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

4AAs not giving 6v!

  1. Jun 3, 2008 #1
    I am making an external charger for my ipod touch that uses 4 AA batteries. Because the ipod requires data communication to charge, i have a 560k resistor from the power source going to D+ and D- of the usb port, both in parallel. I also have a wire connecting directly from the power source to +5VDC of the usb port. I tested the EMF of the 4 AAs (brand new zinc chloride, 1.5v) and it was 5.1v, even though each battery measured 1.6v. When everything is connected up, my power is 4.1v, but my ipod starts charging. Unfortunately the current part of my multimeter is broken so i can't measure it but i will try to find another one. Although my ipod says it is charging, it does not actually charge, and i think this is due to the fact that it needs at least 5v. I know about internal resistance, but is it really that much? I think that as current goes up, terminal voltage goes down when there is internal resistance...Is there a voltage booster circuit to get me back up to 5v, 5.5v ideally. Could it also be the batteries? Again, i'll try and find some more but i live in the middle of nowhere and can only get to places at the weekend.

    Any help appreciated!

  2. jcsd
  3. Jun 3, 2008 #2
    Try measuring the batteries when they are not connected to your circuit. You'll probably find about 6V.

    A diagram could be helpful to help you diagnose the problem.

    Have you heard of Minty Boost? It's an open project that does the same thing you are trying to do.
    (I can't post URLs yet, but search for "Minty Boost" and you'll find it)
  4. Jun 3, 2008 #3
    Like i said i tested the EMF (batteries with no circuit) and it was 6v. Yes i have heard of minty boost but is there a way to it with less components? I want to minimize this at all costs...Is there no other way around this?
  5. Jun 3, 2008 #4
    I think the iPod has a built-in voltage regulator, so you could probably get away with a very simple setup. However, a quick search indicates that the iPod Touch needs some data sent to it to commence charging. You'll have to send that appropriate data, not just tie the data lines to +5V.
  6. Jun 3, 2008 #5
    Not to be rude or anything, but can you not read? First, i said i had tested the EMF of the batteries, which is them unconnected from the circuit, and you ask if i had measured that. Then i get told it requires data communication and i put "Because the ipod requires data communication to charge, i have a 560k resistor from the power source going to D+ and D- of the usb port, both in parallel." - this gives me 2.2v and the ipod says it is charging. I think you are missing the point. My ipod touch commences charging...it says it is charging all the time, but does not charge. The battery simply drains slower and the voltage at +5vdc is only 4.1v when it should be more...
    Last edited: Jun 3, 2008
  7. Jun 3, 2008 #6
    Look, I don't know anything about your level of education, so I started simple. I missed the line when you mentioned already knowing about the data, apologies. good luck.
    Last edited: Jun 3, 2008
  8. Jun 3, 2008 #7
    Test each AA battery by placing a 30 Ohm (plus or minus some) resistor across it while measuring the Voltage. If the voltage falls below 1.5 V, throw the battery into the recycling bin. Then say "Thank you" to asynchronous13 and go buy a new battery.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook