Internal resistance (IR) tells you more about a battery's condition than capacity does. High IR means voltage sag under load, heat, and wasted energy. Here's how to measure it with whatever gear you have.

What internal resistance is

Every battery has some resistance to current flow. That resistance is measured in milliohms (mΩ). When current flows, the voltage drop across that resistance is wasted as heat. A fresh cell might read 20 to 100 mΩ. A degraded cell can read 500 mΩ or more.

IR matters most in high-current applications. A cell with 200 mΩ IR drops 2V at 10A discharge. That's 20 watts of heat instead of power to your load.

What counts as good IR

These are approximate ranges for common cell types at room temperature:

  • Quality 18650 NMC (new): 50 to 150 mΩ
  • Quality 21700 NMC (new): 20 to 60 mΩ
  • LFP prismatic (100Ah+): 0.2 to 0.5 mΩ
  • LFP cylindrical (38120, etc.): 5 to 20 mΩ
  • Used 18650 (Grade A): under 200 mΩ
  • Used 18650 (marginal): 200 to 400 mΩ
  • Used 18650 (retire): above 500 mΩ

Manufacturer specs use AC IR measured at 1kHz. DIY methods give DC IR, which runs higher. Don't compare your DC reading directly to the datasheet — compare cells against each other within your batch.

Method 1: Dedicated IR meter (best)

A YR1030+ or similar 4-wire milliohm meter is the most accurate and fastest method for spot-checking cells. It clips onto the cell terminals, applies a small AC signal, and reads resistance directly. Total time per cell: under 5 seconds.

Some capacity testers (OPUS BT-C3100, SkyRC MC3000) include IR measurement built into the charge/discharge cycle. This is slower but means one tool does both jobs.

Method 2: Multimeter with load resistor

If you don't have a dedicated IR meter, you can calculate DC internal resistance with a multimeter and a known load resistor.

What you need

  • Multimeter (preferably with 4-digit resolution)
  • A load resistor — 1Ω at 5W for 18650s is a good starting point
  • Alligator clips or a cell holder

Procedure

  1. Charge the cell to a known state (50% SoC is conventional; voltage varies less at midpoint).
  2. Measure open-circuit voltage (Voc) with no load. Record it.
  3. Connect the load resistor across the cell terminals.
  4. Immediately measure the loaded voltage (Vload) — do this within 1 second. Voltage continues to drop as the cell heats.
  5. Calculate current: I = Vload / R (use your measured resistor value, not the label)
  6. Calculate IR: IR = (Voc − Vload) / I

Example

Voc = 3.72V, R = 1.00Ω, Vload = 3.58V

I = 3.58 / 1.00 = 3.58A

IR = (3.72 − 3.58) / 3.58 = 0.14 / 3.58 = 39 mΩ

That's a healthy cell. The same math works for any resistor value.

Method 3: Two-point voltage comparison (quick screen)

If you just need to quickly sort a batch, measure open-circuit voltage, then measure again under a small load (even just a flashlight bulb). Cells that show a larger voltage drop under load have higher IR. This won't give you a number, but it quickly separates bad cells from good ones in a batch.

Temperature effects

IR is strongly temperature-dependent. A cell at 0°C can show 3 to 5x higher IR than the same cell at 25°C. Always test at room temperature and let cold cells warm up for at least 30 minutes before testing.

IR vs state of charge

IR is lowest at mid-charge (30 to 70% SoC) and rises at both extremes. Testing at a consistent SoC level — 50% is conventional — gives the most reproducible results when comparing cells.

Using IR to grade a pack build

When building a series-parallel pack, match cells within each parallel group to within 20 mΩ of each other. High-IR cells in parallel pull less current than low-IR cells, creating imbalance. Over time, that imbalance causes uneven degradation. For a series string, IR mismatch causes voltage sag differences between cells, which can trigger BMS cutoffs on the weakest group first.

Use the Cell Database to look up typical IR specs for name-brand cells before buying salvaged batches — it helps you know what you're aiming for.

FAQ

Can I measure IR with a standard multimeter's resistance mode?
No. Standard multimeter resistance mode applies a tiny current that won't give you a useful reading on a battery. You need to use the voltage-drop method above, or a dedicated IR meter.
My reading seems high — is the cell bad?
Check temperature first. Cold cells read high. Also check your connections — poor contact adds resistance. If it's still high at room temperature with good connections, the cell is degraded.
Do LFP cells have lower IR than NMC?
Large-format LFP prismatic cells (100Ah+) have extremely low IR — fractions of a milliohm — due to their large electrode area. Cylindrical LFP cells are in a similar range to cylindrical NMC. Compare within the same form factor.