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.
(Please note the credits for this section.)
There are currently three types of batteries commonly used for portable computers.
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)
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).
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)
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. A good indication for an external recharging unit is when your ac adapter uses a proprietary connector with a lot of power lines.
Other manufacturers put the recharging unit inside the laptop case where users cannot simply replace it with a newer technology. If your ac adapter only uses two power lines to connect to the computer (just like mine), the recharging unit is probably inside the laptop.
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.
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...
(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.'')
(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 machines' little batteries have to be replaced within minutes...)
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.
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.
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.