I'm in a similar situation to hilton8r in this post: viewtopic.php?t=28111, remotely managing a number of XU4 odroids.
Could someone please confirm if the RTC battery is discharging also when the board has power (odroid running or shutdown)? This would help me determining a policy for battery swaps.
I'm looking into getting a voltmeter to measure the state of the battery before commissioning and during annual service - I'll update this thread on the procedure as it doesn't seem trivial.
If it helps someone, I found that, on ubuntu 18.04 at least, after a boot from a period of no power supply, the failure or absence of the RTC battery can be detected using the command
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journalctl -b 0 -o short-iso
The RTC clock seems to be functioning as long as there is any power supply to the board. I have even managed to do a shutdown, switch off the power at the wall for 60 seconds, then booted up the board on a rtc wake a few minutes later, without having a battery attached. The XU4 PSU takes up to 97 seconds to dissipate the red light, which seems to be enough to keep the clock ticking.
Thank you for your help!