Energy storage in an uninterruptible power supply (UPS) is one of the most frequent applications of batteries. This can be found in hospitals, communication centers, public centers, ships, trains, etc. Most frequent industrial methods for battery state-of health estimation require a technician to move to the battery's location and, in some cases, require the use of heavy equipment and disconnection of the battery from the UPS. For example, in railway applications, trains must stop at the maintenance depot producing significant total costs. This article proposes a new method to assess a battery's health by measuring the battery's internal resistance, based on the measurement of its voltage ripple in response to the current ripple imposed by the charger which in most UPS applications is permanently connected to the battery. Unlike most traditional methods, this system makes it possible a continuous on-line and on-board monitoring, and, therefore, it eases condition-based maintenance (CBM). To verify its viability, a low cost measuring prototype has been built and measurements in a railway battery with its charger have been carried out.Energies 2019, 12, 2836 2 of 13 spent long periods of time floating without activity. Therefore, it is crucial to know aspects such as the process of battery degradation, how much energy they actually store, how much energy they will be able to provide, and how much time they have left before a failure in order to carry out an optimized preventive maintenance. These characteristics are summarized by the state of charge (SOC) and the state of health (SOH) of the battery [5-10].State-prediction techniques (prognostics and health management, PHM) have evolved in line with the knowledge of the internal operation of batteries, their chemical reactions and the process of degradation of their components. The existing methods to determine the state of health and the charge of a battery can be divided into direct methods and computational methods.Direct methods are based on tests, which usually require the intervention of a technician; that by direct measurements of a physical parameter of the battery can give directly or can estimate the battery state of charge SOC and state of health SOH. Some of these methods are: a.