I have an external portable USB-C screen (a Lenovo ThinkVision M14d).
If I plug things like that : wall power ↔ screen ↔ Tachyon ↔ battery (the screen has 2 USB-C ports); then every things works correctly. When I boot up the Tachyon, the desktop mode starts and displays correctly on the screen.
However, if I unplug the wall power, then the Tachyon does not power the screen. I have tried scenarios where I would start the Tachyon both bofore or/after the wall power is removed. In either case the screen is never powered by the Tachyon.
The screen requires less than 10W to operate and can accept many standard USP-PD input voltage (5, 9, 12, or 20V). So the battery should be able to power it easily (I have tried that with Particle 3 cells 9300mAh battery).
Is this scenario intended to be supported? Can the USB1 port also output USP-PD, or can it only power the Tachyon but not power other devices?
Tachyon can output power over the USB1 port, but I believe it's restricted to less than 10W (and potentially 5W). Saying this, I don't have an exact piece of information to hand (or the Linux configuration that specifies this!) so we'll have to loop back here as well as update our docs!
The background is that Tachyon can take up to 4A at 5V at peak operation capability and I will guess that the USB1 port is configured to supply something like 1A (enough to power USB-C hubs, peripherals etc...) but not 2A (such as 10W display at 5V). Now, it might be possible to modify the SW as long as you restrict the Tachyon module usage (and what that means exactly I don't know, but could be changing the CPU governor or the cellular radio behaviour) to supply more current, but we have not gotten into that level of configuration default at the moment.
If you want to play around, we'll update the docs and this ticket - a word of concern that browning out a module repeatedly with high external loads can damage the health of the battery and/or electronics.
Thanks for the quick response. Yes, if there are ways to try this, I’m interested in playing around. Damaging the battery wouldn’t be a big deal (damaging the electronic a little more, but I’ll be careful).
The screen spec says that it needs a 1.5A input and that it’s peak power usage is 6.85W. So hopefully it’s low enough to limit the risk of browning the device.