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Battery Internal Resistance Calculator

Calculate battery internal resistance from open-circuit and loaded voltage measurements, determine power loss and maximum short-circuit current.

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Formula

Rint=Vocv−VloadIloadR_{int} = \frac{V_{ocv} - V_{load}}{I_{load}}
Vocv— Open circuit voltage (V)
Vload— Loaded terminal voltage (V)
Iload— Load current (A)
Rint— Internal resistance (Ω)

How It Works

Battery internal resistance calculator determines Rint from open-circuit and loaded voltage measurements — essential for EV battery management, UPS health monitoring, and portable device optimization. Battery engineers use this to predict remaining capacity, as internal resistance increases 20–50% over a cell's lifetime per IEC 61960.

Internal resistance comprises ionic resistance (electrolyte), charge transfer resistance (electrode-electrolyte interface), and ohmic resistance (current collectors, tabs). For Li-ion cells, fresh 18650s measure 20–80 mΩ; automotive prismatic cells 0.5–2 mΩ. Lead-acid batteries: 3–15 mΩ per cell. Per USABC standards, EV battery end-of-life is defined as 80% capacity OR 2× initial internal resistance.

Temperature strongly affects Rint: at 0°C, Li-ion resistance doubles compared to 25°C; at -20°C it increases 4–6×. This explains why EVs lose 20–40% range in winter — not primarily from heating loads, but from increased IR drop during acceleration.

Worked Example

Tesla Model 3 cell health check (per SAE J2464 test procedure)

Given: 2170 cell, OCV = 4.18 V, V_load = 4.02 V at 10 A discharge

Step 1: Calculate internal resistance R_int = (V_OCV − V_load) / I = (4.18 − 4.02) / 10 = 16 mΩ

Step 2: Compare to spec

  • New cell: 12 mΩ (Panasonic datasheet)
  • Current: 16 mΩ → 33% increase
  • EOL threshold: 24 mΩ (2× initial)
Step 3: Estimate remaining life
  • Resistance growth is approximately linear with cycles
  • At 500 cycles: 16 mΩ → ~750 more cycles to EOL
  • Estimated total life: ~1,250 cycles (within 1,000–1,500 typical range)
Step 4: Power loss at max discharge (100 A) P_loss = I²R = 100² × 0.016 = 160 W per cell → For 4,416-cell pack: 700 kW loss at peak power (explains thermal management needs)

Practical Tips

  • ✓Use 4-wire (Kelvin) sensing to eliminate lead resistance error — critical when measuring <50 mΩ cells
  • ✓Allow 30+ minute rest before OCV measurement; Li-ion voltage relaxation can be 50–100 mV immediately after charge/discharge
  • ✓For pack-level testing, measure cell-to-cell variation: >20% spread indicates weak cells needing replacement
  • ✓Track Rint vs temperature: create lookup table at 0°C, 25°C, 45°C for accurate SoH estimation year-round

Common Mistakes

  • ✗Measuring at low current (
  • ✗Ignoring SoC dependence: Li-ion Rint increases 20–30% below 20% SoC and above 90% SoC due to concentration polarization
  • ✗Single-point measurement: AC impedance at 1 kHz gives only ohmic component; DC pulse (10 ms–1 s) captures full Rint
  • ✗Confusing Rint with impedance: EIS shows frequency-dependent behavior; 1 kHz ≈ DC Rint ± 10% for most cells

Frequently Asked Questions

Every 1 mΩ increase causes 1 mV drop per amp of current. At 100 A (typical EV acceleration), a degraded cell with 30 mΩ (vs 15 mΩ new) loses 1.5 V extra, reducing available power by 15% and generating 150 W more heat. This creates thermal runaway risk in high-power applications.
Per IEEE 1188 (VRLA) and IEC 61960 (Li-ion): replace when Rint reaches 2× initial value. Typical thresholds: Li-ion 18650 >100 mΩ, automotive pouch >3 mΩ, lead-acid >25 mΩ/cell. Some BMS trigger alerts at 1.5× for proactive replacement.
Li-ion internal resistance follows Arrhenius relationship: ~2× at 0°C, ~4× at -20°C vs 25°C baseline. At 150 kW charging, a cold battery with 4× Rint dissipates 4× more heat internally while accepting less current (to stay within voltage limits). Tesla/Rivian pre-condition batteries to 15–25°C before DC fast charging to restore normal Rint.

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