Title: The Life and death of a laptop battery
Tags: english
Date: 2015-09-24 20:00

<p>When I get a new laptop, the battery life time of the lap top at
the start is OK.  But this do not last.  I got a feeling that within a
year, the life time is just a fraction of what it used to be, and it
slowly become painful to use the laptop without power connected to it.
Because of this, when I got a new Thinkpad X230 laptop almost two
years ago, I decided to monitor its state to have more hard facts when
the battery started to fail.</p>

<p>First I tried to find a sensible Debian package to record the
battery status, assuming that this must be a problem already handled
by someone else.  I found
<ahref="https://tracker.debian.org/pkg/battery-stats">battery-stats</a>,
which collects statistics from the battery, but it was completely
broken.  I sent a few suggestions to the maintainer, but decided to
write my own collector as a shell script while I waited for feedback
from him.  Via
<ahref="http://www.ifweassume.com/2013/08/the-de-evolution-of-my-laptop-battery.html">a
blog post about the battery development on a MacBook Air</a> I also
discovered
<ahref="https://github.com/jradavenport/batlog.git">batlog</a>, not
available in Debian.</p>

<p>I started my collector 2013-07-15, and it has been collecting
battery stats ever since. Now my
/var/log/hjemmenett-battery-status.log file contain around 115,000
measurements, from the time the battery was working great until now,
when it is unable to charge above 7% of original capasity.  My
colletor shell script look like this:</p>

<pre>
#!/bin/sh
# Inspired by
# http://www.ifweassume.com/2013/08/the-de-evolution-of-my-laptop-battery.html
# See also
# http://blog.sleeplessbeastie.eu/2013/01/02/debian-how-to-monitor-battery-capacity/
logfile=/var/log/hjemmenett-battery-status.log

files="manufacturer model_name technology serial_number \
    energy_full energy_full_design energy_now cycle_count status"

if [ ! -e "$logfile" ] ; then
    (
	printf "timestamp,"
	for f in $files; do
	    printf "%s," $f
	done
	echo
    ) > "$logfile"
fi

log_battery() {
    # Print complete message in one echo call, to avoid race condition
    # when several log processes run in parallell.
    msg=$(printf "%s," $(date +%s); \
	for f in $files; do \
	    printf "%s," $(cat $f); \
	done)
    echo "$msg"
}

cd /sys/class/power_supply

for bat in BAT*; do
    (cd $bat && log_battery >> "$logfile")
done
</pre>

<p>The script is called when the power management system detect a
change in the power status (power plug in or out), and when going into
and out of hibernation and suspend.  In addition, it collect a value
every 10 minutes.  This make it possible for me know when the battery
is discharging, charging and how the maximum charge change over
time.  The log file look like this:</p>

<pre>
timestamp,manufacturer,model_name,technology,serial_number,energy_full,energy_full_design,energy_now,cycle_count,status,
1376591133,LGC,45N1025,Li-ion,974,62800000,62160000,39050000,0,Discharging,
[...]
1443090528,LGC,45N1025,Li-ion,974,4900000,62160000,4900000,0,Full,
1443090601,LGC,45N1025,Li-ion,974,4900000,62160000,4900000,0,Full,
</pre>

<p>I even wrote a small script to create a graph of the charge
development over time.  This show the slow death of my Lenovo Thinkpad
X230 laptop battery:</p>

<img src="images/2015-09-24-laptop-battery-graph.png"/>

<p>But why is this happening?  Why are my laptop batteries dying,
while the batteries of space probes and satellites keep working year
after year.  If we are to believe
<ahref="http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries">Battery
University</a>, the fix is to not charge the Lithium-ion batteries to
100% all the time, but to stay below 90% of full charge most of the
time.  I've been told that the Tesla electric cars
<ahref="http://my.teslamotors.com/de_CH/forum/forums/battery-charge-limit">limit
the charge of their batteries to 80%</a>, with the option to charge to
100% when preparing for a longer trip (not that I would want a Tesla,
but that is another story), which I guess is the option we should have
for laptops on Linux too.</p>

<p>Is there a way with Linux to tell the battery to stop charging at
80%, unless requested to charge to 100% once in preparation for a
longer trip?</p>