In February 2007, the U.S. Nuclear Regulatory Commission (NRC) issued Regulatory Guide (RG) 1.129 Rev. 2, "Maintenance, Testing, and Replacement of Vented Lead-Acid Storage Batteries for Nuclear Power Plants." In this RG, the NRC staff endorsed the Institute of Electrical and Electronics Engineers (IEEE) Standard 450-2002, "Recommended Practice for Maintenance, Testing, and Replacement of Vented Lead-Acid Batteries for Stationary Applications." This standard provides the recommended practices, test schedules, and testing procedures including recommended methods for determining a battery's state-of-charge to maintain permanently installed vented lead-acid storage batteries (typically of the lead-calcium type) for their standby power applications. Previous versions of this standard suggested that either float current or specific gravity could be used for determining the battery's state-of-charge. The NRC sponsored the research project described herein to validate the use of float charging current as a measure of a battery's state-of-charge for batteries that are used in the nuclear industry. This report describes the approach taken, the specific activities performed to achieve the objectives of this research effort, and the results achieved. It provides analysis of the data and offers observations and recommendations for use by the NRC and its licensees.