When a cell is subject to a voltage pulse before degradation occurs, the resulting current response is denoted as ī; upon degradation, the degraded response is i. We establish a relationship between the responses with the fitness W = i/ ī, which is the ratio between the degraded and nondegraded current. This describes the change in rate capability of the battery 50 by linking it to physical degradation parameters. The exact solution of the current ratios for one electrode is