The EPS subsystem uses the PyCubed battery board, also called Sapling Seedpod, which was developed at Stanford and flight proven on the NASA V-R3x mission. The battery board meets ISS specifications and maximum capacity requirements of common CubeSat integrators. It uses components with radiation testing heritage, including R5460N233AF for protection against over-charge, over-discharge, short-circuit, excess discharge-current, and excess charge-current. Any PyCubed Battery board may be trickle charged via a USB-C charger on the PyCubed flight computer. Sapling Seedpod also contains a thermistor circuit to monitor temperature.
The PyCubed battery board is equipped with 6 Panasonic NCR18650B lithium-ion cells. These cells were selected for their high energy density, long stable power and run time. These batteries were chosen under consideration of the 2015 NASA findings on Safe, High Performing Li-ion Battery Designs.1
The spacecraft ConOps were defined for autonomous switching between modes based on battery voltage and battery temperature. This dual power system health monitoring strategy addresses the cells’ discharge characteristics, which change with temperature. Thermal cycling on orbit, particularly as a result of eclipse time, will affect battery voltage as shown in figure 1.
Panasonic NCR18650B discharge characteristics with temperature
Sapling has 6 custom solar panel PCBs which each host 6 Ixolar SM101K07TF high efficiency silicon solar cells. These cells are thin, lightweight, and much cheaper than typical space rated solar cells. The AnySolar SM101K07TF was selected after conducting a trade study of over 90 available, affordable solar cells from AnySolar. Their open circuit voltage is 4.84 V. With 6 cells in series, voltage at PMPP is 23.46V, and power at Pmpp is 0.924W.
SM101K07TF solar cells
The arrangement of the cells on the PCB was designed to maximize the area used for solar harvesting without adding deployable arrays in order to buy down risk. By including 6 cells on each face, the CubeSat design is robust to failure of one cell, and total open circuit voltage remains high with only 5 operational cells.
Cell arrangements considered
Solar Cell Arrangement
The Sapling-2 revision of the PyCubed flight computer uses an LTC3652 2A battery charger to deliver current at a 13.5V MPPT setpoint voltage. The use of LTC3652 is one of SSI’s recent upgrades, made after an unsatisfactory assessment of solar charging performance for the Sapling Sempervirens mission. LTC3652HV has flight heritage and comes recommended by AnySolar. The current MPPT of 13.5V was selected based on the solar charger’s performance at varying temperature conditions on orbit. In the future, SSI plans to implement a temperature controlled MPPT which would increase power generation.
All remove before flight (RBF) elements of the Sapling Giganteum satellite are shown below. RBFs are only utilized when all four of the rail switches (which serve as the spacecraft’s battery inhibit) are down (occurs in the case when the CubeSat is in its deployer). All are located at the RBF header in the access port at the +X face.
Sapling Giganteum RBF element locations
List of RBF elements shown in Figure 5
|Ground pin: Inserted at the far left side of the RBF header, necessary for both USB charging or mounting the PyCubed drive while the rail switches are down.
|Charging pin: Inserted at the center of the RBF header, used to enable charging over USBC while the rail switches are down.
|Code update pin: Inserted at the far right of the RBF header, used to allow a laptop to mount the PyCubed drive over USBC while the rail switches are down.