Shorted Cell:

Cell short circuits could be caused through various conditions that involve the following: faulty separators; other metals forming or lead particles a circuit among the negative and positive plates; buckling of the plates; or excessive sediments in the bottom of the jar.  The main cause of some of these occurrences is overcharging and over discharging of the battery that causes sediment to build up due to flaking of active buckling and material of cell plates.

Overcharging and over discharging should be prevented at all costs. Short circuits cause a wide reduction in battery capacity. Along with every shorted cell, battery capacity is decrease through a percentage equal to one over the total number of cells.