Edited to reflect status of trying to find solution: last: 2019-sep-19:
After trying hard, I found no software reason or workaround and sent back my Atom to Amazon.
I got a refund because Unihertz now sells it directly, so I had the opportunity
to think over my decision for that phone again: I ordered a replacement Atom,
updated to Android 9.0 and it now works. Happy again. There still is no other
phone like it. I don't know if it was an early version with a hardware/rom
not compatible with 9.0 or if it coincidentally broke near update time.
I'm very happy with my Atom.
However, starting with the update from Android 8.1 to 9.0 and also after the first update to 9.0
I experienced sporadic reboots several times, with no apparent reason ..
It's not completely random, though: it always happend when I did not use the phone,
mostly overnight, but also during other longer non use times of 1 to several hours.
Everything else seems to work as before, with 8.1and 8.1 update:
I did not install additional apps, change settings, ..
Since it's very sporadic, it's difficult to tell the reason.
Will add to this post if I know more, see below.
Does anybody else experience this? Are there workarounds?
(it's weird if you expect a call or use the phone as a wake up clock ..)-
1) I tried by turning off "intelligent energy saving in standby mode"
and thought this solved it, but unfortunately it did not.
2) One reboot the phone was plugged in (energy saving is off then),
so I guess energy saving is not the problem at all:
EDIT: I forgot that I turn off my power plug array over night,
so the phone was not charging. That brings me to attempt 7),
together with an answer from user UnderSky (see below)
3) I deactivated the Google Carrier Services and uninstalled the Google app which
allows to start an app during inactive screen (Daydream): did not help.
4) I used WLAN (with still the above trials active), turned it off, turned LTE on,
rebooted and in the morning the device had rebooted (energy saving mode on).
After the reboot, google play told me updates are available.
5) Restarting the device (turn off, restart) with only LTE did not help either:
it rebooted again while in standby, overnight (reboot does not electrically turn off the device,
so I suspected bad WLAN hardware state to be the culprit: this is not the reason).
6) I now deactivated Google Play Store (but could not deactivate Google Play Services),
with only LTE on and restarted the device (shutdown, start, not only reboot).
This did not help: it rebooted 2 times during some few hours, daytime.
However, I rethought 2 and found an answer from user UnderSky below
also hinting to my next attempt 7)
7) I suspected unplugging from power (and whatever may follow,
switching back to energy saving, which Android 8.1 did not do) is the culprit.
I charged the device, unpluged, turned it off, restarted with only LTE active:
it still rebooted
8) I uninstalled Avira Antivirus.
It still rebooted.
9) I uninstalled Syncmate
It still rebooted (then reinstalled: after all this is my only phone ..)
10) Disabled USB debugging (which was needed for syncmate)
and increased log buffer size (will have to reenable USB debugging
for viewing them using adb, after a possible reboot and check
if I then can see logs from before the reboot).
>25h uptime but then reboot overnight.
11) Turned log buffers off and USB debugging on again.
System, Development options, Extended, Log buffers: off).
Installed Android 9.0 update from August 27. Left settings as they were
Reboot 2.5 hours after the update ..
12) turned of USB debugging again to see if it influences uptime.
To have an electrically defined state, turned device off and on again.
13) turned on intelligent energy saving in standby mode.
Reboot after less than 1h. turned off the mode and rebooted:
Android was hung during boot animation. Turned off/on: booted.
14) turned off and energy saving mode. Turned off+on device
(to make sure energy saving mode did not place any hardware
in electrically bad state. Still it rebooted some hours later.
15) Reset device to factory settings (=9.0 update): it still rebooted
did not add account, not connect to WLAN, not install apps.
Uptime is sometimes <1h, mostly some h, and rarely up to 25h:
I can't use a phone that reboots:
if I expect calls or use it as a waking clock, booting makes it useless.
I will now review if I can send it back to seller and get replacement
which hopefully is not broken or still has 8.1 / 8.1 update on it or works with 9.0.
Yesterday I have reset the phone again, thankfully it is not my main phone (still hope it will though).
Did not install any applications, not connected to my google account, only updated google apps.
Put on charger in the evening, no reboots while on charger.
In the morning I removed from charger and put it in a pocket. I was driving for ~30 minutes and once parked I checked the phone and it had restarted.
Right now I've installed couple logging apps, not sure though they can catch the reboot cause though, but I'm giving them a try. If nothing helps I'm going to contact the seller for replacement, and if the replacement works the same way, then, well, it won't work for me, unfortunately.
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