<para>There are currently two competing standards for providing power management: APM and ACPI. Both cannot be used at the same time, so which one is best for your situation? If you have a relatively recent (>2.4.10) kernel and are not intimidated by kernel builds and patching source code, you'll find many benefits with the flexibility of ACPI. If you just want to enable generic power management, or are using an older machine, choose APM. Neither method spins down idle hard drives; use hdparm for that instead. Either way, your system's BIOS must correctly support the power management scheme you'd like to use as well; if your system does not fully support either standard, some of the power management options might crash your system and/or cause data loss. You have been warned!</para>
<para>Regardless, you should be aware that even if you don't enable any power management on your laptop, on the x86 architecture Linux will always issue the "hlt" instruction to your processor whenever nothing needs to be done. This results in lowering the power consumption of your CPU. Note that the system doesn't power down when it receives the hlt instruction; it just stops executing instructions until there is an interrupt.</para>
<para>Some system manufacturers may have omitted the pre-ACPI tables used for
SMP configuration. In this case, ACPI is required. If a system supports
HyperThreading, it must use ACPI tables to discover all the virtual processors
present. IA64 machines require ACPI, and NUMA servers are starting to require
it for proper initialization as well. There is generally no advantage to
enabling either type of power management on servers or workstations that do not fall into
<para>The basic goal of any power management technique is to reduce an entity's consumption. In the case of laptop power management, our focus is on decreasing CPU and hard drive usage. To make things a bit simpler, this is broken down into <emphasis>obvious, semi-obvious,</emphasis> and <emphasis>non-obvious</emphasis> techniques. Granted, your mileage may vary.</para>
For example, the following provides 32-bit IO support with sync (-c3), DMA support (-d1), Advanced Power Management (-B128), write-caching
(-W1), disk spin down after five minutes (-S60). gains me tremendous performance with added power savings. Note that your mileage may vary, and you'll want to adjust this for your specific system to prevent data loss (especially the -B and -m flags!).</para>
<para>In the following example, we run some read/write benchmarks of our hard drive before and after using <command>hdparm</command>.
Note that while our cache reads remain about the same, our actual physical reads from the drive increase tremendously!
If you like living on the edge, you can play with the -m, -c, -B, and -u switches with caution (see the man page).
<para>Screen blanking under X-Windows is primarily controlled five configuration options in
<command>/etc/X11/XF86Config</command>. The most basic, and useless setting, is <command>BlankTime</command>, which creates a fake blanking effect on the screen.</para>
<para>If you have a DPMS-compliant monitor, you might want to try enabling support for it under the
<emphasis>Monitor</emphasis> section of your <command>XF86Config</command> file:
<para>Additionally, it's worth considering anything in the following areas:</para>
<para><itemizedlist>
<listitem><para>Adjust your system's BIOS settings to decrease or turn off your display's backlight.</para></listitem>
<listitem><para>Adjust your system's BIOS settings to reduce the CPU clock speed while on battery.</para></listitem>
<listitem><para>Avoid using PCMCIA devices while on battery. Better yet, eject your PCMCIA cards when not in use.</para></listitem>
<listitem><para>Avoid using external devices with your computer while on battery.
This includes printers, external monitors, zip drives, and portable cameras.</para></listitem>
<listitem><para>Avoid using built-in devices while on battery. This includes cdroms and floppy drives.</para></listitem>
<listitem><para>Use simple software. A full blown multimedia application will create a lot more system load and disk activity than a small simple word processor</para></listitem>
<listitem><para>Use a simple window manager. While Gnome and KDE are nice, the extra time it takes to load and run is not worth it while on
battery power. One nifty idea is to use a different <command>xinitrc</command> script to launch a different, more simple window manager based on
whether or not your system is on battery power.</para></listitem>
</itemizedlist></para>
</sect2>
<sect2 id="myth">
<title>Power Saving Myths</title>
<para>One popular pastime involved modifying the update/bdflush
<para>There are currently three types of batteries commonly used for
laptops: Nickel Cadmium, Nickel Metal Hydride, and Lithium Ion.</para>
<sect2 id="nicd">
<title>Nickel Cadmium (Ni-Cd)</title>
<para>Nickel Cadmium (Ni-Cd) batteries were the standard technology for years,
but today they are out of date and new laptops don't use them anymore. They
are heavy and very prone to the "memory effect". When recharging a NiCd
battery that has not been fully discharged, it "remembers" the old charge and
continues there the next time you use it. The memory effect is caused by
crystallization of the battery's substances and can permanently reduce your
battery's lifetime, even make it useless. To avoid it, you should completely
discharge the battery and then fully recharge it again at least once every few
weeks. As this battery contains cadmium, a toxic material, it should always
be recycled or disposed of properly.</para>
<para>NiCad batteries, and to a some degree NiMH batteries, suffer from what's
called the <emphasis>memory effect</emphasis>. Memory Effect means that if a battery is
repeatedly only partially discharged before recharging, the battery will
forget that it can further discharge. The best way to prevent this situation
is to fully charge and discharge your battery on a regular basis.</para>
</sect2>
<sect2 id="nimh">
<title>Nickel Metal Hydride (Ni-MH)</title>
<para>Nickel Metal Hydride (Ni-MH) batteries are the cadmium-free replacement
for NiCad. They are less affected by the memory effect than NiCd and thus
require less maintenance and conditioning. However, they have problems at
very high or low room temperatures. And even though they use less hazardous
materials (i.e., they do not contain heavy metals), they cannot be fully
recycled yet. Another main difference between NiCad and NiMH is that NiMH
battery offers higher energy density than NiCads. In other words, the capacity
of a NiMH is approximately twice the capacity of its NiCad counterpart. What
this means for you is increased run-time from the battery with no additional
bulk or weight.</para>
</sect2>
<sect2 id="liion">
<title>Lithium Ion (Li-ion)</title>
<para> Lithium Ion (Li-ion) are the new standard for portable power. Li-ion
batteries produce the same energy as NiMH but weighs approximately 20%-35%
less. They do not suffer from the memory effect unlike their NiMH and Ni-Cd
counterparts. Their substances are non-hazardous to the 0. Because
lithium ignites very easily, they require special handling. Unfortunately,
few consumer recycling programs have been established for Li-ion batteries at
this point in time.</para>
</sect2>
<sect2 id="smart">
<title>Smart Batteries</title>
<para><emphasis>Smart</emphasis> batteries are not really a different type of
battery, but they do deserve special mention. Smart batteries have internal
circuit boards with chips which allow them to communicate with the
laptop and monitor battery performance, output voltage and temperature.
Smart batteries will generally run 15% longer due to their increased
efficiency and also give the computer much more accurate "fuel gauge"
capabilities to determine how much battery run time is left before the next
recharge is required.</para>
</sect2>
<sect2 id="batcomp">
<title>General Battery Care</title>
<para>Even if the battery case looks the same, you cannot just upgrade to
another battery technology unless your laptop has been pre-configured from the
manufacturer to accept more than one type of battery type, since the recharging
process is different for each of the three types of batteries.</para>
<para>A battery that is not used for a long time will slowly discharge itself.
Even with the best of care, a battery needs to be replaced after 500 to 1000
recharges. But still it is not recommended to run a laptop without the battery
while on ac power -- the battery often serves as a big capacitor to protect
against voltage peaks from your ac outlet.</para>
<para>As the manufacturers change the shapes of their batteries every few
months, you might have problems to find a new battery for your laptop in a few
years from now. This is somewhat of a concern only if you anticipate using the
same laptop several years from now. If in doubt, buy a spare battery now - before it's out of stock.</para>
<para>New batteries come in a discharged condition and must be fully charged
before use. It is recommended that you fully charge and discharge the new
battery two to four times to allow it to reach its maximum rated capacity. It
is generally recommend that you perform an overnight charge (approximately
twelve hours) for this. Note: It is normal for a battery to become warm to the touch during charging and
discharging. When charging the battery for the first time, the device may
indicate that charging is complete after just 10 or 15 minutes. This is a
normal with rechargeable batteries. New batteries are hard for the device to
charge; they have never been fully charged and are not broken in. Sometimes the
device's charger will stop charging a new battery before it is fully charged.
If this happens, remove the battery from the device and then reinsert it. The
charge cycle should begin again. This may happen several times during the
first battery charge. Don't worry; it's perfectly normal. Keep the battery healthy by fully
charging and then fully discharging it at least once every two to three weeks.
Exceptions to the rule are Li-Ion batteries which do not suffer from the
memory effect.</para>
<para>Batteries should be stored in a discharged state since they
can self-discharge and may become inactive after a long storage period. They
should not be stored for any length of time while connected to the laptop.
High humidity and temperatures can cause the battery to deteriorate, so these
should be avoided during storage.</para>
<para>Do not remove and carry a battery pack in your pocket, purse, or other container where metal objects (such as car keys or paper clips) could short-circuit the
battery terminals. The resulting excessive current flow can cause extremely
high temperatures and may result in damage to the battery pack or cause fire