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NIHs

50%

To compute a battery's capacity, we divide the discharge energy (watt-hours) by the depth-of-discharge. The ratio of battery weight to battery capacity is 30 to 40 W-hr/kg for NiCd batteries and 35 to 50 W-hr/kg for NiH2.Often, several batteries operate in parallel to provide the needed capacity. By using several small batteries, we can add some redundant batteries for backup with less weight penalty than for a second large battery.

Spacecraft primary power—power produced by the solar array and batteries—is not well regulated (28 ± 5 V is typical). Furthermore, we must match the solar array's electrical output to the battery's charging requirements and provide switching equipment that allows the battery to supply power when needed. Section 11.4 describes various ways to meet these needs. Significant features include limiting the battery's charge rate, limiting overcharge, providing for low-impedance discharge, and providing for reconditioning. The controller or regulator must cope with the voltage swings between charge and discharge. The power control unit must isolate faults and switch to redundant units while also serving as the center of the power distribution network. An estimate of the power control unit's weight is 0.02 kg/W of controlled power.

Most electronic equipment, for both the payload and the spacecraft, requires voltage regulation tighter than that provided by the arrays and batteries. We must either regulate the primary power or convert it to secondary power, which we can regulate more tightly. In either case, power dissipates in die regulator or the power converters. This dissipation typically amounts to 20% of the power converted, which may be all of the spacecraft's operating power. In sizing the power subsystem, we must therefore include the weight of the power conversion equipment—typically 0.025 kg/W converted.

The power subsystem includes wiring for distribution and may have components for switching and fault isolation. The power dissipated in wiring losses and switching equipment is 2% to 5% of the operating power, and the wiring harness takes up 1 % to 4% of the spacecraft dry weight. Spacecraft which must operate in high radiation environments may require shielded wire to distribute power. Table 10-27 summarizes the weight and power requirements of the power subsystem.

TABLE 10-27. Weight and Power Budget for Power Subsystem. P= required power in watts. Note that M^ is used here as in Table 10-10.

Component

Weight (kg)

Power (W)

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