Lead-acid batteries aren't going away in the foreseeable future. And given their ubiquity, it makes sense to look for an electrically noninterfering way to determine their state of charge. Sandia's Jonathan Weiss has devised an optical method that provides an instantaneous determination; neither an integration over time nor a voltage measurement is required.
As a lead-acid battery discharges, the concentration of sulfuric acid diminishes from its value at nominal full charge to its value at nominal discharge. Accompanying this reduction in sulfuric acid concentration is a corresponding reduction in the electrolyte's refractive index. Weiss's sensor detects the refractive index of the electrolyte as a measure of the battery's state of charge. Although a similar technique based on plastic optical fibers has been tried before, the low value of the electrolyte's refractive index necessitates the inclusion of bends in the fiber that add to the sensor's complexity of fabrication and its size. When that is done, some sensitivity is achieved, but it is less than the optimum. Furthermore, if these bends change during operation, so will the sensor's calibration.
A better way to monitor battery charge is to use a straight optical fiber with a core refractive index as low as necessary to achieve optimum dynamic range, or zero signal at full charge and 100% signal at full discharge. This refractive index would be equal to that of the electrolyte at full charge, which is 1.386 for an electric vehicle, or less for other battery types. A solid optical fiber of this core refractive index may be impossible to obtain, but the corresponding liquid is obviously not. The detector is based on a thin-walled clear tube containing a liquid with a refractive index equal to that of the electrolyte at full charge. (It could be the electrolyte itself, for that matter.) The tube is immersed in the electrolyte, which will not damage glass and certain other materials, such as Plexiglas. Because it has an index higher than that of the liquid inside and outside of it, the tube cannot serve as the cladding. Ignoring the negligible optical influence of the tube, it is in effect a liquid core–liquid clad fiber with the core having the optimum value for this application. Since the core and cladding are essentially made of the same material, they are closely matched thermally. Temperature-induced errors in the measurement are therefore minimized.
In operation, a small reflector is placed at the far end of the closed tube, which sends the light back to a detector along the original optical path. This doubles the path, allowing the device to approach the ideal optical behavior and maintain its compact size (~5 cm long and 1.48 mm in dia.).