4 Battery State of Charge Estimation and Remaining Capacity Prediction

With properly estimated Qmax and battery impedance parameters, the gauge can predict the remaining charge in the battery pack for a variety of load currents as shown in Figure 4-1. The challenge in this prediction is to estimate the difference between the battery OCV and the output voltage across the battery terminals due to the load current and battery impedance. For the purpose of this prediction, the gauge assumes that there is a minimum voltage needed to successfully operate the overall electronic system connected to the battery pack. The prediction of the battery capacity is the difference between the most recent SoC estimate, and the SoC when the battery terminal voltage is predicted to reach the minimum voltage for system operation. The value of Qmax is equal to the battery capacity in the limit of very small discharge current, when the IR voltage drop is negligible. The remaining capacity of the battery is the difference between the total capacity estimate shown in Figure 4-1 and the estimate of the battery DoD.

The remaining capacity prediction depends strongly on the load current and resistance. The IR voltage drop across the internal battery resistance makes the amount of charge that can be provided by the battery a function of the load current. For larger currents, the battery terminal voltage reaches the minimum value sooner.

Gauge algorithms that use only voltage measurements without compensation for the IR drop do not capture this effect and produce inaccurate estimates of the remaining battery capacity. Additionally, gauge algorithms that do not estimate the increasing battery resistance with age can underestimate the IR voltage drop. This generates inaccurate estimates of the remaining capacity.