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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? http://forums.androidcentral.com/acer-iconia-a100/112270-charge-via-usb-port.html#post1696260

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.

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Creating a generic "list of things" is typically not a good fit for Stack Exchange sites (and this one seems particularly prone to become outdated). They fall pretty squarely into the every answer is equally valid part of the what not to ask section of the FAQ. –  eldarerathis Aug 3 '12 at 20:35
    
+1 OK I re-wrote the question. –  therobyouknow Aug 3 '12 at 20:40
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3 Answers 3

up vote 3 down vote accepted

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.

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+1 Thanks @TomG your answer corroborates with others here including earlier poster Tom. Credit to you for providing the detail on power data to support the answer. Thank you ! –  therobyouknow Aug 4 '12 at 15:15
    
Yeah, I opened the question last night and didn't write the answer until this morning, so I didn't see his, otherwise I would have just added the extra info as a comment. –  TomG Aug 4 '12 at 15:38
    
I've accepted your answer now because of your points about the differences between tablets and phones and the power transmission challenges therein. Not something I'm comfortable with in this wonderful community in transferring accepted status, I seldom do it. But I feel that your answer is the most comprehensive, though credit to the other answer as they are all useful too. –  therobyouknow Aug 4 '12 at 16:01
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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.

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@therobyouknow, yes, I've seen devices that do that: allow you to charge via micro-USB port either quickly, when the cable is connected to the supplied AC adapter, or less quickly when same cable is connected to USB host. I really like that arrangement. –  Tom Aug 4 '12 at 14:06
    
+1 yes @Tom ! Indeed just today, I have received confirmation by email from Ergo Electronics that their GoTab Android Tablet range can charge via microUSB. They also provide a specific power port for charging with their own adapter. So, you can either charge them via the tablets' microUSB port from PC, standard USB mains adapter or via the tablets' specific power port. Mains adapter options will be faster at recharging however. –  therobyouknow Aug 4 '12 at 14:27
    
+1. My primary goal is to not need a separate charger for the tablet, to be able to use any charger with micro-USB. The secondary goal to be able to charge off of a USB 2 or USB 3 host is also useful. Presumably, if the Android tablet detects the usual handshaking from the host device then it would know that it is being charged from a USB Host where the current is limited compared to from an adapter. In the case of attachment to a USB host then it seems apparent that the Android tablet can accept this lower current and just charge the battery slower. –  therobyouknow Aug 4 '12 at 15:55
    
@TomG expands on this, explaining the greater power demands of a tablet, so I'd like to make his the accepted answer. But I am keeping my upvotes on yours Tom too. I don't like to move accepted answers and appreciate your early response and don't want to disappoint (sorry about that). However I need to choose the answer that provides the answer that has the highest coverage of the issues in the question. –  therobyouknow Aug 4 '12 at 15:59
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@therobyouknow No problem - I don't mind you transferring the accepted answer. I think that users should feel open to contributing new, possibly better, answers even if another answer is already accepted. –  Tom Aug 4 '12 at 20:52
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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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+1 This is close to what I'm looking for, it satisfies my main requirement whereby at least this a specific charger is not needed for this tablet. The Nexus 7 is a nice tablet - powerful with 4-cores but a pity no microSD input for storage expansion or USB Host/USB On-the-go (OTG) for external storage. These 2 would be particuarly useful to me for backing up photos taken from my DSLR camera and audio recordings made on my Zoom recorder. –  therobyouknow Aug 3 '12 at 20:33
    
I love the fast bootup of an Android device compared to Windows but for it to be serious contender to Windows it needs to be friendly to content creators to connect devices to the tablet. So I couldn't buy the Nexus 7 for that reason, it remains predominantly a content consumption device. –  therobyouknow Aug 3 '12 at 20:36
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Micromax Funbook, a clone of AllWinner A10 tablet has separate 5V DC input for charging through mains and a mini USB port for PC and interface connectivity. I am able to charge the device both using 5V DC input and mini USB (not concurrently ;) ). I have observed that charging the battery through USB doesn't heat up the battery as the other way do and it seems battery stamina is improved as well, –  Narayanan Aug 4 '12 at 4:57
    
+1 Thanks @Narayanan good to be aware of another device that supports USB charging. –  therobyouknow Aug 4 '12 at 10:40
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