Your LiPo shows full voltage on the charger but sags to 3.2V per cell under load. That’s not a bad balance lead — it’s elevated internal resistance, and it means the pack is dying. Here’s how to measure IR, interpret the numbers, and decide whether that battery earns another flight or the discharge pile.
How to Measure LiPo Internal Resistance Accurately
Internal resistance is the single best predictor of battery health — more reliable than looking at voltage, cycle count, or puffing. A healthy 1300mAh 6S pack reads under 5mΩ per cell at room temperature. Once cells drift past 15mΩ, performance drops sharply. Past 25mΩ, the pack becomes a safety risk.
Step 1: Choose Your Measurement Tool
You have three paths into IR measurement, and they differ in accuracy:
- Charger-based IR measurement (ISDT, HOTA, ToolkitRC): Most modern chargers include an IR function. It works by pulsing a small load across each cell and measuring the voltage drop. Accuracy is ±0.5-1mΩ when the pack is at storage voltage and room temperature. Not lab-grade, but good enough for field decisions.
- Dedicated IR meter (Wayne Giles ESR Meter, SM8124A): These apply a 1kHz AC signal and measure impedance directly. Accuracy drops to ±0.1mΩ. Worth the $40-80 if you test packs weekly.
- Multimeter + load method: You can calculate IR manually — discharge at 1C, measure voltage drop, apply Ohm’s law. Tedious but free. I only use this when I don’t trust my charger’s reading.
What happens if you skip this: Flying packs with unknown IR means you’re guessing whether that mid-throttle sag is a bad pack or a tuning issue. I spent two weeks chasing a “PID problem” that was just a LiPo sitting at 22mΩ per cell.
Verification: Measure the same pack three times at the same temperature. Readings should be within ±1mΩ. If they drift, your tool’s contacts are dirty or the balance lead has corrosion.
Step 2: Control Temperature Before Measuring
IR readings are temperature-dependent — a pack measured at 10°C can read 40-50% higher than the same pack at 25°C. Always bring packs to room temperature (20-25°C) before testing. If you flew within the last 2 hours, let the pack cool first — warm cells artificially lower the reading and mask degradation.
Step 3: Interpret IR Values by Cell Count and Capacity
Smaller packs naturally have higher IR because there’s less electrode surface area. A 4mΩ reading on a 450mAh 2S whoop pack is fine; the same reading on a 1300mAh 6S indicates a problem.
The FOM (Figure of Merit) normalizes this: FOM = 12000 / (IR × Capacity in mAh). A FOM above 1.0 means the pack is healthy. Below 0.5, retire it.
LiPo Internal Resistance Threshold Table — 2026 Reference
| Pack Type | Capacity | Healthy (<10mΩ) | Warning (10-20mΩ) | Retire (>20mΩ) | Typical FOM Range |
|---|---|---|---|---|---|
| 1S Whoop | 300-450mAh | <40mΩ | 40-80mΩ | >80mΩ | 0.3-1.0 |
| 4S Freestyle | 1300-1500mAh | <8mΩ | 8-18mΩ | >18mΩ | 0.6-1.2 |
| 6S Freestyle | 1100-1300mAh | <6mΩ | 6-15mΩ | >15mΩ | 0.7-1.5 |
| 6S Long Range | 1800-3000mAh | <5mΩ | 5-10mΩ | >10mΩ | 0.4-1.0 |
| 6S Race | 1050-1250mAh | <5mΩ | 5-12mΩ | >12mΩ | 0.8-2.0 |
Common IR Testing Mistakes & How to Avoid Them
Mistake 1: Testing Immediately After a Flight
A warm pack reads 20-30% lower IR than the same pack at room temperature. You’ll convince yourself a dying battery is still healthy. Always wait until the pack reaches ambient temperature — at least 30 minutes after landing, longer in summer.
Consequence: You keep flying a pack that’s delivering half its rated current. Under heavy throttle, voltage collapses, ESCs desync, and you’re walking to pick up a quad that fell out of the sky.
Fix: Label each pack with a test date. Only record IR when the pack has rested at 20-25°C for 30+ minutes.
Mistake 2: Ignoring Cell-to-Cell IR Imbalance
A 6S pack where five cells read 4mΩ and one reads 12mΩ is dangerous regardless of the average. The weak cell heats up faster, sags harder, and will eventually puff or vent during a hard punch-out.
Consequence: The imbalance accelerates — the weak cell gets weaker every cycle. Eventually it drops below 3.0V under load while the others stay above 3.5V. The charger may not even flag it because total pack voltage looks fine on the bench.
Fix: Flag any pack where the highest cell IR exceeds 150% of the lowest. Write the date and IR on the pack with a silver Sharpie. Retire when the spread exceeds 2×.
Mistake 3: Using IR Alone Without Capacity Testing
A pack at 4mΩ per cell that delivers only 800mAh from a 1300mAh rating has lost capacity without gaining resistance — common in low-C packs stored at full charge for months.
Consequence: You’ll land early, wondering why flight time dropped from 4 minutes to 2:30, while your IR meter says everything is fine.
Fix: Every 20 cycles, do a full charge-discharge cycle on your charger and note actual delivered mAh. Retire at 80% of rated capacity, even if IR is still good.
Mistake 4: Testing Through the Main Discharge Lead
The balance lead is where IR measurement happens — some chargers can measure through the main lead, but the added connector resistance (XT60 contacts, solder joints, wire length) adds 1-3mΩ to every reading. You’re measuring the whole system, not the cells.
Fix: Always connect the balance lead. If your charger doesn’t support balance-lead IR measurement, it’s time for a charger upgrade.
Mistake 5: Storing Packs Fully Charged Between Test Cycles
The single biggest killer of LiPo IR is storage at 4.2V/cell. A pack left fully charged for a week at room temperature gains 20-30% IR. Leave it in a hot car and it’s trashed in a weekend.
Fix: Storage charge to 3.80-3.85V/cell immediately after flying. Don’t charge for a session you might cancel — charge the morning of, not the night before.
⚠️ Regulatory Notice: The battery handling 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 LiPo transport, storage limits, and disposal requirements. Regulations vary significantly between the FAA (US), EASA (EU), CAA (UK), CAAC (China), and other authorities. Damaged or retired LiPos must be discharged and disposed of at certified recycling facilities — never in household waste.
As we covered in our detailed guide on LiPo C-rating selection, even the highest-rated pack degrades over time. Pair IR testing with what we discussed in the voltage sag troubleshooting guide to isolate whether the problem lives in your battery or your power wiring.
For accurate IR measurement at the field, the ISDT Q6 Nano charger reads individual cell resistance through the balance lead and fits in a pocket. Pick one up at the uavmodel store — it pays for itself the first time you catch a dying pack before it kills your quad mid-flight.