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Home > News > How to measure the internal resistance of 18650 cells? What is considered normal?

How to measure the internal resistance of 18650 cells? What is considered normal?

2026-11-12

How to measure the internal resistance of 18650 cells? What is considered normal?

Many people only look at capacity but ignore this:internal resistance (IR) is more indicative of cell health than capacity.


 Why IR is more critical
Capacity indicates how much charge a cell can store; IR represents the cell’s internal resistance. Cell aging, SEI thickening, active-material shedding, and tab cold solder joints all increase IR. Many cells still have 80% capacity, but their IR is already out of spec. They underperform under load and are also warning signs of heating and runaway voltage spread.


Typical signs of high IR
▪ Large voltage drop under load: loaded voltage falls quickly and rebounds after load removal
▪ Noticeable heating at the same current
▪ Charger indicates full early; current drops quickly during the constant-voltage (CV) stage
▪ In packs, voltage spread widens rapidly and drags down the whole pack


Two self-test methods
At room temperature, 25°C, with cells rested and stabilized; preferably test at 3.6–3.8 V (around half charge).

AC IR tester | industry standard, recommended】
1 kHz four-wire (Kelvin) measurement; commercial battery IR testers use this standard.
Key points: use four-wire probes to avoid contact resistance; clean the terminals; press the probes firmly; compare at the same SOC.
Advantages: fast, no discharge required, suitable for batch sorting and matching.
Note: all reference values in the table are AC internal resistance.

【DC voltage-drop method | simple test without an IR meter】
Formula: Rdc = ΔU ÷ I

  1. Measure the rested open-circuit voltage U₀.

  2. Perform a short 0.5–1C discharge (1–3 s; avoid heating) and read the loaded voltage U₁.

  3. Calculate the voltage drop and then the DC internal resistance.


 Note: DC IR is higher than AC IR; the two must not be mixed in comparisons!

18650 AC IR reference ranges
Standard capacity type (2000–2500 mAh, low current): new 40–60 mΩ | warning 70–90 mΩ | ≥100 mΩ discard
Conventional power type (2500–2600 mAh, 1–3C): new 25–40 mΩ | warning 45–60 mΩ | ≥70 mΩ discard
High-rate power type (25R/30Q, etc., 5C+): new 12–25 mΩ | warning 30–40 mΩ | ≥45 mΩ discard
High-capacity type, 3500 mAh: new 50–70 mΩ | warning 80–100 mΩ | ≥110 mΩ discard
LiFePO4 18650: new 20–40 mΩ | ≥60 mΩ discard


Sorting & pitfalls

  1. A two-wire multimeter in resistance mode cannot measure cell IR; the error is huge.

  2. IR must be compared at the same model, same batch, and same SOC.

  3. IR spread among cells in the same pack: ≤3 mΩ for high-power packs, ≤5 mΩ for ordinary packs.

  4. Cells in the warning range may only be used at low current/light load; they must not be used in high-power series packs.

  5. Swollen, leaking, or terminal-corroded cells must be scrapped regardless of IR.

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