Lead-acid secondary batteries voltage

BATTERIES AND FUEL CELLS (SECTION 20.7) A battery is a self-contained electrochemical power source that contains one or more voltaic cells. Batteries are based on a variety of different redox reactions. Batteries that cannot be recharged are called primary cells, while those that can be recharged are called secondary cells. The common alkaline dry cell battery is an example of a primary cell battery. Lead-acid, nickel-cadmium, nickel-metal hydride, and lithium-ion batteries are examples of secondary cells. Fuel cells are voltaic cells that utilize redox reactions in which reactants such as H2 have to be continuously supphed to the cell to generate voltage. 

Note Actually not the true equilibrium voltage but only the open circuit voltage can be measured with lead-acid batteries. Due to the unavoidable secondary reactions of hydrogen and oxygen evolution and grid corrosion, mixed potentials are established at both electrodes, which are a little different from the true equilibrium potentials (cf Fig. 1.18). But the differences are small and can be ignored. 

In battery systems based on aqueous electrolyte, water decomposition, which occurs above a cell voltage of 1.23 V, is such an unavoidable secondary reaction. But under certain conditions the resulting water loss can be avoided, and the system is used as a sealed one, as achieved with sealed nickel/cadmium, nickel/hydrogen, and nickel/metal hydride batteries. In lead-acid batteries corrosion is an additional unwanted secondary reaction with the consequence that lead-acid batteries cannot be made virtually sealed, but must have a valve, and a certain water loss cannot be prevented. 

Aluminium-air batteries are lightweight and produce a higher voltage than a lead-acid battery. They are expensive and are not true secondary cells because the aluminium anode has to be replaced from time to time. 

A car battery is a secondary cell that consists of plates of lead and lead(IV) oxide immersed in sulfuric acid (Figure 20.24). The voltage of each cell is 2 V. In order to operate the car s starter motor, a higher voltage is... 

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