Estimating the life of stationary lithium-ion batteries in use through charge and discharge testing
When stationary lithium-ion batteries are used as a telecommunications backup power supply system, they need to be linked in series to achieve -48 V, and serially-connected lines must be connected in parallel to ensure capacity. To improve the reliability of a system, we must reduce the number of parallel lines. Therefore, we have developed a large capacity 200 Ah battery. We have also improved the safety of large capacity cells by using an electrolyte containing a phosphazene-based flame retardant. As a result, the cells have passing various safety standards, verifying their safety. Batteries for a backup power supply system must remain in a high state of charge (SOC). Generally, high SOC leads to battery capacity deterioration. Therefore, we fabricated with a cathode material in which part of the elemental manganese was replaced with [magnesium-spinel oxide?] and have succeeded in suppressing the capacity deterioration even in environments with almost full charge by constant voltage charging. We have also developed a battery management system for connecting the cells in a multi-series-parallel arrangement. This system has various functions such as the monitoring of various voltages and current temperatures, data backup when a fault is detected, and a safety shutdown function to control reserve batteries. We have implemented life estimation by a charge and discharge test using a backup power supply system that employs these lithium-ion batteries. Several factors affect battery life. Well known examples are the number of charge and discharge cycles, running time, temperature, difference in capacity between modules, and charge voltage. By using stationary batteries for backup, the number of charge/discharge cycles is small and the maintained charge voltage is constant. Therefore, we evaluated the difference in capacity between modules and running time and estimated battery life by charge/discharge testing. The test result shows that the developed backup system's life can be estimated by using square root law. Use of the square root law resulted in prediction of expected lifetimes of at least ten years.