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Old 10-30-2008, 01:21 PM   #1
jharker
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iLiad Power Use: A discussion (or, Leave your stylus at home)

As part of my work on battery replacement, I also did some basic power analysis.

Summary:
  • The Wacom digitizer uses almost no power when turned on and idle (i.e. with the stylus out of the slot).
  • Ethernet uses plenty of power.
  • Scribbling uses A LOT of power.
  • You lose about 7-8 seconds of battery time for every page flip.
My data is based on a program I have that queries the iLiad's battery kernel module. It returns the % charge remaining, estimated time remaining, and most importantly the current draw (in mA) averaged over the past 1-2 seconds or so. I recorded the current every second for 4 minutes under various conditions: with the stylus in, with the stylus out, while scribbling, turning pages, etc.

Here are the details:
  • Baseline (everything off): 78.0 mA
  • Stylus out: 80.7 mA
  • Ethernet turned on: 173.5 mA
  • Pen out, scribbling continouously in a PDF: 307.6 mA
  • Page turn forward in PDF: 7.4 seconds lost.
  • Up arrow pressed in ContentLister: 5.1 seconds lost.
These numbers are rough and may be a bit incorrect.

The last two results are interpreted as follows: Let's say that your iLiad would run 10:00 hours idle. If you turn eight pages, your iLiad will shut down one minute earlier, at 9:59. (8 * 7.4 seconds = ~1 minute.)

Similarly, for every minute you spend continuously scribbling you lose three minutes of battery time. For every minute you have ethernet up, you lose about 1m15s of battery time.

The most surprising thing to me is that the Wacom device appears to use almost no power when on. I thought conventional opinion said that having the pen out shortens battery life substantially? My research says that it doesn't.

So if you want to, you can just leave the original stylus in a drawer and carry around a different stylus. Your battery life won't be affected much, and you can use a more comfortable pen without much inconvenience.

I meant to make this work more extensive and comprehensive, but sadly my thesis demands attention, so I decided to just post what I had. Hope you all find it interesting!
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Old 10-30-2008, 01:28 PM   #2
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By Ethernet do you mean WiFi?
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Old 10-30-2008, 01:42 PM   #3
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Nope, I mean hard-wire ethernet. Literally I ran "ifup eth0" and then did the battery test. I haven't had a chance to test wireless yet. That's part of what I meant about my report not being comprehensive. Damn thesis...
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Old 10-30-2008, 01:50 PM   #4
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Quote:
Originally Posted by jharker View Post
The most surprising thing to me is that the Wacom device appears to use almost no power when on. I thought conventional opinion said that having the pen out shortens battery life substantially? My research says that it doesn't.
I think your observation is correct. I've taken the pen out in my daily use, and I didn't see noticeable difference in battery time.
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Old 10-30-2008, 02:47 PM   #5
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I'd be fascinated to see a graph of the current draw over time when you take the stylus out. I've wondered if the Wacom stuff has some sort of timeout that pushes it into a low power mode if it isn't used.

Many thanks for the figure tho' Very interesting.
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Old 10-30-2008, 03:44 PM   #6
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Originally Posted by LazyScot View Post
I'd be fascinated to see a graph of the current draw over time when you take the stylus out. I've wondered if the Wacom stuff has some sort of timeout that pushes it into a low power mode if it isn't used.
iRex originally said it's supposed to turn it off if not used for a couple minutes, but most user's say that it never really happens. I'm not sure if iRex has ever confirmed whether or not it really turns off.
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Old 10-31-2008, 02:01 AM   #7
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Are you also able to return the current CPU speed with the data?
It would be interesting to see how the throttling is handled in each case.
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Old 10-31-2008, 09:14 AM   #8
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Are you also able to return the current CPU speed with the data?
It would be interesting to see how the throttling is handled in each case.
As a matter of fact, the program does. I think the throttling is actually pretty straightforward, though: when the load goes up, the speed goes up. When the load drops, the speed drops.

I don't know if I'll have a chance this weekend, but I'll post the current version of the program on Monday along with some current vs time plots.
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Old 10-31-2008, 09:31 AM   #9
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Quote:
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iRex originally said it's supposed to turn it off if not used for a couple minutes, but most user's say that it never really happens. I'm not sure if iRex has ever confirmed whether or not it really turns off.
Now you mention it, that rings a bell. It would be interesting to see if it can be confirmed with this.

Also thinking about the power when scribbling, is it possible to estimate the power ignoring the display re-draws that are going on? I guess it would be relatively easy to modify something like iPDF so that it doesn't do the display updates.
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Old 11-02-2008, 08:04 PM   #10
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Originally Posted by LazyScot View Post
Now you mention it, that rings a bell. It would be interesting to see if it can be confirmed with this.

Also thinking about the power when scribbling, is it possible to estimate the power ignoring the display re-draws that are going on? I guess it would be relatively easy to modify something like iPDF so that it doesn't do the display updates.
Then you wouldn't know what you had drawn.
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Old 11-04-2008, 05:59 AM   #11
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Then you wouldn't know what you had drawn.
True, but it would at least allow you to separate the power used by re-drawing from the power used by the Wacom and processing of its input. The idea was just a simple alteration to the program simply to see the power split between redraw and touch pad.
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Old 11-05-2008, 03:42 PM   #12
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If the Wacom tablet isn't drawing much power when not in use, why is the battery life so short compared to the Sony and Cybook?
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Old 11-05-2008, 03:57 PM   #13
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If the Wacom tablet isn't drawing much power when not in use, why is the battery life so short compared to the Sony and Cybook?
Do you mean, why is the battery life of the iLiad shorter than Sony or Cybook?
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Old 11-05-2008, 04:02 PM   #14
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Yes, exactly. Even when we're just reading, the battery lasts for a much shorter period of time than the Sony or Cybook. I always thought this was because the Wacom screen was drawing power. Apparently it isn't. Is it because the CPU is always on? Does the Sony actually stop the CPU between page turns?
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Old 11-05-2008, 04:22 PM   #15
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why is the battery life so short compared to the Sony and Cybook?
The closest comparison is to the Kindle, which also uses a PXA25x CPU but lasts a few days to a week if networking is turned off. The Kindle has a suspend mode, which the iLiad does not.
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