Battery is composed of battery cells and each battery cell is composed of different chemical components such as metals and electrolyte in order to produce required chemical reactions.  When a certain voltage is reached, the electrolyte may start to decompose and unable to perform chemical reactions.  And the battery cell gets weakened and then destroyed.  For Li-Ion cell, 1% overcharge triggers electrolyte decomposing. Safety protection installed in ZAPAK Li-Ion battery prevents such hazard.