1U Design Concern with PyCubed voltage

[aharris0n]

My team is using 26.6 cm^2 SpectroLab UTJ cells and we are designing a modular bus capable of integrating payloads quickly and with minimal connections, to increase flexibility of future CubeSats (in the form of a 1U connected with payloads in form of 1U or 2U). We have been asked to design the 1U to be self-sufficient, but the input voltage on a 1U for worst case scenario would be 4.7V, but the minimum voltage the PyCubed circuit can handle is 9V, and I was wondering how strict this requirement is and if it is strict, if it is advised that we use different solar cells.

[maholli]

Hi @aharris0n

PyCubed uses the Analog Devices LTC4121 energy harvesting chip. See the PyCubed page on energy harvesting. The LTC4121 is a step-down regulator, so it'll only ever be able to charge a battery pack at a voltage below the solar panel output.

It sounds like you may want to change your solar cell scheme to raise the Vmp voltage or use your own/different energy harvesting circuit to better fit the needs of your mission.

Sorry I couldn't be of more help!

[Mikefly123]

@aharris0n spring boarding off @maholli's comment, you could possibly change the layout of the PyCubed battery board to be an all parallel configuration rather than the 2s3p configuration that it current is to bring your battery float voltage down. See the attached screenshot of the LTC4121 data sheet that denotes the requirement to have an MPPT set point at some value above the battery float voltage.

Also note that the open circuit voltage for the solar cell circuit will also need to be above the MPPT voltage by some margin too! While you could boost the solar panel output to a higher value and then run it through the LTC4121 at the given 9V input, the compounding efficiency losses would probably wipe out any benefits you get from the system.