In a lead-acid battery, each of the battery cells produces an open circuit voltage of approximately ______

• A. 1.8 V
• B. 2.0 V
• C. 2.7 V
• D. 2.1 V

Answer: (D)

Lead-acid cells have an open-circuit emf determined by their chemical reactions, and for this chemistry each cell sits at about 2.1 volts when no current is drawn. The positive plate is lead dioxide, the negative plate is spongy lead, and the electrolyte is sulfuric acid; the overall cell potential under no-load conditions is around 2.1 V. In practice, you’ll often see around 2.12 V quoted, but 2.1 V is the common rounded value used for open-circuit voltage per cell. The reason this is the best answer is that it matches the actual emf of a charged lead-acid cell. The other numbers don’t fit: 1.8 V is too low for this chemistry, 2.7 V is higher than the typical open-circuit emf, and while some sources use 2.0 V as a nominal figure, the precise open-circuit per-cell voltage is about 2.1 V. Remember that a fully charged 12 V battery (six cells in series) will measure around 12.6 V when open-circuit.