NOVELTY - Monitoring a hermetically sealed battery cell (12) having an exterior (18) and an interior (16), which is normally sealed off from the exterior and contains a chemistry (17) which, in operation, can generate combustible vapors if exposed to air should any of the chemistry exit to the exterior of the cell, comprises positioning a combustible vapor sensor (14) outside of the cell in air flow communication with the exterior of the cell so as to sense combustible vapors escaping from the cell in its operation.    USE - Monitoring a hermetically sealed battery cell i.e. lithium-ion battery cell (claimed) for coupling to a powered device, particularly electrically powered device, e.g. transportation systems and portable devices. Uses include but are not limited to e.g. electric dragster vehicle, passenger-type vehicle e.g. car or truck, or jet liner.    ADVANTAGE - The method is able to anticipate the potential onset of overheating or fire so that steps may be taken to avert or minimize the problem before the battery overheats or catches fire. Sensing for the presence of combustible vapors in the vicinity of the normally sealed cells can provide advance warning of an impending failure or worse, and allow for steps to be taken to avert or minimize the problem. The method can also grade the severity of such problems based upon predetermined levels. It is possible to detect or anticipate a catastrophic failure or fire days, weeks, or even months, before it might occur, although under severe abuse condition, the detection may occur just moments before a catastrophic failure or fire occurs. But the actuated response provides an opportunity to correct the root problem before a fire occurs, for example, or to at take evasive steps to avoid danger or harm.    DETAILED DESCRIPTION - Monitoring a hermetically sealed battery cell (12) having an exterior (18) and an interior (16), which is normally sealed off from the exterior and contains a chemistry (17) which, in operation, can generate combustible vapors if exposed to air should any of the chemistry exit to the exterior of the cell, comprises positioning a combustible vapor sensor (14) outside of the cell in air flow communication with the exterior of the cell so as to sense combustible vapors escaping from the cell in its operation. When the hermetically sealed battery cell is immersed in a liquid, where interior contains a chemistry which, in operation, can release an electrolyte into the liquid which would be a combustible vapor if exposed to air should any of the chemistry exit to the exterior of the cell, the method comprises positioning an electrolyte sensor outside of the cell in fluid flow communication with the exterior of the cell so as to sense the electrolyte escaping from the cell in its operation. An INDEPENDENT CLAIM is included for a hermetically sealed battery monitoring system.    DESCRIPTION OF DRAWING(S) - The drawing is a perspective, partially cut-away view of a hermetically sealed lithium-ion battery cell and a combustible vapor sensor in air flow communication with the exterior of the cell.    Hermetically sealed battery cell (12)    Combustible vapor sensor (14)    Interior (16)    Chemistry (17)    Exterior (18)