mΩ | Poor | Retire to bench use or ground station power |
| > 30mΩ | Dangerous | Discharge and recycle — high fire risk under load |
These thresholds are for 1300-1800mAh packs on 5-inch quads. Larger packs (3000mAh+) naturally have higher IR — a 4mΩ cell in a 4000mAh pack is normal and healthy, while 4mΩ in a 650mAh pack is borderline.
IR by Cell Size — Reference Table
| Pack Capacity | Healthy IR Range (mΩ/cell) | Warning Level (mΩ/cell) | Retire At (mΩ/cell) |
|---|---|---|---|
| ————– | ————————— | ———————— | ——————— |
| 300–450mAh (1S Whoop) | 15–25 | 35 | 50+ |
| 525–650mAh (2S-4S micro) | 8–15 | 20 | 30+ |
| 850–1100mAh (3S-4S) | 5–10 | 15 | 25+ |
| 1300–1550mAh (6S freestyle) | 2–6 | 12 | 20+ |
| 1800–2200mAh (6S) | 2–5 | 10 | 18+ |
| 3000–4000mAh (Li-Ion) | 15–30 | 50 | 80+ |
Li-Ion packs (18650, 21700) run much higher IR than LiPos — 30mΩ per cell is brand new on a Samsung 40T. Don’t compare Li-Ion IR to LiPo IR.
What Most Pilots Get Wrong
Mistake 1: Measuring IR when the pack is hot.
IR drops significantly as temperature rises. A pack fresh off a flight at 45°C reads 30-50% lower IR than the same pack at 22°C. You’ll think your beat-up pack is healthy. Always measure at room temperature.
Mistake 2: Using IR to compare different capacity packs.
IR is inversely proportional to capacity. A 3000mAh pack with 5mΩ per cell is in worse shape than a 1300mAh pack with 5mΩ per cell. Normalize by capacity (IR × mAh / 1000) to compare across sizes.
Mistake 3: Ignoring cell-to-cell imbalance.
Average IR matters, but a single cell with 18mΩ in a pack of 4mΩ cells is the one that will puff first. That outlier cell works harder on every discharge, gets hotter, and degrades faster. If one cell’s IR is 50% higher than the pack average, retire the pack.
Mistake 4: Storing packs at full charge.
A LiPo stored at 4.20V/cell for a week loses roughly the same IR increase as 10-15 flight cycles. Always discharge to storage voltage (3.80-3.85V/cell) within 12 hours of flying.
⚠️ **Regulatory Notice:** The flight recommendations in this article should be followed in accordance with the latest 2026 drone regulations in your country or region. Always verify local laws regarding flight altitude, no-fly zones, remote ID requirements, and registration before flying. Regulations vary significantly between the FAA (US), EASA (EU), CAA (UK), CAAC (China), and other authorities. Always follow manufacturer safety guidelines for LiPo battery charging, storage, and disposal.
As we explained in our LiPo C-rating deep dive, the label on the pack is marketing — IR is the number that actually governs how much current your battery can deliver. A “100C” pack with 15mΩ per cell will sag harder than a “45C” pack with 4mΩ per cell.
Before you retire a pack that seems weak, check your battery connectors — a worn XT60 with blackened contacts adds 2-5mΩ of resistance that mimics a dying battery. Swap the connector before the pack.
For accurate IR testing, we recommend the ISDT BG-8S battery checker — it reads per-cell IR, voltage, and balance, and fits in your field bag. Available at uavmodel.com for a quick health check between flights.