What Causes a Dead Cell in a Car Battery?
Basic car batteries are composed of six cells, each filled with an electrolyte solution of sulfuric acid and water. Sometimes these cells become deficient and lead to complete battery failure.
Plates of lead and lead dioxide are submerged in each cell's electrolyte solution. In a fully-functioning battery, each cell provides 2.1 volts of electricity, totaling 12.6 volts for the battery. These cells are called "wet cells."
The cells are connected in series and, when the ignition is turned, a chemical reaction occurs between the plates and the sulfuric acid, turning the lead into lead sulfate. This also creates an electrical current, known as discharging the battery. The current then transfers to the alternator and starts the engine. When the car is off, the battery recharges: the lead sulfate slowly releases the sulfur and reforms as lead.
Dead cells may occur if electrolyte solution leaks out, causing an inadequate amount of solution to safely discharge the battery. In this case, the cell will not produce its 2.1 volts and the other cells will have to work harder to produce the 12.6 volts necessary to start the car. Over time, one weak cell can cripple other cells and cause cell failure.
Cells may short-circuit if material from the lead plates sheds and accumulates at the bottom of the cell, causing the reaction to stall. This is the primary reason cells die, as thousands of discharges and recharges inevitably lead to build-up. Battery life-spans depend on how many times they can discharge before too much material shorts a cell.