Laptop Power Efficiency
Joe Shuster
The current slew of inexpensive digital SLR cameras, dedicated astrocameras and good quality webcams has been a great boon to novice astroimagers. People are able to get started in imaging with a relatively small investment in cameras. However, many of the best uses of these cameras require the use of a laptop at the site of the telescope and camera. Supplying power to the laptop at home, near AC power is simple to address: Just get a long extension cord. However, many folks want to use their laptops at dark sites where AC power isn't available. For those people, it's necessary to use some kind of battery power to keep the laptop going during the imaging session.
When you consider a field (not home) use battery to power a laptop, you're faced with four choices: 1) Live and die by the laptop battery 2) Use a 12v battery and an DC/AC inverter 3) Use a 12v battery and a 12v-?? volt converter 4) Use an integrated battery/AC supply (basically #2 in a single package).
Which is best is beyond the scope of this article. But however you supply power to your laptop, you'll want to get the most out of your power source. And that IS the topic of the article.
Most laptops provide three ways to control the efficiency of the power coming from the battery: 1) Where the power goes 2) How the operating system applies power to devices 3) How the operating system manages power in real-time
In this article, I'll present some ideas on making each of these areas most efficient so you can get the most from your battery. (I can only speak for Windows-based PCs, so I won't be considering portable Mac's.)
Where the power goes. When your laptop is plugged into normal AC power, the laptop will use some of the incoming power to operate the computer and it will use some of the of the power to keep the internal battery fully charged. This process of charging the battery is either a large drain on your power source (if the internal battery is really drained) or it's a mild drain on the power source if the internal battery is "fully charged". I've found that even a fully charged battery seems to consume some of the incoming power. So one of the best ways to operate if you are using options 2, 3 or 4 above is to remove the internal battery for the field session. This way, the power source is not trying to charge the battery AND run the computer at the same time. Most internal batteries can be removed without tools. Be sure to check your laptop's instructions to make find out how to remove the battery if it's truly user removable.
Applying power to devices. Under normal circumstances, Windows will operate all the devices attached to a computer. However, you can override that and specify to Windows which devices to ignore by disabling them in the Device Manager. Once you disable a device, system resources (memory, etc.) are not allocated. That's only a small benefit to power consumption, but if it's something like a floppy disk or CD reader, you can get power and operating efficiency by disabling devices in the Device Manager. The benefit is greater if your system is low on memory or if you run a lot of programs at once because the freed memory can help you avoid paging - meaning faster processing and less disk reads and writes.
There are less obvious devices that consume power in a sneaky way. For example, an IR interface on the laptop seems to be dormant when not in use, but if you have an IR-sensitive camera, you can see that many laptops routinely flash IR signals - pretty pointless if you're with your laptop in the field! Disable that IR port!
So I advise disabling all unnecessary devices using the Device Manager. Of course, enabling and disabling devices can be a headache. So to make things simple, Windows provides the concept of a "hardware profile". You can have a standard (normal) profile with all devices enabled and you can create a new profile with many devices disabled for your high efficiency needs in the field. To do this in Windows/XP, use the Control Panel "System" option, and click on Hardware Profiles. Make a copy of the Standard profile (I call my copy "High Efficiency").
Now that you have two hardware profiles, Windows will want you to pick one when you reboot the computer. When you reboot, there should be a dialog asking you to pick a profile. (You can tell Windows to use a default profile if you don't make a choice.) To configure your new profile, pick it when you reboot. Then just go to the device manager and disable things like the Ethernet port, floppy disk, CD, modem, and IR port. Windows will save these settings in the new profile. Then you can reboot normally (using the Standard profile). The next time you want the most efficiency from the laptop, you can just boot the system using your high efficiency profile.
Managing power in real-time. Windows provides for automatic and dynamic power management. Primarily this is used to control the use of the internal battery when AC power is disconnected. After all, it's presumed that AC power is unlimited so there's usually not much need for power management. (From the wall it is unlimited. From your 12v battery it isn't!) Some laptops use the built-in power features of Windows. However, other vendors have a separate set of controls to let you specify how much power is to be used and where it will be used.
If your laptop uses the Windows power management features, you'll be limited to things like how long the disks will spin before being powered down. You can save power by setting this period short so you don't waste battery power spinning the disk when it's not being used. However, if it takes long to respin the disk, you might have some timing problems with imaging. In general I set a spin period slightly longer than my longest long exposure image. That assures me that the disk should be spinning when the camera is ready to record another image. On the other hand, if I'm scratching my head or otherwise occupied, I don't mind if the disk spins down.
On my Toshiba laptop, the Windows power management features are replaced with a proprietary Toshiba panel. (I don't know how many other laptop vendors supply custom power controls.) The Toshiba controls are more robust than the Windows controls. With the Toshiba, you can specify the display brightness, CPU speed, and CPU cooling method. You can collect settings in power profiles. You can switch between power profiles use a control panel applet or via the system tray.
Toshiba also allows you to specify different settings (eg, display brightness) depending on the state of the internal battery (full, high, medium or low charge) so you can squeeze more power out of a weak battery. In my case, I've created a copy of the default "Full Power" (plugged in) profile. I've adjusted the LCD brightness to its lowest setting and it will turn off after 3 idle minute. I've set the disk spindown period to 5 minutes. I've turned off automatic standby mode because I don't want the laptop to shut down. I leave the CPU power at its maximum, because I found that lowering the CPU speed has a horrible impact on overall processing time: program loads are slow and capturing images is extremely slow. I haven't found that changing the CPU cooling method matters much in performance or power efficiency so I've left it at the standard ("Maximum") setting. All these settings are saved in my "Inverter Power" power profile. After I boot the system (with the internal battery removed and using my "High Efficiency" hardware profile) Windows sees that I am "plugged in" (via inverter) to AC, so it presumes I want to use the power to the max. I have to tell Windows to switch from the power-guzzling "Full Power" power profile to my "Inverter Power" profile. From then on, I can operate with very good battery efficiency through the night.
After the night is done and I want my laptop to behave normally, all I need to do is replace the battery, boot up with the Standard hardware profile and let Windows use the "Full Power" power profile.
So you can get a little more out of whatever non-grid power strategy you employ if you take a few steps to make the power environment more streamline.
You should check with your laptop vendor to see if their product has a corresponding special power management system. You can find more about efficiency and Windows Power Management at the following sites:
PC magazine:
www.pcworld.com/howto/article/0,aid,117424,00.asp
Intel: www.intel.com/products/notebook/centrino/enablingbatterylife.pdf
Microsoft: www.microsoft.com/windowsxp/using/setup/learnmore/russel_02march25.mspx
