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, the separator and a battery cell electrolyte can be selected to provide waveguide properties.
GOVERNMENT LICENSE RIGHTS
 Aspects of this invention were made with support from the United States (US) Government under Contract No. DE-AR0000272 awarded by the Advanced Research Projects Agency-Energy (ARPA-E). The US Government may have certain rights in the invention.