Charging the iPhone 4: USB vs Outlet

by Justin Horn on Jul 8th, 2010 @ 4:39 pm

Almost 2 years ago I did this comparison for the iPhone 3G. As the 3GS was similar to the 3G and it’s a time consuming pain in the ass to do this test, I decided to skip it last year. To make up for my break, I tried to make this test a bigger pain by taking readings every 5 minutes. The extra work paid off, the iPhone 4 charging chart is much smoother and accurate!

Test Conditions

  • Started with completely dead iPhone.
  • Wall outlet charged on standard US 120 Volt / 60 Hz using Apple include plug adapter and USB cable
  • USB charged on late 2008 13″ aluminum MacBook.
  • iPhone was checked quickly (1-2 seconds of screen time) every 5 minutes for it’s current charge. This was done for both test so even if it had a minimal impact on charge time, the comparison would still be equal.
  • iPhone screen was kept at very dim level to lessen the impact of checking charge.
  • WI-FI was kept off.
  • iPhone 4 was active on AT&T  3G during charge. I had some activity on the phone while charging, but it was very minimal and should not impact the results.
    • During outlet test I received 2 text, a phone call (canceled call), and 1 voice-mail from the canceled call.
    • During USB test I received 1 text.

Click to enlarge


Stating the obvious, the iPhone 4 takes longer to get a full charge on USB than an outlet. What you may not have realized is how big the difference was, an extra 30 minutes! That’s 23% longer to get a full charge on the USB. If you’ve ever had a completely drained iPhone, you know it takes a bit of time before it turns back on. Plugged into an outlet, it took 3 minutes 11 seconds to come back to life. On USB it took almost 2 minutes longer, 4 minutes 49 seconds.

Obviously the convenience of USB charging is great, but if you’re in a rush to get charged and get out of the house, always go for the outlet.

Compared to the 3G

The 3G took 90 minutes for a full charge on outlet, about 31% faster than a full charge for the iPhone 4′s 130 minutes. On USB the 3G took 140 minutes, about 13% faster than the iPhone 4′s 160 minutes. Keep in mind that the iPhone 4 has about a 20% larger battery, so it makes complete sense that it takes longer to charge.

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View 57 Comments
Featured post, iPhone 4, Tips


  1. [...] Via | WWA [...]

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  6. iPhone 4: USB Vs. Outlet Charging
    July 9th, 2010 9:05 am
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  9. Mark
    July 9th, 2010 10:18 am

    How about new GIGABYTE ON/OFF Charge via USB?

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  12. Justin Horn
    July 9th, 2010 10:45 am

    @Mark Yeah, that would be interesting. I guess it should be closer to power outlet?

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  16. SPL15
    July 10th, 2010 5:16 am

    What you fail to realize (& also ALL Cell phone manufacturers & 99.9% of the Electronics Engineers I’ve worked with) is that Lithium Ion can’t be fast charged at any appreciable rate. This graph shows charge time to 4.2 volts which is only about 70% with a high amperage wall charger or approximately 80% charged with a low amp USB charge.

    The Battery charge gauge cell phones use is based on voltage alone, EVERY rechargeable battery needs a float “bulk” charge at the rated charge voltage. In the case of Lithium Ion it is 4.2 volts, but cellphones tell you the battery is fully charged when it reaches this voltage & NOT when the TRUE full charge is reached which is based on current consumption of the battery. But a current sensing battery meter is impossible on a phone due to a varying load depending on syncing & usage, so phone manufacturers have to use the VERY inaccurate voltage method to display charge. If you force more current into a Lithium ion battery, it will reach 4.2 volts quicker, but it will need a longer bulk float charge at 4.2 volts to reach 100% charge. A slow amperage charge will take longer to reach 4.2 volts but will need less time during the bulk charge. Also slow charging Lithium Ion is better for Battery cycle life…

    So with that, those graphs mean nothing for a REAL full charge, are misleading & just spread misinformation/ignorance of how things REALLY work. ALL high power draw smartphones will exhibit this same charge characteristic. & the battery will be MORE CHARGED if you use USB to charge only to 4.2 volts (100% reading on the phone) & disconnect the charger immiediately when it reaches a suppsed 100% charge according to the phone. Basically, if you charge & unplug the phone when it says 100%, you will have around a 70% charge on a high amperage wall charger & a 80% charge on a low current usb charge. However, if you leave a phone plugged in it will usually do a bulk charge to actual 100% depending on if the manufacturer decided to cheap out on the phone’s internal “smart” charging circuit.

    You sir are speading half ass knowledge across the internet. Just another typical nerd that only enough knowledge to be dangerous. This is how BS in electronics is spread, & it makes my job harder dispelling the BS you people spread. Your article should have read, “iPhone takes 23% longer to reach 4.2 volts on USB than Wall Adaptor using my fairly uncontrolled methods, however time to actual full charge is beyond the scope of my knowledge because the iPhone doesnt tell me this information.”

    By the way, I dont use the iPhone. My open source phone that isn’t locked down by dictator jobs does the same exact thing this article talks about, so did all the other smartphones I’ve owned. None crApple. The full expanation of why what I said is 100% true is beyond what I want to type on here, but email me & I’ll provide you with more info & explanations than you thought existed on the very topic you wrote about. Maybe you could write a good article on the science behind all this instead of posting a pretty graph & making ill informed assumptions that will cause that much more misinformation through the nets.

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  22. Js
    July 12th, 2010 10:14 am

    @SPL15 – First, you’re an asshole.

    Second, almost 100% of phone users (open source or non-open source phones) don’t care about anything you have posted. They care when the phone says it is charged. So his graph is accurate for phone users even if it is not accurate for EE majors and jerks like you.

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  26. iPhone 4 charging: USB vs Outlet
    July 12th, 2010 3:01 pm
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  29. Henry 3 Dogg
    July 13th, 2010 5:46 am

    Or we could invert the thing and say

    “iPhone charges faster from mains than from USB”

    but you choose to look for a negative.

    The reality is simply that the iPhone can charge faster than a USB port can safely deliver power. This isn’t related to the iPhone specification, its related to the USB specification.

    However, USB ports vary. Some won’t deliver the level of power to charge even at the rate that you quote.

    The message is simple. Charge from the mains when you can and charge from the highest rated USB port that you can find otherwise.

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  32. [...] que en el blog de When Will Apple se han decidido a realizar una prueba con un gráfico de por medio comparando la carga del iPhone 4 [...]

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  34. dy
    July 13th, 2010 2:27 pm

    SPL15 – thanks for the information. However, you could be polite instead of antagonistic.

    I will charge using USB to get more juice on board.

    Would a lower current charger, say 50mA allow even greater charge before 4.2V is reached – ie iphone indicating full charge?

  35. 26.
  36. [...] que en el blog de When Will Apple se han decidido a realizar una prueba con un gráfico de por medio comparando la carga del iPhone 4 [...]

  37. 27.
  38. [...] look at this. The iPhone 4 takes an average of 23 percent longer to charge while charging via USB than it does while charging via a standard electrical [...]

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  40. [...] que al hacerlo directo desde el tomacorriente de la pared, sin embargo hasta este día me entero de alguien que decidió hacer la prueba de una forma no tan científica con su iPhone 4G de Apple y los resultados fueron por lo menos [...]

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  42. Nawaf
    August 17th, 2010 2:41 am

    hello to you, nice testing. I just got something that I kinda came up with by myself, and I think it is true, when I charge my iphone with a USB to full 100% it kind of drain out very faster than when I charge my battery using a power outlet to 100%, with power outlet to full i can use the iphone for 2 days then recharge, but when using USB, it ran out after a day. It’s just a conclusion I came up with, dunno about facts by tests, but its just me feeling that way.

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  44. Ravi
    September 27th, 2010 12:15 pm

    Wow. @SPL. What a douche.

    Like the other guy said. Just about everyone who is gonna look at the chart doesn’t give a shit about all that crap u said. While it was informative, it really has no relevance because what we want to know is if it’s faster/ by how much. And we all use the inaccurate battery meter so get over it.

    U wanna hate on him because he’s not an electrical engineer and performed a simple at home experiment and shared it with us?? Get a life.

    It’s fine that u chime in and correct whatever the OP was “wrong” about, but that’s all u needed to do. No point in being a dickhole about it

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  46. Nice
    November 2nd, 2010 4:35 am

    Ravi take your medicines!

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  48. Luigi_PR
    December 5th, 2010 2:47 pm

    Nice graph and info.
    1. If u have time can u do another test with CAR charger??
    2. And to shut up (or confirm) Mr Douchs statement a “charge down” test turning ON flashlight, iPod, and volume and brightness turned up all the way.

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  50. yeah
    December 30th, 2010 1:49 am

    You gotta remember that longer charging time is better for battery life.

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  52. BaSH PR0MPT
    February 28th, 2011 10:07 pm

    Whilst voltage should be stepped down to regulate the power input regardless of socket type, in Australia we have 240V at the wall socket, and it’s never taken anywhere near the times displayed from your test to get a full charge on my phone. Albeit, I never run them entirely flat for the longetivity of their battery.

    I run a pretty popular mixed-theme geek blog, covering gaming, movies, sci fi, just … geek shit. So I will, for the sake of curiosity, replicate this experiment with a new iPhone 4 as soon as I can grab one for testing purposes (so we can agree it’s a fresh battery and thus an ideal test subject), and I’ll drop a line here when I get some results.

    If I don’t remember to, or if you’re curious, feel free to add me on Twitter, @bashpr0mpt, I follow back regardless so bump yo social pool foo’. Here’s hoping my times match yours and I’m not merely in some form of chrono shifting time space distortion and blissfully unaware of it. >_> Black holes, oh my!

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  54. BaSH PR0MPT
    February 28th, 2011 10:23 pm

    Oh, and sorry, I just have to add.

    SPL15. U mad bro?

    It’s okay, you can admit it to strangers on the internet, you’re too poor to own an iPhone. We understand the angst it causes to the peasant class when they can’t get shiny balls of brilliance, comparatively the apex of all human knowledge, science and technology in their chubby little hands.

    I tell you what, @ me on Twitter (address above) and I’ll send you my old iPhone 4 when I replace it simply to run a clean test on a clean battery and because, well … I can. :)

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  56. Michael Freed
    March 9th, 2011 9:01 am

    2 suggestions for next time… put the iphones in airplane mode, and put time on the vertical axis (since that’s what you’re measuring) and charge % on the horizontal.

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  58. Frog
    May 13th, 2011 12:59 am

    This seems strange to me, since the specs for the wall charger allow 1A max current, and the specs for the 2008 13″ MacBook say that one device (the first plugged in) can draw up to 1100 mA (the iPhone draws 1A according to system profiler). So there really *shouldn’t* be any difference…

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  60. Frog
    May 13th, 2011 1:03 am

    @MichaelFreed, you’re right that the phone should be on aeroplane mode, but time should certainly not be on the vertical axis!

    As always, the data from BOTH axes is being measured. Time is the controlled variable, and thus belongs on the horizontal axis. The voltage/charge is the uncontrolled variable and thus belongs on the vertical axis.

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  62. Coach Online Outlet
    May 25th, 2011 3:30 am

    I think both the charging methods have their own advantages. If you have only the option to charge with the usb port of your laptop sitting in a lawn, far from any wall outlet than USB charge is will be a blessing.

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  64. SPL15
    May 30th, 2011 10:29 am

    I lick dog poo.

  65. 41.
  66. [...] time frame and I wonder if 2 hours would really charge my iPhone to full power anyway? Check out Justin Horn’s charging time test of the [...]

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  68. Matt
    November 3rd, 2011 7:34 pm

    Actually SPL15 is very much wrong. While I’m not sure about the iPhone my Treo was able to tell me how much amperage the input was receiving and how much it was being drained. Most cell phone batteries, like any other lithium ion battery does have its own programmed fuel gauge for safety reasons.

    With that said my Treo itself receives the battery voltage from the battery and not the voltage of the charging circuit. There are some devices that do in fact go by the charging circuit voltage and not the battery voltage. Whether or not the iPhone does it doesn’t matter.

    The AC Adapter, surprised no one has mentioned it, is 5V 1A whereas a USB port is 5V .5A. Roughly half the power from a USB port is available. Going from completely drained a cell phone battery will nearly exceed the 1A charging while it will continually peak the consumption off the USB. At around 60 or so percent for either connection is when the battery will take less and less amperage. No matter if there is a .5A current, 1A current or more a lithium ion battery will only accept so much current at a higher percentage. That’s why both graphs are nearly identical at the 90% mark.

    I’ve been able to replicate these behaviors on a Treo, a Samsung Android Phone and multiple ThinkPads. They’re all consistent.

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  70. stephen
    February 6th, 2012 11:37 am

    So, I know I’m pretty late, but even 2 years after published you still get the number one google search spot. That’s pretty sweet and this test is awesome, you definitely helped me make up my mind on with charging method I use mostly lol.

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  72. Daniel
    March 5th, 2012 6:29 am

    I agree, While SPL15 may be right about some phones this is not true of my HTC Wildfire S. It also came with a 1 amp charger, and there is an app (Currentwidget) that will show charge current to the Li-ion cell, cell voltage, and charge %. Right now mine shows 4.237 volts, 95%, and 190mA.

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  74. [...] Taxi of the Future’s power outlet, as opposed to its USB port, to charge your phone. Based on this helpful chart of battery-percentage-by-charge-time, here are some daily life improvements that the Taxi of the [...]

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  76. Don
    May 16th, 2012 1:10 pm

    I’m new to iphone so pls bear with me.

    Have you ever measured the charging current in the 2 different charging modes? I realize that this is more of pain since it would involve cutting the cable but might tell more.

    I have a 3GS and I’ve measured it on a lab bench power supply and all I ever see is 100ma. When you charge off the outlet wall wart do you still use the usb cable plugged into the wall wart or is there a special iphone power cable that supplies a different source of voltage to the iphone which would facilitate higher current than the 500ma USB spec allows, in my case only 100ma for some reason TBD.

    tks for sharing your info.


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    November 27th, 2013 7:42 pm

    iPhone 4S is great works perfectly. I just bought new case. Great website!

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  82. Ruhul amin
    April 2nd, 2014 10:57 am

    Before you invest in this or any other standalone USB charger, you should make sure that your device will charge via USB cable alone. Many phones (Motorola Razr is one) these days have what looks like a standard USB connection but require special software or adapters before they will charge.

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  84. Ruhul amin
    April 9th, 2014 10:07 am

    Each port provides a little over 2.1A under a load…consistently. The device is not small, but fits a power strip without infringing on the adjacent outlets. Runs remarkably cool, considering the power rating. The blue LED power indicator does not overwhelm and mostly only causes the case to glow a little. The blue colored case makes it easy to spot on a cluttered power strip,

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  98. allen downey
    September 29th, 2014 12:58 am

    Good comparison between outlet charger and USB charger. By the result it is clear that by outlet charger we get more speed charging than USB. But now time by USB charger we not only charge one electronic device but also other device. That’s-why I used the USB charger and mainly shop from online store as

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