This work presents the results of experimental analysis of the correlation between open-circuit voltage at 0% and the state of charge of a set (3 x 6) of high-temperature valve-regulated lead acid batteries, which provides a valuable health diagnosis tool when performing predictive maintenance actions. The proposed test could be executed after any emergency event in the battery system. It offers an alternative to the integration ampere hours, simplifying the diagnostic system, and can be used in many applications where the diagnosis can be made by monitoring the discharge voltage to a defined controlled value. By testing three different sealed, high-temperature lead acid battery models, it has been proved that open-circuit-voltage measurement at 0% state of charge is valid to evaluate health status and is applicable to different manufactures. In addition, the first derivative value calculation of the relaxation voltage over time provides accurate correlation with the state of health of the battery. The method proposed minimizes diagnosis times providing an easier way to implement the method in real systems.