[Gta04-owner] uptime of rtc (unusual usage pattern)

NeilBrown neilb at suse.de
Sun Feb 10 06:18:38 CET 2013

On Sat, 09 Feb 2013 18:44:40 +0100 "e.waelde" <ew.ng7125 at nassur.net> wrote:

> Hello,
> after talking briefly to someone at the FOSDEM stand, it
> was suggested to raise this here.
> Q: How long is the onboard RTC supposed to run, when the
> battery is removed from the GTA04?
> Background:
> I do carry the phone (GTA04) with me almost all the time,
> however, it is switched off. I don't need it to be available
> to others. After finding out that the battery on gta04 drains
> rather fast even after shutting down the device, I do remove
> the battery from the case.
> So when I want to use the phone, I boot it. However, if I
> remove the battery for more than 12 h or so, the time on
> the device is back to 2000-01-01 00:00:00. I used to switch
> off the phone for maybe 2 weeks on average, but losing time
> annoys me.
> Any ideas?

According the the schematics, on page 5, the backup battery (B501) is rated
at 60mF.
We charge it to 3.2V and it becomes ineffective at 1.8V, so that is 1.4 volts
of change.
This corresponds to 1.4*60 millicoulombs of charge that will be discharged
while the backup battery is maintaining the RTC.
1 coulomb is 1 Amp*second, so that is 84mAsec.

If you are getting 12 hours, that suggests the current draw is
84000/(12*3600) uA or about 2uA, which seems credible.  I couldn't find the
and specification for how much the 32 kHz clock plus the RTC uses.

If you need longer, the only real option would be to add more capacitance at
the backup battery.  I have no idea if that is practical.

We cannot charge the backup battery any higer than 3.2V.

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