Negative strap corrosion in valve-regulated lead acid batteries and its analysis by accelerated life testing

The corrosion and ultimate fracture of the negative strap of VRLA cells has resulted in open circuit battery strings. These open strings have caused loss of reserve power and service breaks in user equipment and systems. The expense and trauma of these interruptions have made it imperative to determine the cause for this corrosion and prevent it from occurring. This failure mechanism is not related to a previously well identified corrosion problem caused by differences in the antimony content between the strap alloy and plate lugs. It is the intent of this study to elucidate mechanisms that define the path to strap corrosion and failure, and to determine if the cures applied based on proposed failure mechanisms have eliminated the fracturing of negative straps. The effectiveness of the cures were evaluated in accelerated life tests and field service. One important result of this experimentation was the determination of an activation energy (AE) for strap corrosion. Comparison of this AE to that for other failure mechanisms revealed that hot testing may not reveal all failure mechanisms that occur at room temperature, and that remedies based on hot test studies may not achieve the desired results. This study included the following areas: strap quality and metallurgy; strap surface voltage and cathodic protection; and sodium sulfate additive.