A short circuit between the plates of one cell.
A plate closure can be detected by measuring the voltage of a battery. If the voltage is lower than the nominal voltage exactly by the value of one cell, this is most likely due to a plate short circuit. The battery is irreparably destroyed.
There are several possible causes for direct contact between negative and positive plates, i.e. a short circuit: excessive sulphation causes the plates to grow too much and damage the separators; mechanical stress causes the grids to bend; corrosion causes the grids to grow excessively; battery sludge forms an undesirable bridge between plates of different polarity.