Lead-Acid Battery Selection for Electronic Switching

In considering the maximum and minimum permissable voltages on any DC power system for telephone power use it is necessary to review the designs of switching equipment that have been available for the past 30 years. Over the years the most common number of hours used in calculating the capacity of batteries has been 8 hours. This was predicted on overall telephone systems: center city, urban, and rural. And superimposed on this 8 hour duration was an allowance for "busy hour" operation of the batteries. Since the early 1960s the alloys used in the design of both "antimony" and "calcium" batteries have been changing to keep pace with the improved grid casting techniques and with the increased life expectancy of flat plate batteries. At the commencement of the design change under consideration, flat plate battery systems had a life expectancy of around 12 years. Today this has been improved by rigid control of grain size and alloy content to more than 20 years. This has reduced float current which in turn has demanded the use of higher floating voltages to insure that sufficient floating current is drawn into the battery to maintain its operating stability. To obtain the full life expectancy from any of the products there is a restriction on the number of equivalent 8 hour discharges per year. For guidance we provide a table giving the overall comparisons of the four industries telephone batteries available.