The purpose of this study was to investigate the method of residual capacity estimation for lead–acid batteries used in automobiles. First, relation charts for the internal resistances of a battery at various load currents to residual capacity percentages were established, and the relation charts for all load currents were then combined to obtain the corresponding residual capacity by calculating medians. The experimental equipment included lead–acid batteries for automobiles, an electronic loader, an internal resistance tester, and test cables. The experimental procedures were discharging the battery with the electronic loader, using the internal resistance tester to record the internal resistance, voltage, and temperature of the battery, and then transmitting the data to a computer via the test cables for analysis. The experiment obtained nine sets of data, which were recorded in Excel and illustrated using charts. The medians obtained from combining the internal resistance with the residual capacity percentages were used to generate the relation charts for the internal resistances at various load currents to the residual capacity percentages. Finally, 60 Ah was used as the normal capacity to estimate the residual capacity discharging time. Furthermore, a curve-fitting approach for determining the relation equation between internal resistances and capacities was used to replace the table look-up method for residual capacity estimation. The results revealed that the estimation errors after correction were acceptable.
