Capacity Discharge Testing of VRLA Batteries

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 80Ahr, the three-hour rate discharge would have the equation

(13.054 - Vt)(3.7 - t) = 2

for the VRLA battery VOC = 13.054V 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,

loss of mechanical strength occurs, or electrolyte dry out begins, and the battery loses its capacity fairly rapidly. The deterioration of the battery is not as rapid at the end of life. Rather the deterioration begins at close to 80% of the capacity rating and falls-off rapidly from that point.

If a new battery pack is stored at delivery without a maintenance charge for an extended period of time, it may lead to the development of sulfation of the battery electrodes. This phenomenon will contribute to the additional loss of useful battery capacity during the service life of the battery pack. It is strongly recommended to place the battery pack on a maintenance charge as soon as possible. Also, the manufacturer of the battery must be consulted in case the battery pack exhibits less than 90% of its useful capacity.

The only accurate test of the useful battery pack capacity is the capacity discharge test. The test measures the amount of power removed from a fully charged battery over a rated time period. A capacity discharge test is performed on the battery pack while maintaining a constant current (or constant power) discharge on a battery bank using a regulated resistive load. The cell voltage and the battery pack voltage are monitored during the period of the discharge. Both the voltage values decrease over the discharge period. The time taken to reach the cell lower cut-off voltage (determined by the manufacturer) is noted and used to determine the overall battery pack’s capacity.

Before the capacity test is performed on the battery pack, proper load and monitoring equipment is installed along with resistive load units. The resistive load units must be capable of manual or automatic control. Resistive load units are typically a series/parallel configuration of resistor banks with forced cooling fans. The resistor configurations allow adjustments of the load currents in small increments through relays and switches. In addition, a main circuit breaker switch provides protection, while ampere- and volt-meters monitor the cell/battery pack current and voltages, respectively.

The load units are specified and selected based on the Ahr rating and voltage levels of the battery pack. Data published by the battery manufacturer in the form of curves or tables for specific model types and Ahr ratings are a useful reference to determine the load unit specifications. The individual cell performance data sheets provide both discharge times that can range from one minute to eight hours with various constant current (or constant power) loads leading to a specified final battery voltage. Selection of a load unit with a large ampacity (ampere capacity) allows for shorter duration discharge tests simulating city driving conditions with greater user flexibility to profile the test.