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And what is roughly the ratio of the charging speed in USB vs charger? My phone is Samsung Galaxy S, if that matters.

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Not an exact dupe but check out this question android.stackexchange.com/questions/1887/… –  gary Jan 26 '11 at 19:40
    
I find that charging from a powered external USB hub goes pretty fast. –  Zan Lynx Jan 26 '11 at 23:25
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6 Answers

up vote 40 down vote accepted

A computer USB port is limited to 500 milliamperes (or 900 mA for a USB 3.0 port). The dedicated wall charger, on the other hand, is usually able to provide 1000 mA or more.

Let's take the case where a connected phone uses 200 mA. Then, the USB port would have 300 mA left to charge the battery while the wall USB charger would have 800 mA left. A typical smart phone battery has a capacity of about 1500 mAh (milliamperes-hour) nowadays. This means that in a perfectly efficient world, a current of 1500 mA would charge the battery in 1 hour. So the same battery would then charge in 5 hours over the USB 2.0 port and in about 1h50 with the wall charger.

Of course in reality power losses would also affect charging times but these numbers are quite close to what I get with my actual phone.

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As a side note, if you're plugged into an unpowered USB hub that other devices are sharing, you'll have even less available for charging the phone. –  BBlake Jan 28 '11 at 18:55
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Charging from a USB port of a computer is slower than charging using a charger because the voltage and amperage provided by the computer is considerably lower than in the case of using a normal charger, so the mobile is using “slow charge mode”

I don’t know which is the charging speed ration between USB and normal charger in the case of a SGS, but in my case (HTC Desire) with a normal charger I need approximately 2 to 3 hours for a full charge, but with a USB connection that time only charges 15 to 35 percent.

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The voltage is actually the same (5V) its the current (amperage) that's lower. –  Chris Thompson Jan 26 '11 at 22:59
    
@ChrisThompson while definitely true for now, the USB org decided to confuse people even further, and added multiple supply voltages to the USB 3.1 spec. Only after lots of confusing negotiation, of course... –  endolith Feb 10 at 15:34
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Consider that computers often have other things inside of them or connected to them that need the same charge. The power within a computer itself is always lower than direct power from a outlet from a wall. When power is distributed through a computer to your phone, that same amount of power is also being used throughout the whole computer slowing the voltage to your phone, and other things that are plugged up. Always remember, if you want direct power and a quicker charge, its always good to use to wall outlet instead of a computer or labtop.

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The USB standard officially limits the current flow to 500mA. Most mainboards can supply more, but today's phones only take 500mA when they sense a PC-USB port. –  ce4 Jan 11 '13 at 22:19
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USB from computer delivers only 500mA. So, a 2100mA battery will take 5 hours to charge from a computer.

The charger outlet delivers 700mA to 1A. So, the battery will charge within 3 to 3.5 Hrs.

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Samsung bridges pins 2 and 3 facing the phone. If pins 2 and 3 are open, the phone limits your charging capability to about 350 mA or so. Check it out: http://forum.xda-developers.com/showthread.php?t=1384253

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Shorting pins 2 and 3 together is the official USB charging spec, but not all phones and chargers follow it. Sony and HP and Apple have their own proprietary connections. –  endolith Feb 10 at 15:37
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Most of the answers here are very informative on the battery issue, I don't want to touch on that, though I should let you know that if you have to charge your android with your laptop or desktop, make sure it gets full before you unplug it, otherwise you will be slowly ending the long life of your battery by unplugging every time you have to leave because it kind of takes longer. I take a major precaution when it comes to charging because I changed my phones charging adapter more than thrice due to charging using my laptop. I was finally advised by my techie to use the specified charger and since then, i have no battery issues. Just a precaution.

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