The round cell: promises vs. results 30 years later

In the late 1960´s Bell Laboratories developed a novel cylindrical design for a stationary reserve lead acid battery to provide back-up power in the Bell Telephone System´s central offices. The extensive laboratory test and evaluation data suggested that this design should significantly extend the expected life and reduce maintenance in telephone float service vs. conventional designs available at that time. Limited commercial production of this design was initiated in 1972, with full-scale production at two manufacturers underway by 1974. To date more than 1.5 million cells have been made, installed and used by the divested telecom heirs to their Bell System forefathers, as well as nonBell companies. Actual discharge test, field inspection and overall performance evaluation results are presented on more than 14,000 individual round cells, ranging from less than one to more than 24 years in service. Round cell string capacity tests show a level trend with no string failures for the full 24-year period. Individual round cell tests of the 14,568 round cell population indicate a worst case failure rate of 19 round cells in the 14,568 cell population, or 0.1% failure. Eliminating cells of questionable suitability (not recharged, etc.) results in a failure rate of only 2 cells of the 14,551 remaining round cell population, or 0.01% failure. The 1970´s accelerated tests had projected plate growth to reach only 2% in 70 years. Plate growth data from round cells removed from field service for 7, 15 and 23 years appear to confirm that projection. The data also indicates zero post seal or jar cover seal leakage in any of the more than 14,000 round cells tested, in stark contrast to the traditional field expectations of post seal and jar/cover seal leakage in either rectangular flooded or valve regulated lead acid (VRLA) cell designs. An actual post seal leakage rate of 4 to 10 percent was found on the more than 28,000 rectangular flooded cell population tested. Capacity test results on the >14,000 VRLA cell test population follow results of other recent publications. VRLA average string capacities trend linearly downward with age, reaching 80% in only 5 years. This study lends further credence to what appeared at the time to be exorbitant promises of the 1970´s wit- h regard to both reduced capacity aging and grid growth rates. This study also reports a zero rate of post seal and jar/cover seal leakage, unique in the industry.