A method and apparatus for determining the instantaneous state-of-charge of a battery in which change in composition with discharge manifests itself as a change in optical absorption. In a lead-acid battery, the sensor comprises a fiber optic system with an absorption cell or, alternatively, an optical fiber woven into an absorbed-glass-mat battery. In a lithium-ion battery, the sensor comprises fiber optics for introducing light into the anode to monitor absorption when lithium ions are introduced.
The present invention was conceived and developed in the performance of a U.S. Government Contract. The U.S. Government has certain rights in this invention pursuant to contract No. DE-AC04-94AL85000 between the United States Department of Energy and Sandia Corporation.