Battery Voltage State of Charge Calculator

Limitations & assumptions (read before relying on the %)

• Open-circuit voltage: This estimate is intended for resting batteries. For best results, stop charging/discharging and let the battery rest (no load) for ~20–60 minutes before measuring.
• Not accurate under load/charge: Voltage sag (under load) and surface charge (right after charging) can shift readings significantly and make SOC look lower/higher than it really is.
• Temperature matters: Cold batteries read lower voltage at the same SOC; hot batteries may read higher. The reference curves assume roughly room temperature.
• Cell health & variation: Aging, internal resistance, and cell imbalance (especially in multi-cell packs) can cause the pack voltage to misrepresent true remaining capacity.
• Chemistry/model differences: Different manufacturers and cell formats have different voltage–SOC curves. This calculator uses typical reference points and piecewise linear interpolation, so results are approximate.
• Lead-acid specifics: Lead-acid SOC from voltage is especially sensitive to recent charge/discharge history and needs longer rest for a stable open-circuit reading.

Formula used (piecewise linear interpolation)

The calculator converts pack voltage to per-cell voltage by dividing by the number of cells in series, then interpolates between the nearest reference points for the selected chemistry.