blog/data/2015-09-24-laptop-battery.txt

   1 Title: The life and death of a laptop battery
   2 Tags: english, debian
   3 Date: 2015-09-24 16:00
   4
   5 <p>When I get a new laptop, the battery life time at the start is OK.
   6 But this do not last.  The last few laptops gave me a feeling that
   7 within a year, the life time is just a fraction of what it used to be,
   8 and it slowly become painful to use the laptop without power connected
   9 all the time.  Because of this, when I got a new Thinkpad X230 laptop
  10 about two years ago, I decided to monitor its battery state to have
  11 more hard facts when the battery started to fail.</p>
  12
  13 <img src="http://people.skolelinux.org/pere/blog/images/2015-09-24-laptop-battery-graph.png"/>
  14
  15 <p>First I tried to find a sensible Debian package to record the
  16 battery status, assuming that this must be a problem already handled
  17 by someone else.  I found
  18 <a href="https://tracker.debian.org/pkg/battery-stats">battery-stats</a>,
  19 which collects statistics from the battery, but it was completely
  20 broken.  I sent a few suggestions to the maintainer, but decided to
  21 write my own collector as a shell script while I waited for feedback
  22 from him.  Via
  23 <a href="http://www.ifweassume.com/2013/08/the-de-evolution-of-my-laptop-battery.html">a
  24 blog post about the battery development on a MacBook Air</a> I also
  25 discovered
  26 <a href="https://github.com/jradavenport/batlog.git">batlog</a>, not
  27 available in Debian.</p>
  28
  29 <p>I started my collector 2013-07-15, and it has been collecting
  30 battery stats ever since. Now my
  31 /var/log/hjemmenett-battery-status.log file contain around 115,000
  32 measurements, from the time the battery was working great until now,
  33 when it is unable to charge above 7% of original capacity.  My
  34 collector shell script is quite simple and look like this:</p>
  35
  36 <pre>
  37 #!/bin/sh
  38 # Inspired by
  39 # http://www.ifweassume.com/2013/08/the-de-evolution-of-my-laptop-battery.html
  40 # See also
  41 # http://blog.sleeplessbeastie.eu/2013/01/02/debian-how-to-monitor-battery-capacity/
  42 logfile=/var/log/hjemmenett-battery-status.log
  43
  44 files="manufacturer model_name technology serial_number \
  45     energy_full energy_full_design energy_now cycle_count status"
  46
  47 if [ ! -e "$logfile" ] ; then
  48     (
  49         printf "timestamp,"
  50         for f in $files; do
  51             printf "%s," $f
  52         done
  53         echo
  54     ) > "$logfile"
  55 fi
  56
  57 log_battery() {
  58     # Print complete message in one echo call, to avoid race condition
  59     # when several log processes run in parallel.
  60     msg=$(printf "%s," $(date +%s); \
  61         for f in $files; do \
  62             printf "%s," $(cat $f); \
  63         done)
  64     echo "$msg"
  65 }
  66
  67 cd /sys/class/power_supply
  68
  69 for bat in BAT*; do
  70     (cd $bat && log_battery >> "$logfile")
  71 done
  72 </pre>
  73
  74 <p>The script is called when the power management system detect a
  75 change in the power status (power plug in or out), and when going into
  76 and out of hibernation and suspend.  In addition, it collect a value
  77 every 10 minutes.  This make it possible for me know when the battery
  78 is discharging, charging and how the maximum charge change over time.
  79 The code for the Debian package
  80 <a href="https://github.com/petterreinholdtsen/battery-status">is now
  81 available on github</a>.</p>
  82
  83 <p>The collected log file look like this:</p>
  84
  85 <pre>
  86 timestamp,manufacturer,model_name,technology,serial_number,energy_full,energy_full_design,energy_now,cycle_count,status,
  87 1376591133,LGC,45N1025,Li-ion,974,62800000,62160000,39050000,0,Discharging,
  88 [...]
  89 1443090528,LGC,45N1025,Li-ion,974,4900000,62160000,4900000,0,Full,
  90 1443090601,LGC,45N1025,Li-ion,974,4900000,62160000,4900000,0,Full,
  91 </pre>
  92
  93 <p>I wrote a small script to create a graph of the charge development
  94 over time.  This graph depicted above show the slow death of my laptop
  95 battery.</p>
  96
  97 <p>But why is this happening?  Why are my laptop batteries always
  98 dying in a year or two, while the batteries of space probes and
  99 satellites keep working year after year.  If we are to believe
 100 <a href="http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries">Battery
 101 University</a>, the cause is me charging the battery whenever I have a
 102 chance, and the fix is to not charge the Lithium-ion batteries to 100%
 103 all the time, but to stay below 90% of full charge most of the time.
 104 I've been told that the Tesla electric cars
 105 <a href="http://my.teslamotors.com/de_CH/forum/forums/battery-charge-limit">limit
 106 the charge of their batteries to 80%</a>, with the option to charge to
 107 100% when preparing for a longer trip (not that I would want a car
 108 like Tesla where rights to privacy is abandoned, but that is another
 109 story), which I guess is the option we should have for laptops on
 110 Linux too.</p>
 111
 112 <p>Is there a good and generic way with Linux to tell the battery to
 113 stop charging at 80%, unless requested to charge to 100% once in
 114 preparation for a longer trip?  I found
 115 <a href="http://askubuntu.com/questions/34452/how-can-i-limit-battery-charging-to-80-capacity">one
 116 recipe on askubuntu for Ubuntu to limit charging on Thinkpad to
 117 80%</a>, but could not get it to work (kernel module refused to
 118 load).</p>
 119
 120 <p>I wonder why the battery capacity was reported to be more than 100%
 121 at the start.  I also wonder why the "full capacity" increases some
 122 times, and if it is possible to repeat the process to get the battery
 123 back to design capacity.  And I wonder if the discharge and charge
 124 speed change over time, or if this stay the same.  I did not yet try
 125 to write a tool to calculate the derivative values of the battery
 126 level, but suspect some interesting insights might be learned from
 127 those.</p>
 128
 129 <p>Update 2015-09-24: I got a tip to install the packages
 130 acpi-call-dkms and tlp (unfortunately missing in Debian stable)
 131 packages instead of the tp-smapi-dkms package I had tried to use
 132 initially, and use 'tlp setcharge 40 80' to change when charging start
 133 and stop.  I've done so now, but expect my existing battery is toast
 134 and need to be replaced.  The proposal is unfortunately Thinkpad
 135 specific.</p>