Power supply system reliability taking battery failure into account

This paper reports a reliability evaluation method of a power supply system with battery backup. When one evaluates the reliability of a power supply system, the failure rate of contentional flooded batteries can be neglected because this type of battery is very reliable. There have been, however, some failures with valve-regulated lead-acid (VRLA) batteries losing their capacity within a few years after installation. So, it is necessary to clarify the power supply system reliability evaluation method to consider battery failure. First, by analyzing the distribution curve of operating time before failure using the Weibull plot method, it is shown that VRLA batteries have a failure mode similar to that of common electronic components. A reliability evaluation method is then presented that considers battery failure. This reliability can be evaluated by adding both unavailabilities within and outside of the battery reserve time. The former is obtained with the probabilities of the battery failing within the reserve time and of the power conversion block repair time not exceeding the battery reserve time. The latter is obtained with the probability of the power conversion block repair time exceeding the battery reserve time.