XO power management

Ismael Schinca ischinca at plan.ceibal.edu.uy
Tue Apr 5 09:33:32 EDT 2011

Thanks! This is very useful information. I'll try to run some new tests and
try to look at the data more thoroughly. The new batteries' capacity is
pretty puzzling. I'll repeat the tests on these batteries and add a couple
more of new ones to get more results.
I think it's a good idea to think about discarding the 07 batteries. They
are pretty old anyways.
Any information regarding the total number of charge/discharge cycles would
be really useful.


2011/4/4 Richard A. Smith <richard at laptop.org>

> On 03/31/2011 03:43 PM, Ismael Schinca wrote:
>> Richard, Ceibal about to start working heavily in reworking how to
>> classify used batteries. Due to the number of batteries involved, we
>> were asked to devise a "good enough" procedure to estimate the battery
>> condition in less than 10 minutes. The plan would be to get several
>> parameters from the batteries and try to estimate it's condition based
>> on them. Have you been able to analyze the results from the logs I sent
>> you?
> Sorry.  I've been very busy lately and its taken me a while to look at
> them.  I'm currently in route to Taiwan where I'll be for the next 2 weeks.
>  I've been looking at your logs on the plane.  Note that after about 4pm PST
> I'll be out for a day or so . After that I'll be out of sync with your
> timezone  (I'll be +11 or +12 or so from you) and I'll be very busy so my
> responses my lag quite a bit.
> I'm afraid its difficult to make any judgments from the data you have sent
> so far.   You are going to have to test many more batteries to get more
> data.
> I'm going to have to do some work on my processing tools to help with the
> comparison.  I'll try to get to that later on.
> Here's a quick look at the capacity of the batteries you have tested so
> far.  Thank you very much for the battery serial number in the filename that
> is very helpful.  Please continue to do that.  I've broken up the serial
> number so that we can see the age easily. The 2nd column is the ammount of
> Watthours that was extracted from the battery sorted into ascending order of
> Watthours.
> 00401 071229 110002160OLD.csv   -4.177
> 00401 071229 110002221OLD.csv   -11.663
> 00602 080608 110000600.csv      -13.915
> 00802 100751 10002447NEW.csv    -13.992
> 00602 080616 110002384.csv      -14.405
> 00802 100804 110003201NEW.csv   -14.803
> 00802 090331 110002400.csv      -15.066
> 00802 081130 110001133.csv      -15.768
> 10102 080109 310000524.csv      -15.863
> 00702 080710 110000901.csv      -16.009
> 00802 090829 110001062.csv      -15.914
> 00802 081231 110000683.csv      -16.576
> 00702 080714 110000474.csv      -16.826
> 00802 090415 110001059.csv      -16.995
> 00802 090429 110002159.csv      -17.705
> 00802 090206 110001307.csv      -18.489
> The first thing to notice is that it roughly correlates with age which
> makes sense.  The both the 004's are suffering from charge balance.
> Something you can see from the charging voltage logs.  If you want a quick
> first pass of segregating the batteries then you can use the first 3 digits
> as guide.  Any battery with the first 3 digits that are less than 008 (ie,
> 007,006,005,004...) is subject to charge balance.  I recommend you group
> your batteries int < 008 and >= 008.  Depending on how many you have you may
> or may not want to even try to analyze the < 008 group.
> The next thing to notice is that your batteries that you marked as NEW are
> showing a capacity that is much less than expected.  Those should have had a
> capacity of 18Wh or more.  Please re-run the test on those batteries and see
> if the data is consistent.
> I've estimated that the capacity loss should be .025% per/cycle. If we
> assume the rough age of the batteries is 2 to 3 years and that they average
> 1 cycle per day then I'm expecting the available capacity to be in the 73%
> to 82% range or 15-16 Wh.  Looking at your numbers I say its right on par.
>  So perhaps you can also use the battery age as a guide.
> What is the minimum capacity you want to allow to be re-used?
> The attached graph shows your charging voltage in detail.  As you can see
> the the screen turning off and on has an effect on the voltage. This graph
> also illustrates why this is going to be a difficult thing to accomplish in
> only a 10 minute test.
>  Another question. Is there information in the battery controller
>> regarding total number of charge/discharge cycles?
> There is an attempt to do that and the logs extract that information from
> the EEPROM.  However,  I don't have any idea how accurate it is. Its legacy
> code that I've never had a reason to examine closely.  I'll examine the code
> that does that in detail and then add decoding of the values to my
> processing script next so I can take a look at the numbers.
> --
> Richard A. Smith  <richard at laptop.org>
> One Laptop per Child

Ismael Schinca
Centro Ceibal - Depto. T├ęcnico - I+D
Avda. Italia 6201 - Edificio Los Ceibos
Montevideo - Uruguay.
Tel.: 2601 57 73 Int. 2232
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