A battery management system includes one or more fiber optic sensors configured to be disposed within an electrochemical battery. Each fiber optic sensor is capable of receiving input light and providing output light that varies based on the input light and an amount of free or dissolved gas present within the battery. A detector detects the output light and generates an electrical detector signal in response to the output light. Battery management circuitry determines the state of the battery based at least in part on the detector signal.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with government support under contract DE-AR0000274 awarded by ARPA-E (Advanced Research Projects Agency-Energy). The government has certain rights in the invention.