1 December 1998 Using optical absorption to measure the state of charge of lead-acid batteries
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Proposed here is a method of measuring the state of charge of lead-acid batteries based on the assumption that the addition of sulfuric acid to water will reduce the optical absorption at certain absorption peaks of pure water. This reduction is expected to result from the accompanying drop in the concentration of water molecules. Measurements of the absorption in the vicinity of 0.97, 1.20, and 1.45 ?m indicate that this is indeed the case, although deviations from a linear dependence on water concentration are noted. Two ways of implementing this phenomenon in a lead-acid battery are presented; one involves an absorption cell in a flooded battery, while the second involves an optical fiber woven into an absorbed-glass-mat battery. In the second case, the absorptive electrolyte serves as the cladding of the optical fiber and introduces attenuated total internal reflection into what would otherwise be perfectly guided modes.
Jonathan D. Weiss, Jonathan D. Weiss, Brian R. Stallard, Brian R. Stallard, Manuel J. Garcia, Manuel J. Garcia, } "Using optical absorption to measure the state of charge of lead-acid batteries," Optical Engineering 37(12), (1 December 1998). https://doi.org/10.1117/1.601999 . Submission:

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