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I have a Samsung Galaxy S3 and a laptop with 2 USB 3 ports. When tethered to my laptop (to provide Internet access via GSM/HSPA data), the phone charges very slowly, especially if I am using it for anything else in addition to data tethering. While connected to a USB 3 port using the OEM cable, it took about 5 hours to charge while tethered but not otherwise being used. Given the 2100 mAh battery, this works out to a bit under 500mA¹. It has a micro-USB 2 port as its sole means of power input.

The S3's USB-to-AC Adaptor indicates that it provides 5.0 V at 1.0 A.

The Battery HD app, when running on my phone, tells me that an S3 will charge just under twice as fast via AC power as via USB.

I haven't found any apps which indicate, or any other way to determine, the phone's current draw from the power supply.

If I'm reading wikipedia correctly:

  • Maximum current draw for a normal USB 2 port is 500mA.

  • Maximum current draw for a normal USB 3 port is 900mA.

  • Dedicated Charging Ports (which I gather is what the Samsung USB-to-AC adaptor has) can provide over 500mA but do not support data transfer. I didn't see any indication in the article of a maximum current.

  • Charging Downstream Ports can also provide over 500mA along with high-speed data transfer. It sounds like the maximum current was originally 900mA in the 2007 spec but was upped to 1500mA when version 1.2 was published in 2010.

¹: Wikipedia gives battery specs for the S3 as "2,100 mAh, 7.98 Wh, 3.8 V". The rate of charge of the battery over 5 hours works out to 420mA at 3.8V. At USB voltage of 5V, this is only 320mA. I'm not sure what the efficiency ratio for charging a Li-Ion battery is—80% perhaps?

How can I increase my phone's charging rate when connected to my computer? Is it possible for a USB 2 device to draw 900mA from a USB 3 port? Is it possible (and safe) to connect my phone to two ports at once to increase the total current?

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  • Good recherche on USB specs. So correct me if I'm wrong, but: considered USB-2 provides you with ~500mA, and you use the phone while charging, "a bit under 500mA" should be more than fine – as your device at the same time uses power itself, which then cannot be used for charging. See my answer here for the S3's power consumption details to make up your own calculation.
    – Izzy ♦
    Nov 22, 2013 at 7:09
  • @Izzy: The 5-hour charge period I mentioned was while the phone was not in use other than for tethering. I've updated the question to make this more clear. The current draw actually works out to 420mA, assuming that we're using the battery voltage (3.8V) rather than the nominal USB voltage of 5V (which would give 320mA).
    – intuited
    Nov 22, 2013 at 14:44
  • @Izzy: The chart is interesting, though they don't seem to have researched the power consumption for GSM data tethering. I guess it might be comparable to the Wifi Tether action, which would put it at about 80mA (at 5V).
    – intuited
    Nov 22, 2013 at 14:54
  • So combining your two comments with the original question: USB delivers ~500mA. ~420mA go to charging, ~80mA to tethering – summing up perfectly to the ~500mA. But I'm not involved deeply enough in this to give a full/exact explanation, just wanted to point out some backgrounds ;)
    – Izzy ♦
    Nov 22, 2013 at 15:49
  • Not all of the 420mA will go to charging; some will become heat waste in the charging process. But that's 420mA at 5V rather than at the 3.8V used by the battery (and presumably the phone). But the focus of my question on how to increase the supply current above 500mA.
    – intuited
    Nov 22, 2013 at 16:41

3 Answers 3

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Charging rate is set by hardware and software, and can't be modified by software only.

Charging rate depends on the following:

  • resistance between D+ and D- (determines is it a official, unofficial or USB charging)
  • hardware technology and limitations (QuickCharge 2.0, 3.0)
  • cable length and quality (bad cables usually have more resistance)

For the S3, as far as I know, you can charge slow (USB, 5V 500mA) or fast (wall charger, 5V 900mA).

The charging type is set depending on the resistance, which is different for wall chargers (fixed to activate fast charging functionality in phones chip) and standard USB charging. If you want to charge quickly while tethering, the only valid option is to have your phone connected to an official Samsung wall charger which will ensure a higher charging current. There may be an option to set the USB connection to "Charge only" but that doesn't guarantee that the fast charging will be turned on.

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Look into the usb battery charge spec 2.0 for cdp. A simple accessory to test with is an evaluated kit for the tps2546. Available from digikey for about $25.

Note i believe my siii recently stopped supporting cdp after a vzw update...

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To anyone still reading this. There is an app on play (Galaxy Charging current Lite) that shows the charging current while charging. Uppon reaserching i find out that the phone software limits charging current. The default current for AC charging is set to 800mA, for USB charging to 480mA. But if you are rooted,you can apply new Kernel (mine is AGNI pureCM kernel) and you can change that up to AC: 1500mA, USB: 1000mA. But you must have AC charger that can produce that current. Original Samsung is 1000mA.

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