Today, I went ahead and carried out a few simple energy and thermal measurements with the Joule. Since the board is supposed to fly in a high-powered rocket, it must be battery powered. And as decided previously, it will not have active cooling. So it was important to go beyond the simple “does it burn” the fingers test to check that it will be possible to use the board as intended. For the workloads, I didn’t go wild – so no power viruses –, I just used some simple loads. A basic command such as
for i in 1 2 3 4; do while : ; do : ; done & done
and one of the many stress software one can find on the web such as ./stressapptest -s 120 -M 256 -m 8 -C 8 -W (https://github.com/stressapptest/stressapptest.git). I do not expect to have any heavier load at this point (this is still ~100% CPU user load). The ambient temperature during the measurements was 23.2C.
I was very pleased by the numbers – but that’s probably because the power supply requirements listed in the documentation are a bit biffy (12VDC@3A, that’s 36Watts). So it turned out – check out the videos & pics – that the Joule on idle (with the OS, drivers, etc. of this specific configuration) draws ~1.7W. Once shut down, prior to power off, it draws ~0.120W. In load, it never exceeded 8W. Pretty good. This will allow me to pick any LiPo battery pack available on the market. Based on the specs, I did pre-order a 5.2Wh 6S LiPo battery. Although the main battery will power most of the electronics on-board (with the exception of the flight computer and main recovery telemetry – you don’t want to put all your eggs in the same basket), it is still too high of capacity (and the extra weight is not required).
So I ordered a 1.4Wh 3S LiPo battery as an alternate option. Both of them, with respectively 10 and 40-80 C ratings, even if the Joule peaks at 3A, will be perfectly able to take the load. On the temperature side, measured at the heatsink, it never crossed 56C. So up to now, all is good, even a tad better than expected. As a next step, while waiting for the batteries to be delivered, I will put the power distribution together and power the Joule from its battery. Have a great WE and week!