The PREPA was a fully commercial battery system that was acquired for daily operation in a frequency control and spinning reserve mode for the island grid of Puerto Rico. The grid had ongoing stability issues, and routinely had frequency and voltage excursions that could only be controlled by aggressive load-shedding, unless new generation was added to provide regulation and stability. The choices for new generation included fast-acting combustion turbines or battery energy storage. PREPA's analysis showed that battery energy storage systems offered superior operational benefits due to their faster reaction times, both for frequency regulation and spinning reserve requirements.

The comparable slower response times of combustion turbines required more installed capacity, whereas the faster reaction time of a battery system meant that a much smaller battery could offer the same functionality as larger sizes of combustion turbines. Typically, battery systems can reach full operating power in less than 1 second, whereas mechanical systems such as combustion turbines need several seconds to minutes to reach their full power output. The seemingly small difference in reaction times translates into very large consequences for the stability of the electric grid, where events that lead to outages propagate within cycles, and a difference of 1 min translates to a complete blackout under some conditions. Thus, it was shown that battery systems were a more cost effective option compared to combustion turbines because a smaller battery could outperform a much larger block of combustion turbines. This is particularly applicable to an "island" system, where there is no interconnection with a neighboring system to balance the momentary shortage of generation resources during an operational contingency.

The PREPA battery was patterned after the BEWAG battery in application as well as battery type. Valve-regulated lead-acid (VRLA) batteries were commercially available by the time the PREPA battery project was started, but PREPA chose a flooded, flat-plate cell because it had a proven track record at BEWAG. However, once utility operations began in 1994, the battery was cycled more frequently than planned, which caused the battery to age more rapidly than expected. This use led to positive-plate growth, which caused cell / jar cracks, leaks, short circuits, and ultimately early battery failure.

PREPA made the decision in 2001 to repower, or replace, the battery. A tubular positive plate, flooded battery was selected, and the new battery was installed in mid-2004. Several problems occurred, however, and the system was taken out of service.

Lead-acid Battery