Saturday, April 16, 2016

Reducing Laptop Power Consumption

As I found out in an earlier post, my laptop uses about 2.1A when its battery is fully charged and about 5.6A when the battery is depleted. At typical lead-acid battery voltage (~13V) these translate to about 27 and 73 watts, respectively. This makes a laptop one of the largest drains on a battery when imaging. Is there a way to reduce this significantly?

The answer is definitely yes!  Let's start with a review of laptop power consumption as measured by another blogger. He found these amounts for a laptop very similar to mine:
  • Laptop off, battery charging: 54w
  • Laptop on, idle (display at full brightness, WiFi on, power management "balanced": 20w. 
Notice that an idle laptop that's also charging the battery will consume 74w, essentially the same amount I found. Your laptop may have different power use, but it will probably be in the same area.

Next he tried a few methods of reducing the power use with the following results:
  • Set screen brightness to dimmest setting: 4w reduction
  • Turn off hard drive: no reduction
  • Disable WiFi: 2.5w reduction
It's possible to completely turn off the display saving a little more (the specifics in in a moment). My guess is that powering down the display will reduce power by about 6w.

The total reduction by turning off the display and disabling WiFi is therefore about 8w or so. That's about 2/3A when running off a 12V battery, and over a four hour imaging session it will amount to 2.7Ah. That's not a big amount, but it's not insignificant. Recall that my summer star party standard was 16 hours of imaging. The saving in that amount of time is 10.7Ah.

So how does one turn off the laptop display? Luckily there's a tiny program for Windows that does just that. It's appropriately called "Turn Off LCD" and it can be found here. Unzip it, put the exe file on your desktop and double click it whenever you want to shut off your display. This does not affect any programs that are running; guiding software and image downloads will continue normally. To restore the display just move your cursor or hit a key.

You can assign the program to a key stroke if you wish. The way I've done this is to use AutoHotkey. Here are the steps needed to do that if:
  • Download and unzip Turn Off LCD, then move the exe file to the folder of your choice.
  • Download and install AutoHotkey
  • Right click your desktop and choose New / AutoHotkey Script. If this options isn't available it's probably because AutoHotkey isn't running; start it using the Start Button / All Programs / AutoHotkey / AutoHotkey
  • Add your hotkey specification after the script's boilerplate. As an example, here's my script for binding Turn Off LCD.exe to the key combination of the Windows key and s:
         run, TurnOffLCD.exe

    Note that I've renamed the exe file to omit the blanks because I don't like file names that include blanks (I was raised in the era of DOS 3 world of 8.3 file names, what can I say?) You may need to specify a path to the exe file--it's just prepended to the exe file name.
    You can bind the exe to any key you want; more information is provided in the AutoHotkey help. Read it and learn.
  • Save the script file with a name that makes sense to you. I called mine ScreenOff.ahk
  • Right click it and choose Compile Script. This will create ScreenOff.exe.
  • Move the compiled script to the Windows Startup folder so that it will be loaded every time your start your laptop.
  • Hit Windows + s to turn off the display. Move the cursor or hit any key to resume.
Remember to disable WiFi and Bluetooth if you don't need them.

Any software that you don't need should be disabled or turned off, as it will only make your CPU, HDD, and graphics adapter work harder and consume more power. Software that should be disabled while imaging includes all security software, Windows Update, and Windows Defender. If you're not on a network none of that is needed or useful.

Unless your laptop's CPU is slow and feeble you can change its power settings to run slower and use less power. Windows usually supports three modes: High Efficiency, Balanced, and Power Saver (the one you want.) Be sure your laptop's battery is fully charged before you take it into the field.

Do all this and you may be able to reduce your laptop's power consumption by 1/3 to 1/2.

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