Light is transmitted from a light source through or from a separator of a battery cell and received by one or more light detectors. The light that is normally transmitted through the separator is scattered, absorbed, wavelength-shifted or otherwise distorted by an impending fault in the vicinity of or within the separator. The change in light due to the impending fault is measured by a detector and a signal from the detector is processed to identify the impending fault so that a warning can be generated indicative of the impending fault. In particular, one or both of the light source and detector are enclosed within a battery cell housing and receive power from the electrodes of the battery cell.