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102 103 104 105 Cycle Life (Cycles)

Fig. 11 -11. Depth-of-Dlscharge vs. Cycle Life for Secondary Batteries. Increased cycle life reduces the amount of energy available from the batteries during each cycle—DOD decreases with cycle life.

a planetary mission by the year 2000, with space qualification for GEO and LEO applications by the years 2005-2010.

To size a secondary battery, we must identify the parameters and apply the equation in Table 11-40. The parameter values used in die equation can vary significantly with battery type. The ideal battery capacity is the average eclipse load, Pe, times the eclipse duration, Te. This ideal capacity must be increased to include the battery-to-load transmission efficiency, n, and the depth-of-discharge constraints. For LEO, we expect the battery's DOD to be 40-60% for NiH2 technology, compared to 10-20% for NiCd technology. We base these expectations on the average DOD over 24 hours and assume the batteries are fully recharged at least once during this period. The number of batteries, N, may be equal to one for this calculation if you simply require a battery capacity. Two to five batteries are typical. We must have at least two (unless the battery uses redundant cells) because the spacecraft needs redundant operation with one unit failed. But more than five batteries require complex components for recharging. The secondary batteries may be required to help meet peak power loads during full Sun conditions. For some missions, the peak power loads may drive die required battery capacity rather than the eclipse load. To design the Energy-Storage subsystem, follow the steps in Table 11-40

TABLE 11-40. Steps In the Energy Storage Subsystem Design. To obtain the required battery capacity in Amp-hr, divide by the required satellite bus voltage.

Step

Consider

FlreSat Example

1. Determine the energy storage requirements

• Mission length

• Primary or secondary power storage

• Orbital parameters

- Eclipse frequency

- Eclipse length

• Power use profile

- Voltage and current

- Depth of discharge

- Duty cycles

• Battery charge/discharge cycle limits

• Secondary power storage

• 35.3 min per eclipse (Te) •Eclipse load 110 W(Pe)

• TBD—depends on observations taken and downlinked during eclipses

2. Select the type of secondary batteries

• NaS (under development)

• NiCd or NiH2—both are space-qualified and have adequate characteristics

3. Determine the size of the batteries (battery capacity)

• Number of batteries

• Transmission efficiency between the battery and the load

• Cr= 4.5 Amp-hr (26.4 V bus)

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