Life performance and failure mode analysis of improved, large VRLA batteries, used in a 20 kW load-leveling installation

Yuasa has been evaluating the practical performance of a prototype load-leveling (LL) system since November 1997. The installed LL system was connected to the mains supply of our laboratory building and operated with AC output of 20 kW for 8 hours per day. 190 pieces of 2 V-500 Ah valve regulated lead acid (VRLA) cells with horizontal plate orientation are connected in series for the LL system. The system has performed 600 cycles with greater than 80% depth of discharge of the batteries. This demonstrates that long life can be achieved with a large series string of cells in practical operation. Two pilot batteries were selected from the system for more detailed analysis after 600 cycles. The residual capacities were approximately 450 Ah. Positive grid corrosion and negative plate sulfation were found to be the main causes of the battery deterioration. However, positive grid corrosion was low compared with expected levels for the service period, except for areas opposite the negative plate lug. And no serious corrosion was found on the negative strap and lugs. This paper reports further detailed analysis of the system and batteries.