Internal Resistance Calculator

What is Internal Resistance?

Internal resistance (Ri) is the opposition to current flow within a battery cell, caused by ionic resistance in the electrolyte, electronic resistance in electrodes and current collectors, and charge transfer resistance at electrode-electrolyte interfaces.

Internal resistance directly determines power delivery capability and efficiency. Higher resistance means more energy lost as heat (P = I²Ri), lower achievable peak power, and greater voltage sag under load. It increases with age, low temperature, and low state of charge.

For high-power applications (power tools, EVs during acceleration), low internal resistance is critical. Typical values range from 5-20 mΩ for high-power cylindrical cells to 50-200 mΩ for small consumer cells.

Internal resistance is not a single fixed value — it depends on SOC, temperature, frequency of measurement, and current direction (charge vs. discharge). DCIR measured with a 10-second pulse at 50% SOC is the most commonly reported metric, but EIS-derived values at 1 kHz will differ. Always compare values measured under the same conditions.

Formula: R_internal = (V_open - V_load) / I_load Voltage Drop = I × R_internal Power Loss = I² × R_internal

Example Calculation

A cell shows 4.15V open-circuit voltage and drops to 3.95V under a 10A load. Internal resistance = (4.15 - 3.95) / 10 = 0.02 Ω = 20 mΩ. Power loss = 10² × 0.02 = 2.0 W. At 40W output, efficiency = 40/(40+2) = 95.2%.

When to Use This Calculator

• Characterizing a cell's power capability by measuring voltage sag under a known load pulse during incoming inspection
• Diagnosing battery degradation by tracking internal resistance growth over time as part of a predictive maintenance program
• Estimating thermal load from I²R losses to size the cooling system during pack thermal design
• Comparing cells from different vendors or production lots to verify consistency and quality

Common Mistakes to Avoid

• Not allowing sufficient rest time before measuring open-circuit voltage — OCV needs 30-60 minutes of rest to stabilize after any charge or discharge event; measuring too soon gives artificially high or low resistance
• Using too low a test current — at very low currents, voltage drop is small relative to meter resolution, introducing large measurement error; use a current that produces at least 10-50 mV of drop
• Confusing cell resistance with pack resistance — pack-level resistance includes busbar, connector, contactor, and wiring resistance in addition to cell resistance; subtract these when diagnosing cell health
• Measuring at a different SOC than the reference condition — internal resistance varies with SOC, typically being lowest around 50% SOC and highest near empty; compare measurements at the same SOC

Frequently Asked Questions

How does internal resistance change with age?

Internal resistance typically increases 50-100% over a battery's lifetime. SEI layer growth, electrode particle cracking, and electrolyte decomposition all add resistance. This is why old batteries have reduced power capability and generate more heat, even if capacity fade is moderate.

What is the difference between DC resistance and AC impedance?

DC internal resistance (DCIR) is measured with a simple load pulse and includes all resistance components. AC impedance (EIS) uses varying frequencies to separate bulk electrolyte resistance, charge transfer resistance, and diffusion impedance, providing more diagnostic information about degradation mechanisms.

Why does internal resistance increase in cold weather?

Low temperatures slow down lithium-ion diffusion in the electrolyte and reduce the kinetics of charge transfer reactions at the electrode surface. At -20°C, internal resistance can be 2-5× higher than at 25°C, which is why EVs experience reduced range and power in winter — much of the extra energy is lost as heat in the cell.