Life testing of plastic optical fibers for lead-acid battery fast charge equipment

Our research has been focused in the past years on the development of efficient fast battery chargers of application to electrical vehicles. One of the challenges that we are facing is selecting adequate sensing components to accurately monitor the charging process, due to the harsh environment in the batteries that use an electrolyte with sulfuric acid (H₂SO₄) at an elevated temperature during the charge process. The sensor must be able to measure the electrolyte density at different places within the battery -density varies with location. The accuracy of the density readout must be maintained through the useful life of the vehicle. In this paper we review our current line of research in fast battery charges as it pertains to their reliability. We present our results on accelerated tests using plastic fiber optics as sensing elements for electrolyte density. The test duration was over one calendar year (9,552 hours) using a sensor with four optical fibers. Three of the optical fibers are used to measure density, and the fourth one as a reference to account for common-mode variation. The charge level of the battery can be derived from this density measurement, as well as the overall health of the battery. For the test, the electrolyte acid density was set to 35%, and the temperature to 70 degrees Celsius. An electronic system for emission and detection of light collected data periodically to assess the transmission loss in the fibers as they aged.