The simplest way to estimate a cell’s state of charge is to read its voltage while it is actually delivering or accepting current, rather than waiting for it to settle at rest. This is the terminal voltage method: the measured voltage under load is used directly, or with a simple correction, as a stand-in for the Open Circuit Voltage, which is the real basis for most voltage-based SoC lookups.
The catch is that terminal voltage isn’t OCV. Current flowing through the cell’s internal resistance and polarisation effects pulls the measured voltage away from the true rest voltage — down on discharge, up on charge — so a raw terminal-voltage reading systematically under- or over-states SoC unless that offset is estimated and removed. Because the error scales with current, this method is weakest during high-rate charge or discharge, and least useful for chemistries with a flat OCV-SoC curve, where even a well-corrected voltage reading maps to a wide range of possible SoC values.
Its appeal is cost and simplicity: no current integration, no cell model, and a result available the instant a voltage reading is taken — which is why it is typically the fallback a BMS uses at start-up, before coulomb counting or a Kalman filter has enough data to run.
References
- Wen-Yeau Chang, The State of Charge Estimating Methods for Battery: A Review, ISRN Applied Mathematics, Volume 2013, Article ID 953792
Part of a series expanding on the SoC Estimation Techniques overview.