blog/The_life_and_death_of_a_laptop_battery.html

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  14  <h1>
  15      <a href="http://people.skolelinux.org/pere/blog/">Petter Reinholdtsen</a>
  16
  17  </h1>
  18
  19 </div>
  20
  21
  22     <div class="entry">
  23       <div class="title">The life and death of a laptop battery</div>
  24       <div class="date">24th September 2015</div>
  25       <div class="body"><p>When I get a new laptop, the battery life time at the start is OK.
  26 But this do not last.  The last few laptops gave me a feeling that
  27 within a year, the life time is just a fraction of what it used to be,
  28 and it slowly become painful to use the laptop without power connected
  29 all the time.  Because of this, when I got a new Thinkpad X230 laptop
  30 about two years ago, I decided to monitor its battery state to have
  31 more hard facts when the battery started to fail.</p>
  32
  33 <img src="http://people.skolelinux.org/pere/blog/images/2015-09-24-laptop-battery-graph.png"/>
  34
  35 <p>First I tried to find a sensible Debian package to record the
  36 battery status, assuming that this must be a problem already handled
  37 by someone else.  I found
  38 <a href="https://tracker.debian.org/pkg/battery-stats">battery-stats</a>,
  39 which collects statistics from the battery, but it was completely
  40 broken.  I sent a few suggestions to the maintainer, but decided to
  41 write my own collector as a shell script while I waited for feedback
  42 from him.  Via
  43 <a href="http://www.ifweassume.com/2013/08/the-de-evolution-of-my-laptop-battery.html">a
  44 blog post about the battery development on a MacBook Air</a> I also
  45 discovered
  46 <a href="https://github.com/jradavenport/batlog.git">batlog</a>, not
  47 available in Debian.</p>
  48
  49 <p>I started my collector 2013-07-15, and it has been collecting
  50 battery stats ever since. Now my
  51 /var/log/hjemmenett-battery-status.log file contain around 115,000
  52 measurements, from the time the battery was working great until now,
  53 when it is unable to charge above 7% of original capacity.  My
  54 collector shell script is quite simple and look like this:</p>
  55
  56 <pre>
  57 #!/bin/sh
  58 # Inspired by
  59 # http://www.ifweassume.com/2013/08/the-de-evolution-of-my-laptop-battery.html
  60 # See also
  61 # http://blog.sleeplessbeastie.eu/2013/01/02/debian-how-to-monitor-battery-capacity/
  62 logfile=/var/log/hjemmenett-battery-status.log
  63
  64 files="manufacturer model_name technology serial_number \
  65     energy_full energy_full_design energy_now cycle_count status"
  66
  67 if [ ! -e "$logfile" ] ; then
  68     (
  69         printf "timestamp,"
  70         for f in $files; do
  71             printf "%s," $f
  72         done
  73         echo
  74     ) > "$logfile"
  75 fi
  76
  77 log_battery() {
  78     # Print complete message in one echo call, to avoid race condition
  79     # when several log processes run in parallel.
  80     msg=$(printf "%s," $(date +%s); \
  81         for f in $files; do \
  82             printf "%s," $(cat $f); \
  83         done)
  84     echo "$msg"
  85 }
  86
  87 cd /sys/class/power_supply
  88
  89 for bat in BAT*; do
  90     (cd $bat && log_battery >> "$logfile")
  91 done
  92 </pre>
  93
  94 <p>The script is called when the power management system detect a
  95 change in the power status (power plug in or out), and when going into
  96 and out of hibernation and suspend.  In addition, it collect a value
  97 every 10 minutes.  This make it possible for me know when the battery
  98 is discharging, charging and how the maximum charge change over time.
  99 The code for the Debian package
 100 <a href="https://github.com/petterreinholdtsen/battery-status">is now
 101 available on github</a>.</p>
 102
 103 <p>The collected log file look like this:</p>
 104
 105 <pre>
 106 timestamp,manufacturer,model_name,technology,serial_number,energy_full,energy_full_design,energy_now,cycle_count,status,
 107 1376591133,LGC,45N1025,Li-ion,974,62800000,62160000,39050000,0,Discharging,
 108 [...]
 109 1443090528,LGC,45N1025,Li-ion,974,4900000,62160000,4900000,0,Full,
 110 1443090601,LGC,45N1025,Li-ion,974,4900000,62160000,4900000,0,Full,
 111 </pre>
 112
 113 <p>I wrote a small script to create a graph of the charge development
 114 over time.  This graph depicted above show the slow death of my laptop
 115 battery.</p>
 116
 117 <p>But why is this happening?  Why are my laptop batteries always
 118 dying in a year or two, while the batteries of space probes and
 119 satellites keep working year after year.  If we are to believe
 120 <a href="http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries">Battery
 121 University</a>, the cause is me charging the battery whenever I have a
 122 chance, and the fix is to not charge the Lithium-ion batteries to 100%
 123 all the time, but to stay below 90% of full charge most of the time.
 124 I've been told that the Tesla electric cars
 125 <a href="http://my.teslamotors.com/de_CH/forum/forums/battery-charge-limit">limit
 126 the charge of their batteries to 80%</a>, with the option to charge to
 127 100% when preparing for a longer trip (not that I would want a car
 128 like Tesla where rights to privacy is abandoned, but that is another
 129 story), which I guess is the option we should have for laptops on
 130 Linux too.</p>
 131
 132 <p>Is there a good and generic way with Linux to tell the battery to
 133 stop charging at 80%, unless requested to charge to 100% once in
 134 preparation for a longer trip?  I found
 135 <a href="http://askubuntu.com/questions/34452/how-can-i-limit-battery-charging-to-80-capacity">one
 136 recipe on askubuntu for Ubuntu to limit charging on Thinkpad to
 137 80%</a>, but could not get it to work (kernel module refused to
 138 load).</p>
 139
 140 <p>I wonder why the battery capacity was reported to be more than 100%
 141 at the start.  I also wonder why the "full capacity" increases some
 142 times, and if it is possible to repeat the process to get the battery
 143 back to design capacity.  And I wonder if the discharge and charge
 144 speed change over time, or if this stay the same.  I did not yet try
 145 to write a tool to calculate the derivative values of the battery
 146 level, but suspect some interesting insights might be learned from
 147 those.</p>
 148
 149 <p>Update 2015-09-24: I got a tip to install the packages
 150 acpi-call-dkms and tlp (unfortunately missing in Debian stable)
 151 packages instead of the tp-smapi-dkms package I had tried to use
 152 initially, and use 'tlp setcharge 40 80' to change when charging start
 153 and stop.  I've done so now, but expect my existing battery is toast
 154 and need to be replaced.  The proposal is unfortunately Thinkpad
 155 specific.</p>
 156 </div>
 157
 158       <div class="tags">Tags: <a href="http://people.skolelinux.org/pere/blog/tags/debian">debian</a>, <a href="http://people.skolelinux.org/pere/blog/tags/english">english</a>.</div>
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 483  <li><a href="http://people.skolelinux.org/pere/blog/tags/reprap">reprap (11)</a></li>
 484
 485  <li><a href="http://people.skolelinux.org/pere/blog/tags/rfid">rfid (3)</a></li>
 486
 487  <li><a href="http://people.skolelinux.org/pere/blog/tags/robot">robot (9)</a></li>
 488
 489  <li><a href="http://people.skolelinux.org/pere/blog/tags/rss">rss (1)</a></li>
 490
 491  <li><a href="http://people.skolelinux.org/pere/blog/tags/ruter">ruter (4)</a></li>
 492
 493  <li><a href="http://people.skolelinux.org/pere/blog/tags/scraperwiki">scraperwiki (2)</a></li>
 494
 495  <li><a href="http://people.skolelinux.org/pere/blog/tags/sikkerhet">sikkerhet (46)</a></li>
 496
 497  <li><a href="http://people.skolelinux.org/pere/blog/tags/sitesummary">sitesummary (4)</a></li>
 498
 499  <li><a href="http://people.skolelinux.org/pere/blog/tags/skepsis">skepsis (4)</a></li>
 500
 501  <li><a href="http://people.skolelinux.org/pere/blog/tags/standard">standard (49)</a></li>
 502
 503  <li><a href="http://people.skolelinux.org/pere/blog/tags/stavekontroll">stavekontroll (3)</a></li>
 504
 505  <li><a href="http://people.skolelinux.org/pere/blog/tags/stortinget">stortinget (10)</a></li>
 506
 507  <li><a href="http://people.skolelinux.org/pere/blog/tags/surveillance">surveillance (36)</a></li>
 508
 509  <li><a href="http://people.skolelinux.org/pere/blog/tags/sysadmin">sysadmin (2)</a></li>
 510
 511  <li><a href="http://people.skolelinux.org/pere/blog/tags/usenix">usenix (2)</a></li>
 512
 513  <li><a href="http://people.skolelinux.org/pere/blog/tags/valg">valg (8)</a></li>
 514
 515  <li><a href="http://people.skolelinux.org/pere/blog/tags/video">video (55)</a></li>
 516
 517  <li><a href="http://people.skolelinux.org/pere/blog/tags/vitenskap">vitenskap (4)</a></li>
 518
 519  <li><a href="http://people.skolelinux.org/pere/blog/tags/web">web (38)</a></li>
 520
 521 </ul>
 522
 523
 524     </div>
 525     <p style="text-align: right">
 526  Created by <a href="http://steve.org.uk/Software/chronicle">Chronicle v4.6</a>
 527 </p>
 528
 529   </body>
 530 </html>