Failure modes of valve-regulated lead/acid batteries

The decline in the cycle-life performance of lead/acid batteries is often caused by deterioration of the positive plates. When batteries are used in electric vehicles, however, the decline in performance is due not only to positive-plate degradation, but also to serious problems with the negative plates. This is because electric-vehicle batteries are used in the form of a pack and their temperature rises excessively (especially during the hot weather) which, in turn, causes decomposition of the lignin additive in the negative plates. Deterioration of the negative plates limits vehicle-running performance even in hybrid electric vehicles (HEV). When batteries are used in this way, charging is achieved during travel via regenerative current and the generator. Thus, the batteries are charged by large currents, and charging is irregular. This causes the accumulation of lead sulfate in the negative plates. Such behaviour can be suppressed by increasing the amount of carbon, one of the negative-plate additives. As a consequence, cycle-life performance is improved significantly. On the other hand, in small valve-regulated lead/acid batteries for trickle use, contact between the plates and separators is impaired and this results in a decline in the high-rate discharge performance. This occurs because transmission of water vapour from the battery container lowers the amount of electrolyte in the separators, which then contract.