Battery Powered Linux Mini-HOWTO: General information

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This section describes a few technical things about laptop batteries and some general power saving tips. This information is not Linux-specific and if you are experienced with laptops, you might already know all this.

2.1 Be kind to your battery

(Please note the credits for this section.)

There are currently three types of batteries commonly used for portable computers.

  • NiCd batteries were the standard technology for years, but today they are out of date and new laptops don’t use them anymore.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.

    (A sidenote about the memory effect. James Youngman knows of a rather drastic method to – uhm – « repair » batteries: « If your NiCd battery is suffering from the memory effect, remove it from your computer, hold it about 30cm above a desk or the floor, and drop it (make sure it lands flat). » He says that this will break the whiskers that have formed in the battery and that are the cause of the memory effect if your battery is already affected by it. « I don’t know if this works for non-NiCd batteries or not. »)

    Cadmium is a very hazardous poison, but if returned to your dealer, the material can almost be fully recycled.

    Just in case you might be interested, here are some specs for NiCd:

    Cell voltage: 1,2 V
    Energy / mass: 40 Wh/kg
    Energy / volume: 100 Wh/l
    max. Energy: 20 Wh
    Charge temp.: 10 to 35 C (50 to 95 F)
    Discharge temp.: -20 to 50 C (-5 to 120 F)
    Storage temp.: 0 to 45 C (30 to 115 F)
  • NiMh batteries are the current standard used in most low price laptops to date. They can be made smaller and are less affected by the memory effect than NiCd.

    However, they have problems at very high or low room temperatures. And even though they use less hazardous and non-poisonous substances, they cannot be fully recycled yet (but this will probably change in the future).

    NiMh specs:

    Cell voltage: 1,2 V
    Energy / mass: 55 Wh/kg
    Energy / volume: 160 Wh/l
    max. Energy: 35 Wh
    Charge temp.: 10 to 35 C (50 to 95 F)
    Discharge temp.: 0 to 45 C (30 to 115 F)
    Storage temp.: 0 to 30 C (30 to 85 F)
  • The new high performance batteries use LiIon technology. In theory, there is no memory effect at all with these batteries, but on occasion, they seem to have similar problems. Their substances are non-hazardous to the enviroment, but they should be returned for recycling as well.

    LiIon specs:

    Cell voltage: 3,6 V
    Energy / mass: 100 Wh/kg
    Energy / volume: 230 Wh/l
    max. Energy: 60 Wh
    Charge temp.: 0 to 45 C (30 to 115 F)
    Discharge temp.: -20 to 60 C (-5 to 140 F)
    Storage temp.: -20 to 60 C (-5 to 140 F)

Even if the battery case looks the same, you cannot just upgrade to another battery technology. The recharging process is different for the kind of battery you use. Some manufacturers integrate the recharging circuit inside the laptop’s external ac adapter, so you might just get away with buying a new power supply to upgrade. Other manufacturers put the recharging unit inside the laptop case where you cannot simply replace it with a newer technology. When in doubt, ask your manufacturer if your laptop supports a more modern battery.

A battery that is not used for a long time will slowly discharge itself. And even with greatest 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.

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. Buy a spare battery now – before it’s out of stock.

2.2 Power saving – The obvious stuff

There are some obvious things that you can do to reduce your system’s power consumption. Well, maybe not so obvious, since not very many people follow these rules…

  • Decrease or turn off your display’s backlight when you don’t need it. By the way, tft displays use more power than dstn (so now you have a fine excuse why you bought the cheaper laptop…).

    (David Bateman tells me that using a crt screen while on battery and turning off the laptop display will extend battery time by about 30%: « Not that this is a very useful piece of knowledge though, if you’ve got the crt plugged in then why not the laptop too. »)

  • How much processing power do you really need? I doubt that you will be doing very much more than text editing when on the road (well, at least I don’t compile linux kernels then). While on battery, reducing the cpu clock speed will decrease power consumption, too. Quite a few laptops offer a cpu clock selector that will toggle between normal and slow speed.
  • Avoid using external devices (printer, crt screen, zip drive, portable camera etc.) with your computer while on battery. When connected to a standard ink jet printer, my laptop’s battery time is reduced from up to 120 minutes down to 20 minutes.
  • Avoid using any built in device unless necessairy: Diskette drive, harddisk, cd-rom. Especially cd-rom access will dramatically decrease your battery time.
  • Pcmcia cards can also consume a lot of power, so don’t leave your modem or network adapter plugged in when it is not in use. But this is different between the various pcmcia manufacturers, so check the product specs before you buy (e. g. some cards never turn themselves off even when not in use).

    (By the way, I recently read that pcmcia cards are the biggest problem for windows ce palmtops – they drain so much power that the tiny computers’ little batteries have to be replaced within minutes…)

  • Use simple software. A full blown multimedia application will create a lot more system load and harddisk / cd-rom activity than a small simple word processor.
  • Grant Taylor has a tip for those of us who want to upgrade their system: « Newer versions of some upgradable components consume less power. For example, IBM’s Travelstar 2.5 inch 1.6 gigabyte ide harddisk drive consumes 20 percent less than the 500 megabyte toshiba harddisk my laptop came with. »
  • If you are yet about to buy a laptop – don’t buy a laptop with a 2nd level cache if battery uptime is important. A computer with 2nd level cache is about 10% to 20% faster and it will be a lot better with multimedia applications and number crunching, but it consumes a lot of power. Bjoern Kriews tells me that he has two almost identical laptops and the one without cache ram runs 4h30 compared to 2h30 with cache.

    If you already have 2nd level cache installed, turning it off will probably not help you very much. Give it a try and write me about your experience.

  • Another tip for those still buying a laptop – don’t buy the latest, fastest cpu type. Usually, the older generations are optimized by the manufacturer after some time without notice. The « new » versions of old cpu types often create less heat and consume less power than the product’s premiere version.

    There are also frankenstein laptops available that use cpus not optimized for portable systems. As I wrote this in May 97, the newest generation pentium-200 laptops ran about 20 minutes on battery and became so hot that they burnt your lap. When writing the second revision in Oct 97, pentium-233 laptops run two hours or longer without ac power. Go figure.

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Well, you get the idea. Most of these are restrictions that will probably stop you from doing any serious work with your Linux system. (The best way to save power while on battery is… not to do anything at all. That increases my laptop’s battery uptime by almost 100 percent.)

So let’s go ahead to some other, more useful measures that will save power without disturbing your work.

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