10 - E10 Materials Science and Engineering Module Batteries...

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E10 Materials Science and Engineering Module: Batteries and Fuel Cells Fall Semester 2010 Thomas M. Devine Department of Materials Science and Engineering University of California, Berkeley INTRODUCTION The performance of a battery is largely described by plots of charging and discharging curves. The discharge curves were introduced in the second lecture. “Discharge” means that the battery is providing energy. “Charge” means that energy is being stored in the battery. Both charging and discharging curves are graphs of voltage (vertical axis) vs charge (horizontal axis; units of amp hr or coulombs). As shown in Figure 1 , a discharge curve is a plot of the instantaneous cell voltage (e.g., V cell = V 1 ) at a given value of the amount of charge (e.g., Q 1 ) that has been discharged by the operation of the battery at a constant value of cell current(I 1 ). There is a different discharge curve for each value of cell current. Note that the total amount of charge that a battery can provide decreases as the discharge current increases. In other words, the faster you discharge a battery, the less is the total amount of charge the battery provides. A charging curve graphs the instantaneous cell voltage at a given value of the amount of charge that has been stored in the battery as a consequence of a constant value of charging current. As shown in Figure 2 , there is one charging curve for each value of charging current. (Note: the curves should be smooth - I ! m not a good artist.) The higher the current used to charge the battery the larger is the total amount of charge supplied to the battery. Note that the maximum value of cell voltage, V cell max , is identiFed in both Fgures. The numerical value of V cell max is the same in both Fgures. ±urther on in these notes we ! ll discuss how V cell max is calculated. One of the objectives of the Materials Science and Engineering module is to explain the shapes of the charge/discharge curves and the effects of cell current on the charge/ discharge curves. 1
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The selection of the most appropriate battery for a particular application is made by the use of the charge/discharge curves and our discussion begins with analysis of the charge/discharge curves. DISCHARGE CURVES Discussing the shape of a discharge curve is best done by comparing a “real” discharge curve, such as that presented in Figure 1 , to the “ideal” discharge curve. The comparison between real and ideal is presented in Figure 4 . First, note that the “ideal” battery ! s cell voltage is the maximum possible value, V cell max , independent of the battery ! s state of charge (i.e., independent of how much charge the battery has already provided). Second, in the “ideal” discharge curve, the total energy that the battery can provide (i.e., the total amount of work that the battery can do) is independent of the rate at which the battery provides the energy. That is, for the “ideal” battery there is only one discharge curve; it is that same curve independent of the value of the discharge current. The total amount of work that the battery can do is equal to the area underneath the
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10 - E10 Materials Science and Engineering Module Batteries...

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