This article describes a multi-point optical fiber-based sensor for the measurement of electrolyte density in lead-acid batteries. It is known that the battery charging process creates stratification, due to the different densities of sulphuric acid and water. In order to study this process, density measurements should be obtained at different depths. The sensor we describe in this paper, unlike traditional sensors, consists of several measurement points, allowing density measurements at different depths inside the battery. The obtained set of measurements helps in determining the charge (SoC) and state of health (SoH) of the battery.

There are numerous applications for the use of leadacid batteries: automotive applications, telecommunications, UPS, remote-area power-supply systems, etc. Modern energystorage applications require higher reliability than has been demanded in the past; in addition, systems have been developed for monitoring and managing the batteries so as to achieve duty in service. For determination of battery state, it is necessary to know the state of charge (SOC) and the state of health (SOH), one of the physical parameters with information about the SOC is the electrolyte density. An electrolyte density sensor must be able to measure the electrolyte density at different places because the battery-density varies with location. This paper presents a plastic optical fiber sensor developed for measuring in real time the electrolyte density into lead-acid batteries. The sensor measures the density at four different heights.The environment in the batteries that use an electrolyte with Sulfuric Acid (H 2 SO 4 ) at an elevated temperature during the use process is very hard and the accuracy of the density readout must be maintained through the useful life of the vehicle. We have done an accelerated test to check the reliability sensor. The test duration was over one calendar year (9,552 hours) using a sensor with four optical fibers. 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. In this paper we present the test results.