USB Fast Chargers

I’ve been a bit confused lately at what constitutes fast charging versus normal charging, and why newer Android devices complain repeatedly if you use the wrong cable, or the wrong charger, or the wrong cable AND charger. How does it know?

Then I found an article on LifeHacker that partially explains it, but this comment thread clarified in an excellent manner:

According to this article and the super helpful comment thread, the multiple things that affect an Android’s ability to fast charge are:

    1. Battery current rating – I have a 3000mAh / 11.4Wh battery capacity (it is able to provide 3 amps at a nominal voltage for one hour or 11.4 Watts for one hour). P/I=V = 3.8V which is what this battery indicates it’s able to deliver. This battery also has a minimum rating of 2940mAh / 11.2Wh (also 3.8V).
    2. USB Spec – Normally USB is supposed to output 5V +/- 0.25V. To convert from the 3.8V battery, we simply draw less current at the USB and convert it accordingly: (3.8V x 3A) = 11.4W. (11.4W / 5V) = 2.28A. So ideally my battery wants to present a current draw of 2.28A on the charger when acting as a load. With the 0.25V allowed voltage margin, this could be anywhere from 2.17A to 2.4A.
    3. A standard USB 2 port on a computer is rated only for 500mA (0.5A). If too many devices hang off of a USB port, you could start current limiting and the voltage to each device will drop, usually below the allowed 4.75V. This is why powered hubs are recommended. Even with a single device trying to draw more current than the port is capable of, this is when you get those errors that “your phone is charging slowly. Please use the charger and charging cable that came with your phone.” A good charger should be able to provide all 2A needed for a standard charging rate the phone requires.
    4. Diameter of charging cable – USB 2.0 has five lines that it uses:
      • Rx
      • Tx
      • V+
      • V-
      • GND (braided shield)

      Normally the four main lines are all 28AWG. According to, this means that the charging lines will provide 0.23A before they start heating up and creating an appreciable resistance (higher than 213 Ohms per km) which will in essence place a second, noticeable load in series with the battery. The power lines (V+/V-) in the wire can be increased to 24AWG to allow 0.58A at 84.2 Ohms per km and thus decreasing this second parasitic load. Additionally, the load of the charging cable can also be reduced by shortening the cable itself.

      Gauge Test Current Impedance (per km) Wire length (km) (~6 feet) Cable Impedance
      28AWG 0.23 213 0.002 0.426
      24AWG 0.58 84.2 0.002 0.1684


      Volt drop (test) Voltage Left for Phone
      28AWG 0.09798 4.90202
      24AWG 0.097672 4.902328


      Volt drop (0.5A) Voltage Left for Phone
      28AWG 0.213 4.787
      24AWG 0.0842 4.9158


      Volt drop (2A) Voltage Left for Phone
      28AWG 0.852 4.148
      24AWG 0.3368 4.6632

      With a standard 0.5A charger, even a 28AWG 6-foot cable will be able to charge the phone at a standard voltage (4.79V being higher than the minimum allowed of 4.75V). However, if we wanted a full 2 Amps for fast charging, we would need a bigger cable (24AWG), which would even then present enough of a load to drop the voltage on the battery to 4.66V which, while better than 4.15V on the 28AWG wire, is still out of spec.

    5. Androids have the ability to detect a “fast charger.” Fast chargers indicate their ability to provide 2A by a simple short between the unused Rx/Tx lines. If the android sees this “loop-back” connection on its data lines, it is programmed to assume that whatever its connected to is able to provide 2A. Otherwise it will only draw 0.5A.
    6. Additionally, some fast chargers which are paired with a standard length and gauge USB cable are able to do some rudimentary current sensing and output a voltage higher than 5.0V to compensate for the drop on the cable itself.

    So in short, for a proper fast charger, two things should be used:

    1. A cable with higher diameter power lines
    2. A charger that is able to output the current required by the battery being charged

    And an additional feature that could be desireable:

    • A charger that is able to sense the current, calculate the voltage drop across a known cable and boost its voltage output to compensate.

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