A group of valve-regulated lead-acid (VRLA) batteries (12 V, 33 Ah) cycled under high power has exhibited premature failure. The only difference between failed and healthy batteries is the shedding of active material from the positive plates. The dislodged material has been examined by means of X-ray diffraction, scanning electron microscopy, and cyclic voltammetry. It is found that the material has a beta-PbO2 structure. The particles, which are oval in shape with a diameter of about 100 mum, are uniform and well separated from each other. The activity of the material can be restored under pressure. It is concluded that the failure mode of VRLA batteries under high-power cycling is softening of the positive active-material, which eventually results in deterioration of the electrical conductivity and de-activation of the material. (C) 2004 Elsevier B.V. All rights reserved.