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Power Sources
Electronics Unit – Lecture 8
Bench power supply
Photovoltaic cells, i.e., solar panel
Thermoelectric generator
Battery
Power Budget
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Bench Power Supply
Adjustable, regulated output voltage
Often includes a current limiting feature
Great for designing and prototype testing
Not for flight
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Bench Power Supply
Important specifications:
Voltage range - should be 0 V to ~12 to 15 V
Load current - should be a few hundred mA
Regulation - hold output with a few tenths of V
Current limiting - very desirable if adjustable
can protect circuits from accidental damage
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Photovoltaic panel
Potential flight power source
“Free” electricity while Sun shines
Size and weight concerns
Expense
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Photovoltaic panel
Panels made up of an array of individual cells
Each cell produces about 0.5 volt potential
Cell current depends on surface area and illumination
In full sun expect perhaps 10 milliwatts per cm2
For 6 V at 100 mA (600 mW) 12 cells would be needed,
each having surface area about 5 cm2
Must face the Sun - and payload is probably rotating
Multiple panels needed - unless auto pointing
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Photovoltaic panel
Cells are delicate and subject to breakage
Voltage output may vary - electronic regulation needed
Backup battery needed for cloudy intervals
Not the best choice for short duration student flights
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Thermoelectric Generator
Uses the Seebeck Effect to convert a temperature difference
directly into an electric current
Needs a heat source - deep space missions use the heat of
radioactively decaying plutonium
Probably not a good choice for a student project ☺
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Battery
Strictly, a battery is a combination of discrete cells
Best choice for short duration flights
Inexpensive
Reasonably lightweight
Variety of voltages and energy capacities available
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Battery Types
Primary batteries - one time use
carbon-zinc (old fashioned flashlight batteries)
alkaline (most common in consumer products)
silver-mercury (used in hearing aids and older cameras)
lithium - lots of energy for small weight
Rechargeable batteries - multiple charges/discharges
lead-acid (car batteries, or “gel cells”)
nickel-cadmium, Ni-Cd (older style rechargeable chemistry)
nickel-metal hydride, NiMH (popular now in consumer products)
lithium ion, (high end uses - laptop computers, digital cameras)
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Battery Characteristics
Terminal voltage - depends on specific chemistry
Capacity - rated in ampere-hours, or milliampere-hours
3600*(ampere-hours)*(average terminal voltage) = energy capacity in joules
Physical size and weight - energy density in joule/gram
Discharge characteristics - especially at low temperature
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Battery Characteristics
Terminal voltage - depends on specific chemistry
carbon-zinc about 1.5 V per cell
alkaline - about 1.5 V per cell
lead acid - about 2.0 V per cell
Ni-Cd and NiMH - about 1.2 V per cell
lithium - about 1.5 V per cell
but often made as double-cells for 3 V
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Battery Characteristics
Capacity - rated in A-hr or mA-hr for small cells
Usually specified at the “ten-hour discharge rate”
Example - an Energizer AA 1.5 V lithium cell rated 2900 mA-hr
should deliver 290 mA for 10 hr before voltage falls below 1V
But, will not last proportionally as long at higher currents
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Battery Characteristics
Discharge characteristics - the discharge curve
Use this typical lithium battery as an example
Nominal Voltage
3.0V
Rated Capacity
5Ah to 2.0V at 20°C (68 F)
Average Weight
55g (1.94oz)
Volume
26.5cm3 (1.60in.3)
Operating Temp. Range
-20°C to 60°C (-4 F to 140 F)
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Battery Characteristics
Notice the degradation of capacity
at low temperatures, especially at
higher load currents.
And lithiums are about the best!
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Calculating a Power Budget
Given:
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minimum permissible voltage
maximum load current
average off-peak load current
load current versus time data (duty cycle)
mission duration
minimum expected temperature
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Power Budget Example
The GPS radio telemetry package:
Transmitter requires a minimum supply voltage of 9 V
GPS receiver requires from 3 V to 6 V
Flight computer requires 5 V
If a single battery pack powers all three units, its terminal
voltage cannot fall below 9 V.
Voltage regulators will be used to reduce the voltage for the
other units.
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Power Budget Example
The GPS radio telemetry package:
GPS receiver draws 140 mA, continuously (100% duty)
Flight computer draws 50 mA, continuously (100% duty)
Transmitter draws 80 mA when in standby mode (93% duty)
Transmitter draws 1050 mA when transmitting (7% duty)
(since transmitter sends a 2 second data burst every 30 seconds)
Peak current = 1320 mA, minimum current = 270 mA
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Power Budget Example
The GPS radio telemetry package:
140 mA x 1 = 140 mA
50 mA x 1 = 50 mA
80 mA x 0.93 = 75 mA
1050 mA x 0.03 = 32 mA
Add them up….. about 300 mA
If the beacon must operate for at least 8 hours…
300 mA x 8 hours = 2400 mA-hr required from the battery pack
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Power Budget Example
The GPS radio telemetry package:
A sufficient number of cells must be wired in series so that,
even when delivering 1320 mA, the composite terminal voltage
remains above 9 V, even at the minimum expected temperature.
Consulting the battery curves previously viewed, cell voltage
can drop to about 2.2 V at -20 celsius when delivering 600 mA
or more. Therefore, four cells will be needed. Allowing a
reserve, cells should have a capacity of about 3000 mA-hr.
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Power Budget Example
Mission Duration Considerations
A science package must operate through pre-launch preparation
and for the full duration of flight. If data is in non-volatile
storage, it need not operate after touchdown. (~ 4 hours)
A tracking beacon must operate through pre-launch, the entire
flight interval, and be able to continue well after touchdown to
assure recovery. (8 hours or more)
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Activity E8
Perform a cold environment battery test
Compare alkaline, NiMH, and lithium cells
Use HOBO to collect data
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