In power-assist hybrid electric vehicles (HEVs) batteries are required to operate from a partial-state-of-charge baseline and to provide, and to accept, charge, for short periods, at very high rates. Under this regime conventional lead–acid batteries accumulate lead sulfate on the negative plate and fail quickly. This failure mode can be effectively countered by the inclusion of certain forms of carbon at greater concentrations than have been used in lead–acid batteries in the past. So effective is this preventive measure that VRLA batteries benefiting from the inclusion of such carbon have been able to substitute for nickel metal hydride batteries in power-assist HEVs with no significant loss of performance. There has been much speculation about the function of the carbon that is providing this remarkable improvement. This paper aims to review the several mechanisms that have been proposed as possibly playing some contributory role.