Li-ion SOC from OCV Calculator

Estimate Li-ion state-of-charge (SOC) from open-circuit voltage. Uses typical LiCoO2/LiMn2O4 discharge curves — accurate within ±5% at room temperature.

Calculator Electronics Updated Apr 23, 2026
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How to Use
  1. Let the battery rest for at least 1 hour (reach open-circuit equilibrium).
  2. Measure voltage with no load, high-impedance DVM.
  3. Enter OCV; tool returns estimated SOC %.
Input
V (per cell)
Presets
OCV Curve
SOC
—%
Pack V
—V
DOD
—%
Status
—

Show Work

Enter OCV.

OCV Reference

NMC/LCO Full
4.20 V = 100%
Standard 4.2 V chemistry.
NMC/LCO 50%
~3.80 V
Storage target.
NMC/LCO 0%
3.00 V
Discharge cutoff.
LFP Full
3.65 V
Iron phosphate.
LFP Plateau
~3.25 V flat
20-90% SOC all near 3.25 V.
Rest Time
> 1 hour
For OCV equilibrium.

History of Li-ion SOC Tracking

John B. Goodenough's 1980 cathode material (LiCoO2) and Sony's 1991 commercial cell created the modern Li-ion industry. Early state-of-charge estimation relied on open-circuit voltage (OCV) lookup tables. Modern BMS chips (TI BQ34110, Maxim MAX17205) combine high-accuracy coulomb counting with periodic OCV calibration at rest - achieving 1% SOC accuracy over battery lifetime. LiFePO4 (LFP) cells, introduced by A123 in 2005, broke the OCV-SOC method due to their flat voltage curve; LFP BMS designs lean heavily on coulomb counting.

About This Calculator

Enter rested cell OCV (one-hour no-load equilibrium), chemistry (NMC/NCA/LCO have the same 4.2V curve; LFP is different), and cells in series for multi-cell packs. The tool maps OCV to SOC using a piecewise-linear fit to published discharge curves (±5% accuracy).

For precision SOC tracking in production BMS, use coulomb counting (integrate current) anchored by periodic OCV calibration at rest. Everything runs client-side.

Frequently Asked Questions

Why OCV?

Open-circuit voltage maps 1:1 to SOC in a rested Li-ion cell. Under load, terminal voltage drops by I × R_internal (10-50 mΩ typical), so loaded-voltage readings don\'t work — you need rested cells.

Accuracy?

±5% for typical cobalt-based chemistries (LCO, NMC, NCA). LiFePO4 (LFP) has a very flat voltage curve around 3.2-3.3 V — OCV gives poor SOC estimation between 20% and 90%. Use coulomb counting for LFP.

Temperature effect?

OCV-vs-SOC curve shifts slightly with temperature (~1 mV/°C). At -10°C, a cell reads ~30 mV lower than at 25°C for the same SOC. Modern BMS systems compensate with temperature input.

Common Use Cases

Field Battery Check

18650 cell rested overnight: read 3.85 V → ~70% SOC.

Pre-Shipping Spec

Li-ion rules require 30% SOC max for shipping; target 3.75 V.

Storage Prep

Store long-term at 40-60% SOC; target 3.8 V.

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