Power

Overview

Per the design, ELFIN has a 60% power margin against the battery specifications, after itemized contingencies. Careful analysis is also given toward the Energy consumption during orbits in eclipse, with specific requirements. For the longevity of the batteries, discharge and charge cycles must be kept at a minimum. Care must also be taken in predicting the power consumed by the torquer coils for repositioning.

The ELFIN Power system has been designed to provide sufficient power to the systems throughout the entire life of the spacecraft. The system includes Spectrolab UTJ cells and rechargeable Lithium-Ion batteries. These components are controlled by PIC microcontrollers to provide proper power to each of the other systems during the ELFIN mission.

Power Flow

Our source of power production comes from our Solar Panels on the exterior Y and Z faces of the CubeSat. We have 20 UTJ cells on each satellite, all hand soldered on by in house technicians onto custom circuit boards. Those cells transfer power directly into our avionics unit. There it feeds from BETC to our two SBPCBs and into our Lithium Ion batteries. Each SBPCB has two batteries, and only two batteries are needed to provide power to our Ring Bus. The flow can be seen in the figure below!

Grounding

ELFIN’s central grounding point is at the center point of the four batteries on board. The grounding path flows from them into our avionics unit, starting with our SBPCBs. There are two grounding paths on the avionics stack, one for “clean” signals like UART logic, and one for “heavy” channels like radio PA and deployments. The grounding path continues from BETC to our He-82 radio and onto the –X panel and ends on the chassis. But back at our LETC1 board, ground follows along the payload cable and onto the SIPS board, where it splits into three paths; EPD, IDPU, and FGE. As our EPD is very delicate, there is an inductor current filter for ground prior to the pre amp. On the FGE, there are two grounds connected to our FGM, “Digital Ground” at 0V and “Analog Ground” at +4V.

Inhibits

There are four physical inhibits on an ELFIN spacecraft; two deployment inhibits and two remove before flight (RBF) inhibits. The two deployment inhibits inhibit spacecraft power and transmission while in the depressed state (when the spacecraft is in the PPOD). One of the RBF inhibits prevents the deployment of our Stacer Boom. The other RBF inhibits prevents all power flow in the spacecraft.

Solar Panels

ELFIN uses 20 UTJ solar cells with 2.2V potential. They are arranged in a magnetically clean design. There is 1 cell string with 2 in parallel. There are 4 cells on +Y normal, 6 on –Y normal, 6 on +Z normal, and 4 on –Z normal. The cells are lined on top of one another, with traces on the PCB to prevent current from being induced in the opposing direction.

Batteries

Our power system contains a set of 4 Molicel ICR18650J Lithium-Ion batteries. These four batteries satisfy all of ELFIN’s power needs: radio beacons, deployments, data processing, etc. Their volatility and high nominal voltage make Li-Ion batteries perfect for ELFIN’s needs. The batteries are screened at the Aerospace Corporation, but we performed battery cycle tests on all flight batteries to get the most accurate representation of their capacities and characteristics. The batteries are paired in an anti-parallel manner to reduce their magnetic moments.  Each battery is dressed with a battery heater, TMP circuit board, and twisted power lines that feed into our Battery Adaptor Board.

SBPCBs

Our Power management boards are the Solar Battery Printed Circuit Board (SBPCB) from the Aerospace Corporation. These boards match the form factor of our avionics unit and are connected to other circuit boards through mezzanines and flat flexible cables. These boards regulate power consumption, housekeeping, and how the power produced by the panels gets fed into the batteries.

Assembly

All solar panels are printed circuit boards custom made by members of ELFIN. Our solar cell Fabrication method was provided by the Aerospace Corporation but performed in house.

Testing

All of the components of the power system went through testing prior to final integration of the spacecraft. Each test further validated functionality and implementation of the power system design.

Panels

●      Forward Bias Tests

●      IV curves

●      TVAC Cycling

Batteries

●      Charge/Discharge

Integrated

●      Solar Power Functionality Test

●      FM Illumination Test