Within the next decade, there will be major changes in automotive technology with the introduction of several new features which will increase significantly the on-board power requirements. This high power demand is beyond the capability of present 14 V alternators and thus a 42 V power network is to be adopted. The new 'PowerNet' requires the lead-acid battery to be capable of providing a large number of shallow discharge-charge cycles at a high rate. High-rate discharge is necessary for engine cranking and power assist, while high-rate charge is associated with regenerative braking. The battery will operate at these high rates in a partial-state-of-charge condition, so-called HRPSoC duty. Under simulated HRPSoC duty, it is found that the valve-regulated lead-acid (VRLA) battery fails prematurely due to the progressive accumulation of lead sulfate mainly on the surfaces of the negative plates. This is because the lead sulfate cannot be converted efficiently back to sponge lead during charging either from the engine or from regenerative braking. Eventually, the layer of lead sulfate develops to such extent that the effective surface area of the plate is reduced markedly and the plate can no longer deliver the high cranking-current demanded by the automobile. A mechanistic analysis of battery operation during HRPSoC duty shows that high-rate discharge is the key factor responsible for the build-up of the lead sulfate layer. Such discharge causes a compact layer of tiny lead sulfate crystals to form on the surface of the negative plate and subsequent charging gives rise to an early evolution of hydrogen. Hydrogen evolution is further exacerbated when a high charging current is used. (C) 2003 Published by Elsevier B.V.