The car battery contains six separate cells, and every cell has 2.1 V power. If any of these cells are dead, In such a case, the battery can't work correctly. Once any car battery cell is over, the battery needs replacement.
After every six months, check the specific gravity of electrolyte fluid to test the car battery for dead cells. If the specific gravity is low in a cell compared to another cell, it means the cell with low specific gravity is dead.