After first time battery calibration, the battery time left icon is not right

Discussion in 'MacBook Pro' started by iluvnachos30, Aug 16, 2010.

  1. iluvnachos30 macrumors newbie

    Joined:
    Aug 16, 2010
    #1
    i just finished calibrating my battery for the first time this morning. i just opened my macbook pro to see that the time left icon jumps every minute. it goes from 8:50 to 12:38 to 6:54 to 11:37 to 7:40! and now it went from 8:45 to 9:19! the percentage remaining is consistent and correct but the time left is way out of whack! sorry for the slang! now it went from 9:19 to 9:11 then 8:47. i have the screen at the lowest brightness, no backlit keyboard, nothing else running except firefox. i wasnt aware that i was supposed to calibrate my battery and its been about 3 months since i bought this. now its at 8:37 now 8:18. this is quite annoying and i thought battery calibration will give me a more accurate reading! please help! thank you!
     
  2. Repo macrumors 6502a

    Repo

    Joined:
    Feb 11, 2009
    #2
    The percentage won't jump around because it's a physical read out. The time remaining is just an estimate, and will constantly change as you use your MacBook.
     
  3. iluvnachos30 thread starter macrumors newbie

    Joined:
    Aug 16, 2010
    #3
    but it wasnt as crazy as it was with the estimations before the calibrations. at least it was sort of consistent, estimating within the same hour but now it jumps from hour to another hour to 2 or 3 hours! could i have calibrated wrong?
     
  4. Pax macrumors 6502a

    Joined:
    Dec 12, 2003
    #4
    The Mac is probably working in a way which puts a highly variable load on the battery. For example the CPU might be working hard one minute, then idle the next minute. Or the HDD, or the screen.

    If you leave your Mac with no programs open and don't touch it, I'm sure the battery indicator will settle down. Or look in Activity Monitor at your CPU load, is it jumping up and down?

    I am sure calibration didn't cause this, it's probably just a coincidence.
     
  5. iluvnachos30 thread starter macrumors newbie

    Joined:
    Aug 16, 2010
    #5
    @Pax: wow, im sorry but im fairly new with mac and i dont really understand what ur saying! but it hard to believe its a coincidence tho! i mean, right after calibration it got like this! oh well, just as long as this doesnt decrease the performance of the macbook, im okay with it. ill just have it show the percentage instead of the estimated time left!

    Edit: wait, no. after reading it a couple of times, i understand what ur saying. its not jumping huge, sometimes like 1 or 2 bars. its okay. again, just as long as there isnt something seriously wrong with it. maybe it might get fixed after i calibrate again next month or whenever!
     
  6. Pax macrumors 6502a

    Joined:
    Dec 12, 2003
    #6
    OK
    The battery contains a certain amount of power. Measured in milliamp hours (mAhr). That's so many milliamps of current for so many hours

    When full your battery contains eg 6000 mA hr. So it can power something needing 6000 mA for 1 hour, something needing 3000 mA for 2 hours etc. Or something needing 600 mA for 10 hrs.

    Using a car analogy - your gas tank contains 10 gallons. That will drive you 300 miles at 70 mph, or 100 miles at 120 mph.

    The % figure is the amount of the 6000 mA hr which is left in the battery (amount of gas in the tank). The Mac knows that figure pretty accurately, because it can keep track of what goes in and what comes out (pretty good gas gauge). So if you have 5000 mA hr remaining, it will read 80%.

    But how long will 6000 mAhr last you? It depends how much power your Mac uses. And that varies a lot from moment to moment. In particular your CPU can use anything from about 500 to about 3000 mA depending on how hard it's working. Add in other components (screen, WiFi etc) to get 700 - 3500 mA. Note that's anything from about 10 hours to <2 hours depending on what you're doing.

    Every 30 second (?) your Mac looks at how many mA the CPU and other components are using. It then divides the power remaining in the battery by that figure to get a runtime. For example, let's say your CPU is completely idle. Your Mac might use 700 mA. If your battery is full with 6000 mAhr in it your runtime will be 8.5 hours. But let's say 30 seconds later, Firefox is working your CPU harder and your Mac is using 850 mA, runtime will drop to 7 hours.

    The truth is, your Mac has absolutely no idea how long your runtime will be - because it doesn't know what you are going to ask your CPU to do in the future. When it says "runtime 8 hours" it means "if you carry on doing exactly what you are doing now, you will get 8 hours". But if you start gaming, you will only get 2 hours. Because CPU load jumps up and down from second to second, so does your runtime.

    Calibration only affects the % figure - it give the Mac a better idea of how much power can fit in the battery. Calibration does directly not affect the runtime figure.

    It's just the same as your car. If you have one of those fancy driving computers it will predict how far you can drive on the gas in your tank. But only if you drive like you are now. If you start going up a steep hill, it will change the range prediction a lot. When you go down the hill the prediction will go the other way.

    You can see how much power your Mac is using by opening System Profiler and clicking on Power. Look at the Amperage figure. Every 30 seconds that figure will change (but you have to press Cmd-R to refresh it). I expect you will see that amperage figure bounce up and down a lot as your CPU load changes. To get the runtime figure I believe your Mac just divides Charge Remaining by Amperage to get hours runtime.
     

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