Voltage of Lead-Acid Cell

The voltage of a lead-acid battery cell during charge must exceed the open-circuit voltage. During discharge, the case is reversed. The extent of these deviations with respect to the open circuit voltage is determined by 

When electric current is flowing through the cell, the closed-circuit voltage depends on both the direction and magnitude of the current and the temperature. 

FIGURE 1.9 Lead-acid battery discharge voltage curve. 

With a new battery, the voltage normally drops from 2.12 V at open circuit to 1.97 V on load per cell. After passing through a shallow minimum, the voltage increases again and reaches 2.00 V. This value is maintained for a given period of time and then decreases slowly. After reaching 1.7 V, a rapid decay is observed. 

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FIGURE 23.4 Open-circuit voltage of lead-acid cell as a function of electrolyte specific gravity. 

Figure 18.1 VariaUonof voltage of lead-acid cell during recharge...

Dependence of Lead-Acid Cell Voltage on Temperature and H2SO4 Concentration... 

FIGURE 23.2 Typical voltage and specific gravity characteristics of lead-acid cell at constant-rate discharge and charge. 

Table 1 gives the calculated open circuit voltages of the lead—acid cell at 25°C at the sulfuric acid molalities shown. The corrected activities of sulfuric acid from vapor pressure data (20) are also given. 

The first difference between these two batteries is the voltage they produce a watch battery produces about 3 V and a lead-acid cell about 2 V. The obvious cause of the difference in emf are the different half-cells. The electrode potential E is the energy, expressed as a voltage, when a redox couple is at equilibrium. 

Fig. 5.2 Approximate open circuit voltage and electrolyte density as a function of percentage service capacity for the lead-acid cell...

Suppose a battery-powered device requires a minimum voltage of 9.0 V to run. How many lead-acid cells would be needed to run the device (Remember that a standard automobile battery contains six lead-acid cells connected in one package.) The overall reaction of a lead-acid cell is... 

Calculate the free energy change (heat change) of the cell reaction (AH) in calories for two battery systems (a) A lead-acid cell with an open-circuit voltage of 2.01 V at 15 °C and a temperature coefficient of resistance (dE/dT) of 0.0037 V/K. (b) A Zn-Hg cell (Clark cell) with an open-circuit potential of 1.4324 V at 15 °C and a temperature coefficient of 0.00019 V/K. (Bhardwaj)... 

Calculate the voltage A%° of a lead-acid cell if all reactants and products are in their standard states. What will be the voltage if six such cells are connected in series ... 

Fig. 5.9 Lead-acid cell voltage charge for a constant charge as a function of time...

V design and comprises six lead-acid cells in a monobloc container. This battery is compatible with the vehicle electrical system that operates in the voltage range from about 12 to 14 V. Until around 1970, 6-V systems were also in use. In European trucks and buses on the other hand, it is common practice for two, 12-V batteries to be connected in series to achieve a nominal voltage of 24 V. 

The voltage efficiency of the lead-acid cell is about 80 %. This near reversibility is a consequence of the rapidity of the chemical reactions in the cell. As we have seen, the ability to supply large currents at potentials near the open-circuit potential means that the chemical reactions at the electrodes are fast as the charge is drained away by the current, the potential should drop, but the chemical reaction occurs rapidly enough to rebuild the potential. 

Electrolytic gas evolution is a significant and complicated phenomenon in most electrochemical processes and devices. In the Hall process for aluminum production, for example, bubbles evolved on the downward-facing carbon anodes stir the bath and resist the current, both of which directly affect the heat balance and the cell voltage. Bubbles appear as a result of primary electrode reactions in chlorine and water electrolysis, and as the result of side reactions in the charging of lead-acid batteries and some metal electrowinning. Stirring of the electrolyte by gas evolution is an important phenomenon in chlorate production. Electrolytically evolved bubbles have also been used in mineral flotation. Relatively few major electrochemical processes do not evolve gas. 

Experimentally measured and calculated values for lead—acid cell voltage as a function of H2SO4 relative density, as reported by different authors. 

Changes in cell voltage during the initial stage of formation of a lead—acid cell with formation... 

The internal resistance of a Na/S cell is higher than that of a comparable lead/acid cell. This higher resistance leads to reduced voltage at high current drains and limits the voltage efficiency of the Na/S battery to, typically, 65-85% depending upon the power output. This is not entirely wasteful as some heat output is required, in any event, to maintain the battery at operating temperature-... 

Figure 7.15 Discharging current per 100 Ah of nominal capacity versus discharge duration with an end of discharge voltage of 1.75V/cell and 1.05V/cell for lead acid and nickel/ cadmium batteries, respectively.

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