Lead-acid batteries open-circuit voltage

The open-circuit voltage for a battery system is a function of temperature and electrolyte concentration as expressed in the Nemst equation for the lead-acid cell (see also Chap. 2). 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

Typical dimensions for the /5-alumina electrolyte tube are 380 mm long, with an outer diameter of 28 mm, and a wall thickness of 1.5 mm. A typical battery for automotive power might contain 980 of such cells (20 modules each of 49 cells) and have an open-circuit voltage of lOOV. Capacity exceeds. 50 kWh. The cells operate at an optimum temperature of 300-350°C (to ensure that the sodium polysulfides remain molten and that the /5-alumina solid electrolyte has an adequate Na" " ion conductivity). This means that the cells must be thermally insulated to reduce wasteful loss of heat atjd to maintain the electrodes molten even when not in operation. Such a system is about one-fifth of the weight of an equivalent lead-acid traction battery and has a similar life ( 1000 cycles). 

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)... 

An AFC combined with a lead acid battery has been used in a hybrid vehicle. Between operating times the cell was shut down by using the battery to drain the KOH electrolyte. This technique can improve the life expectancy enormously by protecting the fuel cell from a high open circuit voltage, which otherwise could induce carbon oxidation processes to produce carbonates that would destroy the matrix electrolyte. 

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 some battery systems, like lead-acid, the OCV can be used for a rough determination of the state of charge. Other systems again, e.g. some primary lithium systems, offer a rather high initial open circuit voltage obviously due to higher valence oxides, which collapses by several hundred millivolts on the slightest load and never recovers to its initial value. 

Figure 1.17 Typical low rate discharge and high rate discharge curves of a lead acid battery and the main discharge parameters. The broken curve denotes the equilibrium or open circuit voltage.

The charging voltage of lead-acid batteries depends on their open circuit voltage, which can be calculated according to the gravity of the electrolyte by the formula... 

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... 

As discussed earlier, sulfuric acid in electrolyte solution is an active material that participates in the cell reaction. Therefore, the sulfuric acid concentration changes during both battery discharge and charge processes. In addition, as predicted by Equation 5.12, the open circuit voltage of a lead-acid cell is a function of electrolyte concentration according to the Nemst equation. Furthermore, the specific resistance of the electrolyte and its freezing point can also depend on the acid concentration. 

Based on battery capacity, three types of lead-acid battery units are commercially available for different applications, as shown in Figure 10.2 [4-6]. The first type of unit is a large-sized battery with a capacity range of 100 to 3600 Ah (the open circuit voltage [OCV] is normally 2.0 V), the second one is a medium-sized battery with a capacity range of 40 to 200 Ah (OCV is 12.0 V), and the third one is a small-sized battery with a capacity range of 2 to 40 Ah (OCV is 6.0 V). Depending on the power requirements of the application, different types of lead-acid batteries and/or their combinations can be used. 

FIGURE 24.25 Open-circuit voltage vs. residual capacity of sealed prismatic lead-acid batteries at 25°C. 

They found that the battery had a high open circuit voltage of 3.7 volts and gave an initial current of 25 mA cm 2, xhe overall energy density was 176 watt hours per kilogram, compared with a value for the lead acid battery of about 30. 

Even before the Leclanch discovery, a French physicist, Gaston Plants invented the lead-acid rechargeable battery, which came into commercial production in the 1880s. The reaction during the charge and discharge processes is shown in Equation 1.1, whose open-circuit voltage is about 2.08 V. 

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