A simple, robust and low-cost fiber optics system for monitoring the state-of-charge (SOC) in a lead-acid battery is presented. The device is based on measuring light ray trajectory variations produced by the electrolyte media when its density changes. The changes in the electrolyte refraction index are produced due to changes in density, and the system measures such changes by means of an optical sensor coupled with a fiber optics bundle. SOC is indirectly measured on the base of such optical specific gravity measuring. A set of equations based on a paraxial ray model perform the optical sensor design, describing its behavior for different design parameters thereby allowing for the optimization of signal response. The system is applied to an SLI battery discharge-charge cycle obtaining excellent agreement with direct measuring of electrolyte density and estimation of SOC by integration of current. Simplicity, low cost, robustness and the unnecessary use of any signal processing are the main improvements with respect to previous works. (c) 2005 Elsevier B.V. All rights reserved.