Design a NiCd battery for a 10-year calendar life, which is required to (1) power 10-kW load at 240 V_dc for 1 h three times every day (once every 8 h) and (2) charge it back in 6 h, allowing 2-h margin for the battery to cool down after being fully charged and before the next load cycle starts. Assume average discharge voltage 1.2 V/cell, internal resistance 0.02 Ω/ cell, and operating temperature 20°C. You must use three batteries in parallel to share full load and one dormant spare battery for reliability. With one failed battery, the other three must power full load. For each of the four batteries, determine the number of cells in series, cell capacity in ampere-hours, and the required charge current if the c/d ratio is 1.1 and the charge voltage is 1.45 V. Use a diode with a 1-V drop in each battery to prevent a stronger battery charging a weaker battery in case of an internal cell short or for other reasons. See Figure 11.8.

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100 10 Discharge/charge cycles (thousands) 10% DOD 30% DOD 50% DOD 10 20 Cell temperature (°C) 30