Modified Coulomb Counting - Battery Design

Coulomb counting works by integrating measured current over time, but the raw method drifts: current-sensor offset accumulates instead of averaging out, the starting SoC has to come from somewhere else, and the usable capacity it is being measured against changes with temperature and age. Left uncorrected, these errors compound until the reported SoC no longer means very much.

Modified coulomb counting is the family of fixes layered on top of the basic integral. Common approaches include periodically resetting the running count against an OCV or full-charge reference point whenever the cell is genuinely at rest, scaling the usable capacity term for temperature and cycle-count-based ageing rather than treating it as fixed, and adaptive error-correction schemes that estimate and cancel the sensor’s own drift in real time using a recursive filter. None of these remove the underlying need for a rest-point recalibration; they reduce how often, and how badly, the estimate needs one.

In practice a BMS rarely runs coulomb counting alone — it is paired with a periodic OCV check or folded into a Kalman filter as the process model precisely because of this drift behaviour. See also Hybrid SoC Estimation Methods.