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Availability, is usually interpreted as the total time a system is capable of performing its intended function divided by the total time the system is required to be operable. This value is usually expressed as a percentage. Applying this definition of availability to batteries we must be able to determine the battery's state of charge and know a battery's capacity (history) to determine whether it is available or not. There are two common methods for assessing the state of charge of a lead-acid battery discussed in the latest IEEE std. 450. The most common method used in the past has been electrolyte specific gravity (S.G.) measurement readings taken with a hydrometer from one or more cells of the battery. The other method to determine the battery's state of charge is the use of a stabilized charging (float) current measured with a sensitive clamp-on ammeter, or a suitable shunt and voltmeter. Both of these methods are correct and commonly used. Recent downsizing and other cost cutting efforts have reduced the manpower available for maintenance and trending have now made the use of speciftc gravity readings very costly in some cases or impossible to take in others. Some users have turned to automatic monitoring systems with remote reporting capabilities to routinely collect battery data. In addition, many installations have valve-regulated lead-acid (VRLA) batteries which have no provision for measuring specific gravity. With the large numbers of VRLA batteries now in service and being sold today, the use of the float current method of assessing state of charge is expected to increase.