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.
How to Use
- Let the battery rest for at least 1 hour (reach open-circuit equilibrium).
- Measure voltage with no load, high-impedance DVM.
- Enter OCV; tool returns estimated SOC %.
Show Work
OCV Reference
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.
About the Li-ion SOC from OCV Calculator
The Li-ion SOC from OCV Calculator is a simple, free helper for electronics and circuit design that runs entirely on your own device. Estimate Li-ion state-of-charge (SOC) from open-circuit voltage. Uses typical LiCoO2/LiMn2O4 discharge curves — accurate within ±5% at room temperature.
How it works
Enter your figures and the result appears instantly, updating the moment you change anything. There is no submit button and nothing to wait for, so it is easy to try a few what-if numbers and compare the results. Just check each box holds the kind of value it expects.
Want the deeper story? The Knowledge Base explains the ideas behind the tools in more detail.
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.
How do I use the Li-ion SOC from OCV Calculator?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Does it cost anything or need an account?
No. The tool is completely free, there is no account to create, and it keeps working offline after the page first loads.
Is anything I type uploaded?
No. The tool works entirely on your device, so the values you enter never leave your browser.
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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