Off-Grid Electrical Systems in Developing Countries by Henry Louie

Author:Henry Louie
Language: eng
Format: epub, pdf
ISBN: 9783319918907
Publisher: Springer International Publishing

(8.27)

where T d,x is the discharge time.

If we were also measuring the terminal voltage, we would notice that the terminal voltage decreases over time, for example, as shown in Fig. 8.13. As can be expected, the terminal voltage decreases nonlinearly as the discharge progresses, even as the current is held constant. Of particular importance is that the decrease is first gradual and then rapid. The battery is unable to supply x = 8 A after about 390 min (T d,x = 6.5 h). The charge supplied by the battery shown in Fig. 8.13 is therefore approximately 8 × 6.5 = 52 Ah. The discharge ceases when the battery is no longer able to supply 8 A. However, we note that at the end of the discharge cycle, the voltage of the battery was far below its nominal value of 12 V. In practice, it is unlikely that the load connected to the battery would properly function across the range of voltages exhibited. Moreover, the battery is almost surely permanently damaged due to overdischarge. In other words, we are not able to meaningfully extract all 52 Ah of charge. But at what point should the test be terminated and the charge capacity calculated?

Fig. 8.13Discharge curve of a 12 V, 40 Ah, gel battery at a rate of x = 8 A

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