Electric vehicle
Positive electrode changes from lead to lead sulfateElectrons flow round the external circuit LOAD
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| 37 Positive electrode changes from lead to lead sulfate Electrons flow round the external circuit LOAD e.g. electric motor Negative electrode changes from lead peroxide to lead sulfate Reactions during the discharge of the lead acid battery. Note that the electrolyte loses sulfuric acid and gains water. Positive electrode changes back from lead sulfate to lead. + − Negative electrode changes back from lead sulfate to lead peroxide − + Reaction during the charging of the lead acid battery. Note that the electrolyte sulfuric acid concentration increases. External DC supply Pb + SO 2 → PbSO 4 + 2e PbO 2 + 4H + + SO 2 − + 2e − − − → PbSO 4 + 2H 2 O 2H 2 SO 4 → 4H + + 2SO 2 − 4 4 4 − PbSO 4 + 2e → Pb + SO 4 2 PbSO 4 + 2O 2 → PbO 2 + SO 2 + 2e − 2H 2 O → 4H + + 2O 2 − − ; 4H + + 2SO 4 2 − → 2H 2 SO 4 4 Figure 3.4 The reactions during the charge and discharge of the lead acid battery Table 3.1 Nominal battery parameters for lead acid batteries Specific energy Wh kg −1 depending on usage Energy density Wh l −1 Specific power W kg −1 before efficiency falls very greatly Nominal cell voltage V Amphour efficiency ∼80%, varies with rate of discharge and temperature Internal resistance Extremely low, ∼0.022 per cell for 1 Ah cell Commercially available Readily available from several manufacturers Operating temperature Ambient, poor performance in extreme cold Self discharge per day (but see text) Number of life cycles Up to 800 to 80% capacity Recharge time h (but 90% recharge in 1 h possible) 38 Electric Vehicle Technology Explained, Second Edition 3.3.2 Special Characteristics of Lead Acid Batteries Unfortunately the lead acid battery reactions shown in Equation (3.4) are not the only ones that occur. The lead and lead dioxide are not stable in sulfuric acid and decompose, albeit very slowly, with the reactions: At the positive electrode 2 PbO 2 + 2 H 2 SO 4 → 2 PbSO 4 + 2 HO+ O 2 (3.4) At the negative electrode Pb+ H 2 SO 4 → PbSO 4 + H 2 (3.5) This results in the self-discharge of the battery. The rate at which these reactions occur depends on the temperature of the cell – faster if hotter. It also depends on other factors, such as the purity of the components (hence quality) and the precise alloys used to makeup the electrode supports. These unwanted reactions, which also produce hydrogen and oxygen gas, also occur while the battery is discharging. In fact they occur faster if the battery is discharged faster, due to lower voltage, higher temperature and higher electrode activity. This results in the ‘lost charge effect that occurs when a battery is discharged more quickly, and which was noted in Section 3.2.2 above. It is a further unfortunate fact that these discharge reactions will not occur at exactly the same rate in all the cells, and thus some cells will become more discharged than others. This has very important consequences for the way batteries are charged, as explained below. But, in brief, it means that some cells will have to tolerate being overcharged to make sure all the cells become charged. The reactions that occur in the lead acid battery when it is being overcharged are shown in Figure 3.5. These gassing reactions occur when there is no more lead sulfate on the electrodes to give up or accept the electrons. They thus occur when the battery is fully or nearly fully charged. We have noted that the charging and discharging reactions (as in Figure 3.4) involve changing the concentration of the electrolyte of the cells. The change in concentration of the reactants means that there is a small change in the voltage produced by the cell as it Hydrogen formed at positive electrode − Download 3.49 Mb. Share with your friends:
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