Hmmm, I've charged only from the left USB port, which I remember being the fast charge capable. Is the Planet's stock charger a fast charger or an usual one?
The Planet stock charger supports (the oldest, crudest and, thus, cheapest version of) a MediaTek specific fast charging protocol called Pump Express. If you look at the charger itself, it says it can output 5, 7, 9 or 12V, though I've only seen my Gemini and Cosmo use 5 or 9. As any USB charger, it always starts at 5V and only raises the voltage if being explicitly told so by the device. If the charger and the device doesn't speak the same fast-charging protocol, the charger just won't understand the device, and stay at 5V. This is why I can safely plug, say, my 130W (up to) 20V USB-C Dell laptop charger into the Cosmo. It even works.
Should one avoid charging constantly from the left port when using the stock charger? Or, in general, is fast charging more harmful than standard in some way?
It is my understanding (which may be wrong) that higher-current charging does put a bit more wear on Li-ion cells, though well-tuned and sophisticated charge controllers, combined with
modern fast charging protocols, that can regulating the voltage in very fine (like 0.02V) steps already in the charger, should keep the extra wear to a reasonable minimum.
But PumpExpress+ "1.0" just pushes 9V into the Cosmo. Planet Computers themselves may not be experts in optimized charging and "the factory" may not care all that much about device longevity, so in this particular case, I would suggest "slow-charging" your Cosmo when not in a hurry. I cannot prove that would prolong your the battery life, but it's kind of harder to screw up slower charging. Since only the left USB port on the Cosmo supports PumpExpress, using the Cosmo charger in the right port is one way to slow-charge. Another is to use (in either port) a USB charger that doesn't speak PumpExpress, such as your Qualcomm Quick Charge 2.0 Note8 charger, or any garden variety ~2A@5V charger.
There was a unique situation once, when Cosmo started to make a quiet whizzling-buzzing noise while charging, luckily I was sitting very near and heard it right away. Noise went away only after a reboot.
Don't worry. I've had that happen twice. While it may have sounded a bit like a frying battery, I clearly noticed, the second time, that the sound actually came from the speakers. They should, of course, be off when not in use, but if some bug turns them on when no audio is fed to the amplifier, it could pick up any random electrical noise such as digital clock signals or noise from a battery voltage regulator.
Even the CoDi hasn't crashed totally...
While mine HAS "crashed completely" a few times (just staying off, or getting stuck showing half a menu), it tends to lock up and just keep showing the Planet logo, when it should show the clock. I usually just turn it off and on again with the soft button in the Android notification shade, but maybe I should try playing around more with the physical buttons next time.
Edit:Nope, when my CoDi gets stuck on the Planet logo, no amount of button pressing seems to help, only turning the CoDi off and back on again (after which it forgets its brightness setting...) from the Android notification shade.
Edit2:Since my Cosmo apparently happens to work quite well on V20 with stock everything, I decided to try disabling the Planet Voice Assistant (that kept generating unobtrusive, yet slightly annoying notifications). It could, of course, be a coincidence, but I just put my Cosmo on the charger, 44 hours after the last charge, still with 20% left. I do keep it in flight mode at night and I haven't been using heavily (about 135 min screen-on time since last charge) but still, if this wasn't a fluke, with bit of tweaking, such as disabling stuff I won't use anyway, and a few more firmware versions, with further optimizations, two days per full charge, or staying between, say, 30 and 80% with daily charging, as with my Gemini, might be feasible.
Edit3:I suppose it's commonly known that (slow) charging from the right side USB port doesn't work if the CoDi is disabled. If it is enabled, but turned off, I've noticed it can take half a minute or so, after plugging the cable in, before it starts charging. If it's enabled and on at the moment, it seems to just take a few seconds (or maybe I've just been lucky). Anyway, do know, that quite a bit of delay before charging kicks in, can be normal when using the right side port, at least on current firmwares.