Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Cell Phone Charger

  1. Jul 27, 2014 #1
    Hi, I'm currently working on a project: I'm trying to make a cell phone charger from an Altoids tin and batteries. I know there are countless tutorials out there, but I want to understand why everything happens and how it works. I'm getting to each problem individually, so that I can find a solution for that specific problem and understand what's going on. I have two 9V batteries I want to connect to a female USB end, then connect a USB charging cable from the phone to the batteries. I'm using an old phone, so I don't mind accidentally screwing it up, I'm just worried about getting the power to the phone initially; and then I'm worried about getting the correct voltage and amps to the phone.

    Thus far, a single 9V battery connected to the phone via the USB circuit does not cause the phone to respond. Why is this?

    Also, would any other battery combination be more efficient, and for what reason?

    Thanks in advance for any attempted assistance.
  2. jcsd
  3. Jul 27, 2014 #2
    The phone does not respond to the 9v battery because the voltage is to small for the sensors to pickup
  4. Jul 27, 2014 #3


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    I don't think so, considering all the cellphone chargers that I am aware of supply 5V

    9V is too high

    Regenrok. try again with a battery pack that will supply 5V Plus or minus 0.5V

    and did you use the correct wires in the USB cable ?
    The outer 2 wires are the 5V supply wires, the inner 2 are the data wires



    Attached Files:

    Last edited: Jul 27, 2014
  5. Jul 27, 2014 #4


    User Avatar
    Science Advisor
    Homework Helper

    Indeed. USB ports always supply 5V.

    If you apply 9V (or 18V) directly to the phone, you are hoping there is something in the phone to protect against the over-voltage.

    (For completeness, the USB 3.1 specification does include higher voltages and currents, but it also uses a different connector. AFAIK the final version of the spec has not yet been published and so far no USB3.1 devices have been marketed.)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook