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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 capasity. My
54 colletor 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 parallell.
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 mylaptop
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 </div>
149
150 <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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485
486 <li><a href="http://people.skolelinux.org/pere/blog/tags/video">video (54)</a></li>
487
488 <li><a href="http://people.skolelinux.org/pere/blog/tags/vitenskap">vitenskap (4)</a></li>
489
490 <li><a href="http://people.skolelinux.org/pere/blog/tags/web">web (37)</a></li>
491
492 </ul>
493
494
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