Can USB charging (via mains adapter or from PC/Mac) of Android tablets be the main, primary method of charging? Or are there design issues?

  • therobyouknow

    Are there design issues with tablets being charged by USB not seen with Android phones?

    I saw this on another forum, what are your thoughts on this with regard to my question?

    An important consideration in the design is the amount of current the usb connectors can handle. I figure the designers find that it is safer to design a separate charging and power connector because as the devices get larger they draw more current to operate and charge the battery. I think that is the reason they do that. Is there no 12 volt adapter for these devices? I don't own one yet, just leaning towards the acer 7 inch tablet.

    I would prefer charging via USB (via mains or PC) when considering a tablet so that I don't have to carry around a specific charger.

    Many Android phones use their micro USB port for charging - connecting the USB cable to a mains adapter or using the cable to connect to a full size USB port on a computer.

    I see that some major brand Android tablet manufacturers (Motorola, Asus) as well as some smaller less know brands either use a separate power input (non micro-USB) or a dock.

  • Answers
  • TomG

    Tablets have larger screens, and often more powerful CPUs, than phones, which vastly increases power consumption. In order to provide the battery life expected, tablet batteries have much higher capacity than typical phones, and would charge at an unacceptably slow rate with a 5V, 550ma charge provided by a USB 2.0 computer port (some wall chargers are 850ma or 1-2A). At 550ma and 5V, you are getting 2.75 Watts (Volts x amps), at 2A, you are as high as 10 Watts.

    Charging at a higher voltage allows a faster charge while keeping the current down, which minimizes the need for heavier conductors in the cable, etc. The ASUS charger used with the Transfomer series, for example, uses 1.5A at 15V, which provides 22.5 Watts. I believe the iPad charger has similar characteristics. Each of these uses a proprietary connector which provides a signal to the charger that it can safely output the higher voltage, which reduces the risk of damaging other equipment by connecting it to the higher voltage charger. The ASUS will charge, albeit very slowly, from a computer USB port using the proprietary cable; the same may be true of other tablets.

    EDIT: An additional factor is the maximum current rating of the pins in the USB connector. This article states that the maximum rated current on pins 1 & 5 of a micro USB connector is 1.8 amps for a total of 9 watts. Currents beyond that amount run the risk of overheating the pins.

  • Tom

    The USB spec specifies that the power supplied over USB shall be ~5V DC, which is fine.

    A single USB device (e.g. your tablet) may draw up to 500 mA of power in USB 2 and 900 mA of power in USB 3. This is the issue - it's workable but not ideal.

    Using a USB cable as a power connector between a tablet and the AC adapter that came with the tablet is a different story - they can do anything they want - but charging from a USB port is governed by the above limitations.

  • Cornholio

    The Google Nexus 7 charges from its Micro-USB port. I'm not sure if it'll charge plugged into a computer USB port, but plugged into the wall, it works great.

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  • Noah

    I have a Tech_7 from Chinavasion, S3C6410 Android 1.6;

    When I plug it into the usb port, it shows up in the Device manager as HiDROID, but the driver is not installed.

    How can I get the USB driver working?

  • Related Answers
  • Noah
    • Install the avd & sdk manager - I'm assuming you use c:\Android_SDK

    • Edit c:\Android_SDK\usb_driver\android_winusb.inf
      Find the section [Google.NTx86] and add the following:

    %SingleAdbInterface% = USB_Install, USB\VID_18D1&PID_0002
    %CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_0002&MI_01

    Find the section [Google.NTamd64] and add the following:

    %SingleAdbInterface% = USB_Install, USB\VID_18D1&PID_0002  
    %CompositeAdbInterface% = USB_Install, USB\VID_18D1&PID_0002&MI_01  
    • Edit .android\adb_usb.ini and add the following:


    Note: This corresponds to the device id up top

    • Plug device in - you should have an unknown device in the device manager.

    • Update the driver on it and manual select the location of c:\Android_SDK\usb_driver.
      It should install the driver and show up as an ADB Device.

    • Run adb kill-server then adb devices

    Remember to turn on USB debugging from the application settings as well