The higher the discharge rate is, the greater the discharge current density is, the more uneven the distribution of current on the electrode is. The current is first distributed on the surface nearest to the main electrolyte. Thus, pbso4 is first generated on the outermost surface of the electrode. The volume of pbso4 is larger than that of pbo2 and pb. So the lead sulfate discharge product blocks the porous electrode orifice. The electrolyte, on the other hand, does not fully supply the need for the internal reaction of the electrode. The material inside the electrode can not be fully utilized. As a result, the capacity decreases at high rate discharge.
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