3 ELECTRIC VEHICLE BATTERY CAPACITY The valve regulated lead acid-battery (VRLA) is a maintenance-free lead acid battery operating on the principle known as “sealed, recombination,” wherein all the electrolyte is stored in absorptive glass mats (AGM) separators. The battery must remain sealed for its entire operating life and, to achieve maximum cycle life, must be properly recharged to prevent any excessive overcharge. Excessive overcharge results in excessive gas pressure build-up inside the battery, which is relieved by the opening of the pressure relief valve (typically set at 1.5 psi ± 0.5 psi). Everytime the valve opens, water vapor is lost, which in turn reduces battery life. The battery has been developed from extruded lead onto glass-fiber filaments that are woven into grids (mats) for use as electrode plates. This process provides the desirable crystal structure of lead oxide (PbO2) active material. The battery must be maintained, however, under optimal driving conditions. The USABC has outlined the performance requirements for VRLA batteries for the near term and the next few years, especially for use in electric vehicle applications. These requirements are summarized in Table 3–1, which shows that the near term VRLA battery provides up to 95 Whr/L of energy, while the requirements are to increase the energy density to 135 Whr/L over the next few years. This increase in the energy density means that there has to be a significant increase in the battery capacity.
BATTERY CAPACITY The useful available capacity of the battery (in Ahr) is dependent on the discharge current. This relationship can be expressed in the form In ¥ t = K where I is the discharge current in A, t (0.1 < t < 3) is the duration of the discharge in hours and n and K are constants for a particular battery type. 43
