CAPACITY DISCHARGE TESTING OF VRLA BATTERIES
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where VOC is the open circuit voltage of the battery, VT is the on-load voltage of the battery at time t (hours), A and B are the constants to be determined. This is a hyperbolic equation and the curve is identical to the voltage of a battery during a constant current discharge. The values of A and B may be determined iteratively to provide a close approximation to the actual voltages during the rates of discharge between the six-minute rate and the three-hour rate as possible. The resultant equation is where Vt is the voltage after time t hours into the constant current discharge and T is the rate of discharge in hours. As an example for an 80 Ahr, the three-hour rate discharge would have the equation (13.054 - Vt)(3.7 - t) = 2 for the VRLA battery VOC = 13.054 V and A = current discharge rate + 0.7, B = 2. This equation, along with the Peukert equation, provides a voltage through a discharge at varying currents taking into account the SOC. For the case of regeneration, it can be assumed that the battery is 95% efficient in accepting the regenerated charge current. Each VRLA quickly develops its own personality during its formation cycling to the extent that each battery behaves differently during recharge. Thus it is necessary to provide an equalizing charge during the recharging process of a series string of batteries.
CAPACITY DISCHARGE TESTING OF VRLA BATTERIES As recommended by most manufacturers and also industry standards (IEEE 450), a VRLA battery should be replaced if it fails to deliver 80% of its rated capacity. There is a very simple reason for the 80% of the rated capacity value. Based on the typical battery life curve of a lead-acid battery, once the battery capacity begins to deteriorate, the fall-off occurs at a rapid rate. A fully balanced, new traction battery pack will exhibit up to 95% of its rated capacity upon delivery because the active material on the battery plates are still undergoing formation. Once the active materials on the plates reach full formation, the battery capacity rises to its 100% capacity rating. This occurs and is maintained if the battery is under a proper state of charge, typically for a period of six months to several years. Capacity will continue to rise and will exhibit a rating of 100% for almost the entire battery life. As the plates begin to deteriorate and lose active material due to corrosion,
