CR927 RTC Battery lifetime

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Posts: 29
Joined: Fri Dec 21, 2018 7:24 am

Wed Feb 13, 2019 2:21 pm

The RTC battery (CR927) on my first Alpha is already dead, think it have been some time now, but first checked it today.
So my question is, for how long (or short) time should the battery normaly last?

Also wonder if the battery have anything to do with the dead/missing Arduino comport?

PS. expect to get new battery tomorrow, so I can test it šŸ˜‰

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Joined: Sat Oct 13, 2018 9:16 pm

Fri Feb 15, 2019 12:23 am

In one of the CR927 manufacturer's datasheet, it states CR927 has 30 mAh capacity (for comparison: CR2032 has 220 mAh.)
Now the info is hard to get is how much current Intel KabyLake Y will need to run its internal RTC.
(Just a reference, a decade old external RTC IC operates at 50 - 100 uA in clock only mode.)

BTW, AFAIK, Intel CPU/APU's internal RTC just run realtime clock, it does not use that backup battery for other purposes (such as BIOS user settings.)
In another word, (I am thinking) this has nothing to do about booting issues..
Also note, the time keeping function usually can be augmented with the (Windows OS) bootup process (that if set,) can contact Internet NTP server to get an accurate time anyway.

Re: why choose CR927
I am guessing it has a lot to do with keeping this SBC board as small as possible.
The Edge header has a pin that has a label "RTC". I think it's for user to use its own larger battery (e.g., CR2032) for RTC backup purpose.

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Tue Feb 19, 2019 7:19 pm

Now I measured it, the current consumption is no more than 2 uA when LP Alpha does not have any power supplied to it.
When there is power supplied to it (LPA does not need to be powered on), the current drain is negligible, closed to zero (I used a Fluke DMM to measure it.)

CR927 when new, the terminal voltage is around 3.3V. For 3+ months that I have owned my LPA 864 (infrequently use and no power supplied), its voltage dropped to 2.7V.
More on this later.

BTW, CR927 10 pieces price is less than $5, shipped directly from USA. When replacing the battery, be careful on the tiny resistors on the board.

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