Backlight keyboard question

Discussion in 'MacBook Air' started by Mmarzex, Aug 2, 2011.

  1. Mmarzex macrumors member

    Joined:
    Aug 23, 2009
    #1
    Hi all,

    I just received my new air 11" yesterday and I couldn't be happier. I have a couple of questions though about the backlight keyboard.

    First does the light in the keyboard really affect the battery that much? And if it does it there a way for me to set that it auto goes off when I unplug it from the wall?

    Thanks
     
  2. simsaladimbamba

    Joined:
    Nov 28, 2010
    Location:
    located
    #2
    The backlit keyboard does not affect the battery that much, it is hardly noticeable. The display and SSD consume more power than the measly LED powered backlit keyboard, at least according to hundreds of threads about this.

    48
     
  3. iRun26.2 macrumors 68000

    Joined:
    Aug 15, 2010
    #3
    I'm rather surprised that I've never seen a thread that posted results from a 'scientific' backlit keyboard battery drain test. It doesn't seem like it would be too hard for one of the new MBA owners to fully charge their battery and then time how long the computer lasts running a constant drain program (but with the backlit keys enabled in one set of the tests).

    My guess is that the backlit keyboard wouldn't use more than 1% of the battery life when on for a five hour period (3 minutes).
     
  4. flynz4 macrumors 68040

    Joined:
    Aug 9, 2009
    Location:
    Portland, OR
    #4
    I would believe that the test-to-test variability would be greater than the test criteria.

    Some things in life just are not worth worrying about because they are infinitesimal in scope. Example... auto manufactures do not measure the impact of radio volume on gas mileage even though there must be a correlation.

    /Jim
     
  5. iRun26.2 macrumors 68000

    Joined:
    Aug 15, 2010
    #5
    I suspect that you are correct, but it would be nice to have solid data proving that is the case. There are still those who believe it has a significant impact (and were happy the BLK was removed)
     
  6. flynz4, Aug 2, 2011
    Last edited: Aug 3, 2011

    flynz4 macrumors 68040

    Joined:
    Aug 9, 2009
    Location:
    Portland, OR
    #6
    Let's do a simple and conservative analysis.

    The smallest MBA battery is 35 W-h (11" MBA)

    A typical LED flashlight certainly puts out more lumens than would be put into the MBA backlight diffuser. The flashlight will run for about 20 hours on two AA batteries... and therefore... each of the AA batteries has enough energy to power the flashlight for 10 hours.

    AA alkaline batteries have about 2800 ma-hr... or 2.8 W-h

    35/2.8 = 12.5.... so from that we could calculate that the MBA battery could power the flashlight for 125 hours... which is 25X the running time of the MBA

    1/25th of the running time of the MBA's 5 hour battery life is about 12 minutes.

    Since the flashlight is significantly brighter than the backlight... (let's conservatively estimate 4X)... the impact to the battery life on a full charged MBA would be about 3 minutes if the backlight was set to maximum. Running the backlight at a more comfortable 50% would half that to 1.5 minutes.

    I suspect that the backlight of the MBA is more effecient than than a couple of AA's connected to a few flashlight LEDs... so I think the 1.5 minute estimate above is extremely conservative. I bet it is actually under a minute of impact.

    YMMV... but for me, this is an infinitesimal impact to the operation of a laptop. For those who do not like this level of impact... I suggest they hit the F5 key a few times and get their minute back.

    /Jim
     
  7. Lyndo99 macrumors newbie

    Joined:
    Apr 20, 2011
    #7
    Just to backup your argument even more, there is a flaw in the above quote. Most flashlights have their batteries wired in series and not parallel. Meaning that it will have twice the voltage of one AA but the same run time as a single AA. If they were wired in parallel your above statement would be true, that each of the batteries contributes half of the run time, if they are in series though, this is not the case. So in your calculations you should be using 20 hours if that flashlight will in fact run for 20 hrs on 2 AA's. Meaning that the LED backlight in the MBA has even less effect on the battery. I am in 100% agreement with you that the backlight has an extremely small effect on the battery life, LED's use almost nothing for power when compared to the other components of the computer.
     
  8. flynz4 macrumors 68040

    Joined:
    Aug 9, 2009
    Location:
    Portland, OR
    #8
    Series or parallel really doesn't make a difference. You are just trading off current for voltage... but the w-h remain the same... and that is the real capacity of the batteries.

    But... your post made me realize that I failed to convert the ma-hr of the batteries to w-h. So my calculations are off by 20% to 50%.... depending upon the battery type used (Alkaline, NiMH, Li-on, etc). Counter-acting that... I didn't account for the lousy design of AA batteries with their very high internal resistance which no doubt is vastly inferior to the batteries in the MBA.

    Bottom line... the duration is probably still close to, or under 1 minute. Not worthy of any concern. The ambient temperature in your house probably has a larger impact than running the backlight.

    /Jim
     

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