RELATIVE STANDARD DEVIATION OF THE CELL VOLTAGES AS A MEASURE OF THE STATE-OF-HEALTH OF AGM VRLA BATTERIES

Ageing of storage batteries leads to a gradual degradation of their characteristics. Particularly sensitive are valve-regulated lead-acid batteries, when the long-term operation at high temperatures may lead to a significant loss of available capacity, or even to a complete failure of the storage battery. Therefore, it is important to define the procedures for determination of the battery state-of-health (SOH) during its operation or planned maintenance. The paper describes a simple procedure for assessment of the VRLA battery state-of-health. The procedure is based on the monitoring of a cell voltage relative standard deviation in a storage battery chain. In two uninterruptible power supply systems, the state of the batteries was periodically monitored over a period of 12 years, i.e., the nominal exploitation period for these stationary storage batteries. The influence of the exploitation conditions and regular maintenance on battery's state-of-health and service life has been analysed. It has been found that the annual battery cycling, performing the discharge of 80% of the nominal battery capacity, has an indispensable importance for prevention of the premature loss of the VRLA battery capacity. The relative standard deviation of battery cell voltages increases rapidly during the final period of exploitation, simultaneously with spotting of characteristic degradation of the ever increasing number of battery cells. Described phenomenon can be a trustworthy sign for replacement of a worn-out absorbent glass mat VRLA battery.