Battery Capacity Recovery

The ANSI/IEEE 450 standard recommends that a minimum of three sets of readings be taken. One reading may be done at the beginning of the test, one reading upon completion of the test, and then one reading at an interval sometime during the test. This interval could be during the midpoint of the test. These battery pack discharge capacity tests will quickly identify the weak cells and also battery cells that are approaching reversal (displaying a 1V or lesser voltage). The batteries exhibiting weak cells can then be removed from the battery pack and replaced by new batteries. A balancing charge should immediately follow the replacement of the bad batteries to balance the battery pack.

BATTERY CAPACITY RECOVERY

The cycle life of VRLA battery is directly dependent on the depth of discharge (DOD). In addition, the rate of charge of the VRLA also influences the battery life.

Battery cycle life is defined as the number of cycles completed before the discharge capacity falls below 15Ahr (15Ahr is defined as the battery end-of-life). Upon completion of the discharge, a reconditioning charge is applied using the following steps under room temperature conditions.

• Discharge the battery pack at the specified rate to specified depth of discharge • Charge each battery at approximately 2.5V per cell with specified current limit for a specified time • Rest at open-circuit for the specified time • Repeat the steps until the discharge capacity declines below 15Ahr at the cutoff voltage of approximately 1.5V per cell

If a balanced battery pack is maintained at low DOD, the battery cycle life improves to approximately 4,000 cycles. This condition can seldom be maintained for an EV owing to city driving patterns.

Formation of the passivation layer causes the active material to become electrically insulated and/or isolated from the grid. This limits the capacity of the battery available for discharge. The nature of the passivation layer depends on the type of battery grid alloy and by the electrolyte additives. Research results indicate that presence of phosphoric acid in the electrolyte and tin in the grid alloy reduces the passivation (passive reaction) effects.

Under the presence of higher currents, the passivation layer is highly porous. The degree of porosity is determined by the distance between