Measuring the internal resistance of a cell in assembled batteries

The condition of a valve-regulated lead-acid battery can be evaluated by measuring the internal resistance, impedance, or conductance. When the internal resistance of a cell in assembled batteries in active service is measured, however, the measured value is often smaller than the actual value because a portion of the measuring current flows into other cells or equipment connected to the cell. This paper describes methods for measuring the internal resistance of a battery cell without disconnecting the cell. It has been clarified that the actual internal resistance of a cell can be calculated theoretically from the measured values for cells in assembled batteries. A new method (called the correction factor method) for estimating the actual internal resistance is to divide the measured value by the correction factor F, where F=(mn-m+1)/mn, where m is the number of cells in parallel and n is the number of cells in series.