The valve regulated lead acid (VRLA) battery is used extensively throughout the telecommunications industry as a backup energy storage source. Typically, the battery is the only backup energy source in such applications and, thus, the last line of defense against system failure in the event of AC mains failure.
Battery capacity, the energy storage capability, has long been a target of researchers as a definitive battery state of health (SOH) indicator. Conventional standards, such as “IEEE Recommended Practices for Maintenance, Testing and Replacement of Valve Regulated Lead Acid (VRLA) Batteries in Stationary Applications,” IEEE STD 1188-1996, base criteria for determining end of battery operational life on capacity. Typically, when the capacity reaches 80% of the manufacturer's rated capacity, the battery is deemed to be at the end of its operational life.
If the battery fails, the operational security of the telecommunications system is in jeopardy.
A widely accepted method for obtaining an accurate measurement of capacity is to fully discharge the battery. This is time consuming and expensive and leaves the telecommunication system vulnerable to AC mains failure until the battery is fully recharged after the measurement.
U.S. Pat. No. 7,880,438 B1 describes a technique for estimating an uninterruptible power supply battery's capacity. The battery is periodically subjected to a partial-discharge test by using the battery to power a fixed load for a fraction of the theoretical runtime of the battery. Measurements of the battery's voltage are made at various times during the testing. The battery is discharged into a load for substantially less time than to fully discharge the battery. During discharge, first and second voltage values provided by the battery at first and second times are determined. The first and second voltage values are then used to extrapolate a future voltage decrease of the battery to make a first determination of a predicted runtime of the battery .
If the capacity is below a desired level, or if the battery voltage drops below a threshold voltage during the testing, then further testing of the battery is preferably performed after recharging the battery. The same test can be repeated or another test can be performed where the battery is used to power the load for a larger fraction of the theoretical runtime of the battery. If the battery voltage drops below the threshold and/or the estimated capacity is lower than desired in the further testing, then the battery can be determined to be bad, a notification to this effect provided and the battery can be replaced.
As the battery during the partial-discharge test of U.S. Pat. No. 7,880,438 B1 is connected to a fixed load, the discharge current is constant during the time period of the partial-discharge test. When the battery is used as a power supply for the telecommunication equipment in the event of AC mains failure, however, the load and, thus, also the discharge current varies. The capacity of a battery is not constant but depends on the discharge current. In general, a battery has a higher capacity when discharged with a low discharge current than with a higher discharge current. The method suggested in U.S. Pat. No. 7,880,438 B1 is therefore not applicable to a system with variable load conditions.