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Old 01-02-2011, 12:24 PM   #1
elemenoP
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usb power draw question

If I have a USB device (let's say a Sony Reader) and I connect it to my Macbook to charge, does it charge faster if my macbook is plugged in, vs on battery power?

Thanks,
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Old 01-02-2011, 12:32 PM   #2
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Originally Posted by elemenoP View Post
If I have a USB device (let's say a Sony Reader) and I connect it to my Macbook to charge, does it charge faster if my macbook is plugged in, vs on battery power?

Thanks,
eP
NO

USB puts out 5Volts Period
The port has a preset limit on how much current is allowed to flow. That is the sole limiting factor on charge time.


Will it kill your MBP (or any other brand) battery sooner when not plugged in. Yes. Both ways . Charge - Discharge cycles use up the Battery Lifetime (More equals Less )
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Old 01-02-2011, 12:52 PM   #3
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Or it might not charge at all. I believe my netbook will popup a notice that it cannot charge an external device - depending on its battery level if the netbook is not plugged in.
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Old 01-02-2011, 03:15 PM   #4
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ok, thanks!

I have also noticed that if the computer goes to sleep, the reader stops charging. Time to get a wall plug, I think.

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Old 01-02-2011, 03:56 PM   #5
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Most devices only draw at most 500mA (milliamps) so charging from laptop battery itself is minimal. Considering it takes several hours for a full charge for most of our devices, the laptop battery will be low or unusable by that time anyways.
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Old 01-03-2011, 01:44 AM   #6
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ok, thanks!

I have also noticed that if the computer goes to sleep, the reader stops charging. Time to get a wall plug, I think.

eP
Some computers have advanced power settings that let you decide what happens to USB power when the computer is in various states. You might be able to change it on yours.
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Old 01-03-2011, 10:07 AM   #7
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A KINDLE battery's capacity is 1750 mAh. My laptop's battery capacity is near 50 Ah. USB regulated voltage is 5 VDC. My laptop's battery puts out 12 VDC nominal. There are at least three generations of USB standards, with varying power specifications. A high power USB port should be able to charge your e-reader.
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Old 01-03-2011, 12:24 PM   #8
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Actually I just discovered that my ipod can continue to draw power when my macbook is asleep, but my Sony Reader cannot. Why is this?

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Old 01-03-2011, 01:01 PM   #9
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Different devices look for or detect different signals besides just power on usb connections. My Creative MP3 player could only charge from certain usb ports and only using certain ac adapters that provided some signal that the player was seeking. It was not just a matter of having the right voltage.

My Blackberry will occasionally throw up a prompt/message that there was insufficient power to charge it - has happened with both cigarette lighter sockets and usb ports on laptops.

Some devices will not work properly with usb hubs whether they are separately powered or not and sometimes the usb ports on the front of a computer and those on the back will provide different results. USB is not, well, always usb, I gather.
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