Quantify EV state of charge with voltage-curve discipline—not dashboard guesses. Built for fleet engineers, installers, and energy analysts who need auditable SoC baselines before dispatch, V2G, or warranty review.
Benefits
- Maps resting pack voltage to SoC using documented empty/full endpoints—repeatable across vehicles when endpoints are calibrated to OEM BMS tables.
- Surfaces voltage-curve limitations: load sag, temperature drift, and chemistry-specific OCV non-linearity—so teams know when to escalate to coulomb counting.
- Supports ROI planning: quantify usable kWh windows (e.g., 20–80% dispatch bands) before route assignment, depot charging, or demand-charge avoidance.
How it works
- Record resting pack voltage after the vehicle has been idle long enough for surface cell relaxation (typically 15–30+ minutes).
- Enter OEM or field-calibrated empty (~0% SOC) and full (~100% SOC) voltage endpoints for the high-voltage pack.
- The calculator linearizes SoC between endpoints; compare results to the BMS display and log deltas for your audit trail.
FAQ
Why does SoC from voltage differ from my EV dashboard?
Production BMS algorithms fuse coulomb counting, cell balancing data, temperature compensation, and aged OCV lookup tables. A single resting voltage reading gives a field checkpoint—not a replacement for the vehicle's calibrated estimator.
What is the relationship between voltage and SoC on Li-ion packs?
Open-circuit voltage (OCV) rises non-linearly with state of charge, with a flat mid-band and steep regions near empty and full. Linear interpolation between endpoints is a first-order estimate; NMC and LFP chemistries require different curve shapes for warranty-grade accuracy.
Can I use this under load or while charging?
No. Terminal voltage sags under discharge current and rises under charge current (IR drop). SoC from voltage is only meaningful at rest or when using manufacturer-published OCV tables at known temperature.
Technical specifications
- Model: SoC (%) = (V_pack − V_empty) ÷ (V_full − V_empty) × 100, clamped 0–100%.
- Inputs: resting pack voltage (V), empty endpoint (V), full endpoint (V); all positive, V_full > V_empty.
- Reference: align endpoints with OEM BMS documentation; IEC 62619 / UN 38.3 pack datasheets for fleet documentation.
- Limitation: does not replace coulomb counting, temperature-compensated OCV tables, or cell-level balancing data.
OCV curves and EV pack architecture
High-voltage EV packs series-connect hundreds of cells. Pack voltage is the sum of cell voltages; state of charge is a pack-level construct derived from cell-level OCV relationships. NMC graphs show steep slopes below ~15% and above ~85%; LFP holds a flatter mid-band, making voltage-only SoC harder without calibrated tables.
BMS reporting vs. field measurement
The onboard BMS integrates current shunt data, cell voltage min/max, and temperature sensors. Fleet telematics may report SoC with smoothing delays. A resting voltage check validates whether the BMS estimate drifts after long storage or after partial cycles.