A discussion about water loss, compression and the VRLA cell

Various theories have been proposed to explain premature loss of electrical capacity experienced by some valve-regulated lead acid (VRLA) cells and batteries. Most recently, several of the theories have focused on cell dryout, loss of glass mat separator compression and negative plate charge imbalances resulting in undercharged cells. This paper presents a model for water loss using inputs derived from both laboratory and field data collected over the past 15 years. This model is applied to a typical Absolyte IIP cell to predict water loss over a 20 year period. The water-loss prediction is compared to both actual measured values on field-aged product and to a laboratory test that examined the effect of water loss on cell performance and impedance. An alternative failure mode, loss of separator compression, is also presented and discussed. To conclude, this paper proposes that the loss of separator compression, not water loss per se, is a primary contributing factor in premature capacity degradation of VRLA cells. The loss of compression relates to the materials used in, and the design of, the glass mat contained in the cells.