RTC battery drain when power on?

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oddulf
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RTC battery drain when power on?

Post by oddulf »

Hi,

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

Code: Select all

journalctl -b 0 -o short-iso
The first line of the logs will show an easily parsed timestamp, indicating if logs started in 2018 or now - the former of course an indication of RTC battery not working.

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!

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Re: RTC battery drain when power on?

Post by odroid »

XU4 PMIC will not discharge when the system is running. Even it discharges while powered, the battery must run several years with continuous <1uA power consumption.
As far as I know, Lithium coin cell batteries self-discharge rate is also negligible.
Therefore I believe the backup battery must be fine for several years at least.
When did you start running the system?

Anyway, once you measure the battery voltage, please let me know.

Is there any possibility of the RTC register corruption by sudden power-off events?
Or, defective battery?
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oddulf (Thu Oct 15, 2020 7:49 pm)

oddulf
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Re: RTC battery drain when power on?

Post by oddulf »

Thank you, that is very helpful! The first XU4 odroids were commissioned about 18 months ago, so I thought it's about time to make a policy for swaps :-)

I have not seen any RTC corruption by sudden power off events, but that doesn't really mean anything, I suppose. We have a system of swap-in odroids in place, having weighed the risks against the relatively small cost of odroid replacement.

We are also working on an external watchdog with support for graceful shutdown, for the next generation of commissions.

I'll update when I have a system of measuring charge in place

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Re: RTC battery drain when power on?

Post by odroid »

One more possible concern about the coin cell temperature.
If the coin cell is placed near the XU4Q heatsink very closely, that high temperature might accelerate the discharge rate a lot probably.
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oddulf (Fri Oct 16, 2020 5:11 pm)

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Re: RTC battery drain when power on?

Post by oddulf »

Thank you odroid, yes, very good point!

I removed a bit of the leg of the plastic case between the network port and the USB2 port with a chisel, enabling the battery to hang loosely outside the case. I will make a point of monitoring the ambient temperature in the cabinet to see if it is a concern.

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