A fibre-optic density sensor for monitoring the state-of-charge of a lead-acid battery

A fiber-optic technique for measuring specific gravity is described. The method is based on the principle of variation of the critical angle of reflection due to a change in the density of the surrounding medium, which causes an equivalent change in the leakage of light through the walls of the fiber in contact with the fluid. This change can be measured and converted to a related specific gravity value. It is shown how this technique can be used to determine the state of charge of a lead-acid battery as represented by the acid concentration and related index of refraction. Experimental results obtained with this instrument are presented