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Detailed Design
Secondary Power Source - Batteries
Battery Cells
The Sanyo KR-4400D NiCd (Nickel Cadmium) cells will be used for the secondary power source. NiCd cells are rugged and have a long history of space use. Basic characteristics for conventional NiCd cells are a depth of discharge of 20% and a lifetime of greater than 20,000 cycles (from Space Mission Analysis and Design, by Wiley J. Larson and James R. Wertz). MidSTAR-1 will make approximately 11,700 orbits over its two year lifetime. This is well under the 20,000 cycles generally used.
Sanyo KR-4400D Characteristic Summary:
- Each cell has a nominal capacity of 4400 mA-h and a nominal voltage of 1.2 V.
- Charge current is not to exceed 440 mA
- Ambient temperature range for operation:
- Charge: 0 to 45 degrees C
- Discharge: -20 to 60 degrees C
- Storage: -30 to 50 degrees C
- Internal impedance: 3.8 mohm at 1000 Hz
- Cell Specifications (from Sanyo Energy Corporation)
- Cell Material Safety Data Sheet
Battery Cell Layout
There will be two battery boxes of 56 NiCd cells. Our design decisions were guided by the following criteria:
- Current requirement of 0.88 amps: Two sets of 28 NiCd cells will be connected in parallel.
- Energy requirement: The energy capacity for one battery cell is 19 kJ, and 1,064 kJ for the battery box (with 56 cells). Taking an average power requirement of 27 W and the time of one orbit, 153 kJ are needed. At maximum power usage, 31 W and 176 kJ will be drawn. With 31 W required, the depth of discharge for the battery box is 16.5% over one orbit and 6.3% during the longest eclipse period. For details, see Battery Calculations.
- Voltage Requirement: To meet a 28 V total battery voltage under worst case conditions (the cell voltage will drop from 1.4 to 1 V per cell), the series string layout will have positive to negative leads connected in the 4 rows of 7 batteries in series, bound together in an insulating tape material.
Battery Box Layout
The battery box will be 12"x12"x4" with teflon lining inside to provide insulation for the batteries as well as protect from vibrations.
Thermistors
Three thermistors will be placed inside the battery box to monitor the temperature within. They will be used in a resistive mode and the characteristic mathematical models relating resistance and temperature for the thermistors will yield temperature values. Currently, we are looking at 2 vendors with space-qualified thermistors: