The power system of TUMnanoSAT has one integrated Li-Po battery pack with a capacity of 10.2 Wh. The maximum output power from all power buses combined is 10W. A CubeSat has three axes: x, y and z, and each axis has two sides: + and -. The EPS has three Solar Panel Channels, one for each axis. Each Solar Panel Channel on the EPS has two connectors, one for each side of the axis. Therefore, in total the EPS has six solar panel connectors (as a CubeSat has six sides). Each Solar Panel Channel has a DC-DC step-up converter with Maximum Power Point Tracking(MPPT). The output energy for each solar panel is monitored. All three Solar Panel Channels are in parallel and can work at the same time. Therefore, the generated energy is a sum of all solar panels. The Solar Panel Channels can handle input voltages up to 5.5V and the current maximum threshold for overcurrent protection is set to 1.8A. The operating temperature range is from -40 °C to +150 °C and the over temperature threshold is set to +155 °C (the module will turn off if this threshold is reached and restart automatically when the temperature decreases to +130 °C). The efficiency of the step-up convertors is up to 95%.
The step-up convertors work at 100kHz fixed frequency. The duty cycle is controlled by the MPPT algorithm. The boosted output voltage can be accessed through the PC/104 connector for additional functionality such as charging of the external battery pack, super capacitors, etc.
The battery charger operates in both linear and quasi-pulse mode sand this happens according to the Constant Voltage and Constant Current rules. The advantage of the quasi-pulse charging method is that it allows the energy from weakly illuminated solar cells to be very efficiently harvested. Four different charging current mode scan be selected according to the solar panel configuration used on the CubeSat. The efficiency of the battery charger is around 90%. The EPS includes firmware and hardware battery protection. A single battery pack consists of 2 Li-Po battery cells connected in parallel. Each battery cell within each battery pack has an integrated Protection Circuit Module (PCM) which prevents over-current, over-charge and over-discharge.
The electrical budget is presented below:
|Module||Supply volatage, V||Supply current, mA||Consumed power, mW||Consumption time, sec.||Periodicity,h||Periodicity, sec.||Averrage power, mW||Consumed energy, Wh|
|Transceiver (TX mode)||3.3||800||2640||30||0.08||300||338.25||0.33825|
|Power cycle, sec||3600|
|Total consumption, Wh||0.91325|