Does the iPhone run faster on a full battery?

Discussion in 'iPhone' started by Bawstun, Oct 20, 2010.

  1. Bawstun macrumors 65816


    Jun 25, 2009
    OK, either it's my imagination, or when the iPhone comes off of a full charge, or even a 90% charge, it runs a lot faster than when it's down to 30,40,or 50%.

    No, I don't reboot or restart my device once it is fully charged, or before I decide to charge it. I'm talking about the same amount of apps running, same standby stats, etc.

    I'm wondering if it's some feature that Jobs has silently built into the phone where it runs faster/uses more processor speed when the phone is fully charged as to where it slows down when the battery gets depleted and opens apps or tasks a little bit slower to save battery life when it's down to 60,50,40% etc.

    Am I crazy or is this true? I can't help but 'notice' after I'm done charging the phone how fast my Messages app opens, but when my battery is almost drained how slow almost everything opens, including Messages.
  2. QuarterSwede macrumors G3


    Oct 1, 2005
    Colorado Springs, CO
    My battery is currently at 25% and my iPhone 4 still opens up apps with the quickness. There's no difference to me.
  3. murdercitydevil macrumors 68000


    Feb 23, 2010
    Can your car drive faster with a full tank of gas?
  4. chenks macrumors 6502a


    Oct 23, 2007
    some might say that it would be slower with a full tank, due to the extra weight of the fuel.
  5. yamabushi macrumors 65816


    Oct 6, 2003
    Actually many cars do have slightly more available horsepower on a full fuel tank. The extra weight of the fuel will usually more than offset the power gain though. Most cars will drive slightly slower on a full fuel tank. Anyways, the car analogy doesn't work in this case.

    Many electronic devices do adjust performance down to conserve battery life when the charge is low. I do not know offhand if the iPhone does this or not. I suspect it does not.

    The slowdowns the original poster experiences are more likely memory or even heat related.
  6. JulianL macrumors 65816

    Feb 2, 2010
    London, UK
    No. The iPhone is like a digital wristwatch; the iPhone's speed is determined by an electronic clock that ticks away at a defined number of ticks per second, just like the timekeeping on a digital wristwatch is maintained by a (precisely known) number of ticks per second marking the passage of time.

    A non-faulty digital wristwatch will keep just as good a time when its battery is 99% of the way through its working life as it did on the day, perhaps 2 years earlier, when the user bought it with a fully charged battery installed. The same principle applies to the iPhone as it runs through its battery charge.

    Edit: Oops, sorry. I read the full post (really should do that before replying!) and suspect that you understand the principles but I'll leave the above for people that might not.

    As you say, it's just possible that the iPhone downclocks under low battery conditions but if so then I really find it hard to imagine that it would do it at even 30% let alone 40% or 50%. There is still a reasonable safety margin in terms of run time when one gets the first low battery notification at 20% charge so if it was going to go into a "panic" mode and downclock then I would have thought that the notification points (20% and 10%) would be the logical times to do it, certainly not 50%.

    - Julian
  7. sammich macrumors 601


    Sep 26, 2006
    Don't think any modern car that has computerised fuel injectors would have any extra power. They control the fuel/air mixture precisely which controls the amount of fuel and hence the power.

    In any case, it's the software that controls the performance of the hardware. If the hardware (ie battery) decides the performance then there are so many problems that would arise, ie components will turn off if there isn't enough current.
  8. Fliesen macrumors 6502a


    Mar 30, 2010
    It's your imagination, you are crazy ;)
  9. orpheus1120 macrumors 65816


    Jan 23, 2008
    Wahaha. I run faster with a full stomach though.
  10. scaredpoet macrumors 604


    Apr 6, 2007
    iPhones are pretty much known to be underclocked by default. The original iPhone as well as iPhone 3G and 3GS used ARM chips that were underclocked from their rated values, and while we don't have official specs on the A4 chip in the iPhone 4, people suspect the same thing: it runs slightly slower than the 1GHz it can run, and DOES run on the iPad.

    This underclocking is true even with a full battery. So, I have my doubts it would underclock even further when the battery gets low.

    In any case, you'll likely get better battery savings by, say, reducing the brightness on the screen or turning off 3G and going with WiFi/EDGE than you would by slowing the CPU.
  11. kdarling macrumors P6


    Jun 9, 2007
    First university coding class = 47 years ago
    Re: underclocking. No one knows what the actual speed rating is for the chips used by Apple. People are just going by similar available chips. Especially for Apple, it's not economical to pay for testing at speeds that will never be used. Lower speeds = more yield = cheaper parts = Apple profit. For all we know, the max speed used is the max rated speed for these parts.

    Re: faster with a full charge. Almost every smartphone has optional utilities (sometimes requiring a root or jailbreak) that automatically lower cpu speed when the battery is low. Apple does something similar on their MacBooks where they lower the cpu speed if the battery is removed while on A/C, to prevent overloading the power brick.

    So it would not be out of the ordinary if Apple, who has taken extreme measures before to save battery on the iPhone, also clocked the CPU per remaining charge. However, there's been no good empirical evidence (e.g. slower benchmarks) to show this yet.
  12. g35 macrumors 6502a

    Dec 13, 2007
    I haven't noticed anything at 30% etc but definitely have noticed a slowdown in the single digits of battery %. Nothing major, but it stutters when, say, closing an app sometimes.

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