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pichlo's Avatar
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#21
A bit of feedback, nearly 2 years on. I replaced the bupbat on four of my N900s and it worked fine for a while, but I noticed a few months ago that the RTC time is lost again once the main battery is taken out for more than a few seconds. That would suggest that the bupbat has ran out and is not being recharged. Wasn't there supposed to be a fix in the power kernel for that?
 
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#22
hmm, the charging *should* be enabled per default, afaik. You might want to check the registers, and/or probe voltage on bupbat during and after inserting main battery and attaching wallcharger
maybe useful: http://www.ti.com/lit/an/swca025/swca025.pdf
actually seems it's not on per default: http://www.datasheetarchive.com/SWCU050G-datasheet.html

In the boot sequence for the device, going from wait-on to active mode, the backup battery charging sequence must be initiated by setting the BB_CFG[4] BBCHEN bit to 1.
[ SWCU050G – April 2008 – Revised December 2010 p438 "7.4.2.1 Backup Battery Charger"]
I would have expected this getting done by NOLO
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Last edited by joerg_rw; 2015-10-15 at 16:30.
 

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#23
OK, I made a few tests. Please note that I have the package, whatever it is called, that puts up the "enter date" dialog on boot up removed. My phone boots up straight to desktop every time.

Test 1: Remove main battery for 5 minutes. Put it back, power up. System tme is preserved.
Test 2: Remove main battery for 2 hours. Put it back, power up. System time is reset.

So it looks like the bupbat keeps some charge but not for very long.
 

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#24
which bupbat you are using? The polyacene capacitor type is not supposed to provide nearly as long a backup as the original LiMg cell. Anyway I'll try to find the data and do the math how long actually

to start with:


PAS414 CAP PAS COIN-TYPE 3.3V 0.07F
0.07 As/V * 3 V * 3 V = 0.63 VAs; aka Ws aka J (actually * 0.5 since voltage drops linear with discharge)
0.63 Ws / 7.2E-6 W = 87.5E3 s = 24 h (* 0.5; assuming the capacitor could power the VBRTC domain until voltage = 0, so actually probably another time half of that, ~6h in the end)

Anyway BB_CFG register reset default is 0b00000000, for polyacene cap PAS414 (!!!) I suggest setting it to 0b00011111 as soon as possible during boot
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Last edited by joerg_rw; 2015-10-15 at 21:02.
 

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#25
I am using the ones from you. If this is as expected then fine. I think my test proves that the bupbat does get recharged, otherwise the time would be lost right away and not after two hours. I was under a misconception that an RTC backup battery should last for days, if not years. We used some RTC chips back in mid-1990's, can't remember the type number, that had the battery built in. So clearly not vety big. IIRC, the catalogue page claimed the battery should last 10 years.
 

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#26
Originally Posted by pichlo View Post
We used some RTC chips back in mid-1990's, can't remember the type number, that had the battery built in. So clearly not vety big. IIRC, the catalogue page claimed the battery should last 10 years.
Infamous DALLAS chip, had a Lithium battery which had quite a few more Ws than the polyacene. Probably something like 100mAh * 3V = 18Ws. And the clock in DALLAS was less "greedy" than the TPS65950
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#27
Looking for new bup, Tayo Yuiden part is out of production. Could be ELNA DSK-414-HL a suitable replacement?
 

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#28
Originally Posted by jurop88 View Post
Looking for new bup, Tayo Yuiden part is out of production. Could be ELNA DSK-414-HL a suitable replacement?
yes, looks like a perfect match, though I didn't check the exact specs (millimeters etc), anyway generally it should just work as long as it fits.
Thanks for the pointer
/j
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#29
The N900 has a clock backup battery? All these years of ignorance I assumed it was just s lost technology and reseting date/time when changing battery wss a feature. Thanks to this recent activity in this thread I'll see if I can find that potential replacement.

The things one learns every day here!

And digikey stocks the Elna part. Makes it easy to get just one.

Last edited by robthebold; 2016-01-31 at 20:33.
 
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#30
It's probably realted to broken firmware in mine case, but does Nokia N9 got similar RTC battery?
 
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