Lifetime maximization of lead-acid batteries in small scale UPS and distributed generation systems

In this paper, we analyze lifetime maximization problem of lead-acid batteries commonly used in small scale Uninterrupted Power Supply (UPS) and distributed renewable energy systems. The aging process of batteries in these systems is a complex phenomenon and depends upon many factors. We consider the most comprehensive lead-acid battery model that is commonly known as the weighted Ah-throughput (Schiffer) model and identify three key factors affecting the lifetime of these batteries: 1) bad recharge, 2) time since last full recharge and 3) the lowest state of charge since last full recharge. Each factor depends on battery state of charge (SoC). An appropriate weighted sum of these three factors dictating the battery life is considered as optimization objective. The appropriate constrained optimization problems for the two common scenarios are solved and SoC-based charge/discharge algorithms are formulated. Simulation results show significant improvement in the lifetime of lead-acid battery (more than 85% in some cases) as compared to the traditional terminal voltage based charge control algorithms.