Your 6S 1300mAh pack sags to 3.3V per cell on a moderate punch-out, but the resting voltage reads 3.85V per cell — perfectly fine by every surface-level check. The problem isn’t capacity. It’s internal resistance. A pack with IR that’s doubled from its original spec will sag under load, trigger your OSD voltage alarm, and overheat during normal flight. Here’s how to measure IR, what the numbers mean, and when to ground a pack permanently.
What Internal Resistance Actually Tells You
Internal resistance measures how much the battery fights against current flow. Think of it as a resistor wired in series with each cell — as current increases, voltage drop across that resistor increases proportionally. A cell with 5mΩ IR delivering 80A drops 0.4V internally. At 90% throttle, you’re seeing 0.4V less than the cell’s true voltage.
IR increases naturally with age, but it also spikes from:
– Over-discharging below 3.0V per cell
– Storage at full charge (4.2V) for more than 2-3 days
– Physical damage (crashes, punctures)
– Excessive heat (leaving packs in a hot car)
Once a cell’s IR doubles from its original value, capacity is typically down 15-20% and sag under load becomes noticeable. At 3x original IR, the pack is a safety risk — internal heating during discharge can push the cell past 60°C, where thermal runaway becomes possible.
How to Measure IR Accurately
Method 1: Charger IR Measurement (Most Accessible)
Most modern chargers — ISDT, HOTA, ToolkitRC — measure IR during charging. The charger applies a brief current pulse and measures the voltage drop.
For consistent results:
1. Bring the pack to room temperature (20-25°C). Cold packs read higher IR.
2. Charge or discharge to storage voltage (3.80-3.85V/cell). IR varies with state of charge.
3. Measure all cells. Write down the values.
4. Compare cell-to-cell. A single cell with 50% higher IR than the others is failing, even if the absolute numbers look fine.
What happens if you get it wrong: Measuring a cold pack at 5°C can read 3x the actual room-temperature IR. You’ll retire perfectly good packs based on bad data. Always let packs acclimate to room temperature for 30+ minutes before measuring.
Verification: Measure the same pack twice, 5 minutes apart. If values differ by more than 1mΩ, your connection or temperature wasn’t stable. Clean the balance lead contacts with isopropyl alcohol and retest.
Method 2: Dedicated IR Meter
The Wayne Giles ESR meter or the SM8124A are purpose-built IR testers. They apply a 1kHz AC signal rather than a DC pulse. AC-based measurement is more accurate because it doesn’t depend on the battery’s state of charge — it measures true impedance at audio frequency.
For pilots who fly 20+ packs weekly, a dedicated meter pays for itself by catching failing cells before they take out a quad. The SM8124A costs about the same as one 6S 1300mAh pack.
Method 3: Calculated IR from Flight Data
If your FC logs current and voltage (requires a calibrated current sensor), you can calculate IR from the voltage drop during a punch-out:
IR = (Voltage_sag) / (Current_draw)
Example: If voltage sags from 23.4V to 20.8V (Δ2.6V) at 95A:
IR = 2.6 / 95 = 0.027Ω = 27mΩ total, or 4.5mΩ per cell for a 6S pack.
This method includes wiring and connector resistance, so it reads higher than the charger measurement. Use it for trend tracking, not absolute values.
IR Values by Cell Type and Condition
| Cell Type | New (Excellent) | Good (Normal Use) | Marginal (Monitor) | Retire (Unsafe) |
|---|---|---|---|---|
| 6S 1300mAh 100C (per cell) | 2-4mΩ | 4-7mΩ | 7-10mΩ | >10mΩ |
| 6S 1300mAh 150C (per cell) | 1.5-3mΩ | 3-5mΩ | 5-8mΩ | >8mΩ |
| 4S 1500mAh 100C (per cell) | 3-5mΩ | 5-8mΩ | 8-12mΩ | >12mΩ |
| 4S 850mAh 75C (per cell) | 8-12mΩ | 12-18mΩ | 18-25mΩ | >25mΩ |
| 1S 450mAh Whoop (per cell) | 25-40mΩ | 40-60mΩ | 60-80mΩ | >80mΩ |
| 6S 4000mAh Li-Ion (Molicel P42A) | 15-20mΩ | 20-30mΩ | 30-40mΩ | >40mΩ |
Smaller cells have higher IR as a function of their physical size. A 1S 450mAh cell with 40mΩ IR is healthy. A 6S 1300mAh cell with 40mΩ IR is a fire waiting to happen. Always compare within the same cell class.
Figure of Merit (FOM) — When to Retire a Pack
The FOM formula helps standardize IR across different pack sizes:
FOM = (Cell 1 IR + Cell 2 IR + … + Cell N IR) / N × Capacity
For a 6S 1300mAh pack with per-cell IR of 4, 4, 5, 4, 4, 5:
FOM = ((4+4+5+4+4+5) / 6) × 1.3 = 4.33 × 1.3 = 5.6
Interpretation:
– FOM < 8: Excellent. Pack is healthy.
– FOM 8-15: Good. Normal aging.
– FOM 15-25: Marginal. Expect noticeable sag. Not for racing.
– FOM > 25: Retire. The pack sags severely and heats up under load.
Cell balance matters just as much as the average. A 6S pack with five cells at 4mΩ and one cell at 12mΩ has a fine average but a dangerous imbalance. The 12mΩ cell will sag deeper, heat faster, and fail before the others. In flight, that cell hits 3.0V while the rest are at 3.5V — your OSD shows the average and you never see the crisis.
What Most Pilots Get Wrong
Mistake 1: Judging pack health by resting voltage alone.
A severely degraded LiPo reads 4.20V per cell after charging, just like a brand new pack. Resting voltage tells you nothing about IR, capacity, or ability to deliver current. After a flight, if one cell recovers to a different voltage than the others (e.g., 3.78, 3.82, 3.80, 3.79, 3.81, 3.72), that 3.72V cell has elevated IR and is failing. Write down your per-cell voltages after every flight — the pattern reveals pack health weeks before you can feel it in the air.
Mistake 2: Trusting the C-rating printed on the label.
A “100C” label is marketing. At 100C on a 1300mAh pack, you’d be pulling 130A — the wires would melt before the cell delivered that current. Actual continuous discharge ratings for hobby LiPos are typically 25-35C, regardless of what the shrink wrap says. Use measured IR to calculate the true continuous current capability. As we covered in our LiPo C-rating guide, burst ratings and continuous ratings are completely different metrics that manufacturers blur deliberately.
Mistake 3: Storing packs at full charge “because I’ll fly tomorrow.”
LiPo degradation accelerates at higher state of charge. A pack stored at 4.2V for one week loses as much life as 30 charge cycles. IR increases measurably after just 72 hours at full charge. If you’re not flying within 24 hours, discharge to 3.80-3.85V/cell. Your charger’s “Storage” mode does this automatically. A fully charged pack left for a month will have IR roughly 40% higher than an identical pack stored at storage voltage.
Mistake 4: Continuing to fly a pack with one high-IR cell.
A pack with five healthy cells and one failing cell is still dangerous. The failing cell sags deeper, reaches LVC before the others, and gets reverse-charged in the worst case (the other five cells drive current through it like a load). During charging, the charger struggles to balance the bad cell, pushing the good cells slightly higher while the bad cell lags. If charging takes twice as long as it used to because the balance phase never finishes, one cell has elevated IR. Ground the pack.
Mistake 5: Not logging IR values over time.
IR measurement without historical comparison is just a number. Create a simple log: date, pack ID, per-cell IR. After 3 months of tracking, you’ll see which packs degrade faster, which brands hold up, and exactly when IR crosses the retirement threshold. A free spreadsheet is better than the best memory. Packs from the same batch age similarly — when one starts climbing, expect the others to follow within 10-15 cycles. For proper pack care between sessions, see our LiPo storage and maintenance guide.
⚠️ 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. Damaged or degraded LiPo batteries must be disposed of at certified battery recycling facilities — never in household trash.
For pilots replacing aging packs, the CNHL Black Series 1300mAh 6S consistently measures 2-4mΩ per cell fresh out of the box and holds IR below 7mΩ for 100+ cycles with proper storage voltage maintenance. In independent IR testing across five pilots, the Black Series averaged 20% lower IR than similarly priced competitors after 50 cycles. Available at uavmodel.com.
For a practical demonstration of IR measurement using common chargers and dedicated meters, Joshua Bardwell covers the technique and interpretation: