Vanadium Redox Flow Battery 2D: OCV/SoC Strip Chart & Nernst Curve

Stack Current

Applied current 0.0 A

Electrolyte Conditions

Electrolyte temperature 25 °C

Membrane crossover rate Low

How it works

Same electrochemistry as the 3D companion, rendered as instruments instead of a rendered tank: z₋ is the fraction of the negative electrolyte present as V2+ (charged), z₊ the fraction of the positive electrolyte present as VO2+(2+) (charged).

OCV = E0 + (RT/F)·ln(z₊/(1-z₊)) + (RT/F)·ln(z₋/(1-z₋))
dz/dt = I / (Q·3600)      [coulomb counting]

A real battery-management system inverts a single voltage reading assuming z₋=z₊=SoC and a fixed 25°C reference — that inverse curve is drawn as the dashed line on the Nernst chart below, next to the true instantaneous curve at the real electrolyte temperature.

Temperature biases the estimate because RT/F sets the curve's slope; the BMS assumes 25°C no matter what the electrolyte actually is, so the operating dot drifts off the dashed reference curve.
Membrane crossover diffuses V2+/V3+ across the membrane faster than VO2+/VO2+(2+), which equalizes z₋ and z₊ toward each other over time (real diffusion physics) rather than driving them apart — watch the strip chart's z₋/z₊ traces converge, not diverge, as crossover rises. A single blended voltage still can't see this internal split, so the BMS's SoC estimate still carries an error, just from a different mechanism than pure temperature bias.

Charge/discharge use an accelerated demo capacity so tanks fill and drain in seconds instead of hours.

True SoC (avg) BMS-estimated SoC z₋ (negative tank) z₊ (positive tank)