Connect with us

AA battery USB charger - phone is drawing too much current

Discussion in 'Power Electronics' started by moeburn, Jul 26, 2013.

Scroll to continue with content
  1. moeburn

    moeburn

    41
    0
    Jun 25, 2012
    You may have heard of the MintyBoost, a DIY kit that lets you charge USB devices using AA batteries. Well I have a problem, and I was hoping you guys might have a solution.

    The kit is fairly simple, it just uses a DC-DC converter to convert the 3v to 5v that you need to charge USB devices. But unfortunately, my Motorola Atrix MB860 phone is stupid. It tries to draw as much current as possible, whether or not the charger identifies itself as a "Fast charger" according to the USB standard. It can draw up to 1A, and typically draws 850mA, to charge its large 1900mAh battery as fast as possible. So my charger works fine, at first, until the IC gets so hot that it shuts off, and then it cools down, turns back on again, this happens at first every 10 seconds, then every 5, then every 1s, then so fast that the CPU usage on the phone goes to 100%. The PCB of the charger got so hot that it melted the foam sticky pad that was used to mount it in the tin. The 2450mAh NiMH AAs got so hot that if I held my thumb on them for more than 3 seconds, it hurt.

    Is there any way to rescue this circuit to prevent the phone from letting it draw more than 500mA? Some kind of current limiting resistor in there or something?
     
  2. moeburn

    moeburn

    41
    0
    Jun 25, 2012
    I just checked Digikey, and unfortunately there is no 8pin throughole mount 5v step-up DC-DC converter that is capable of higher current than the one they already use:

    Digikey search results
     
  3. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    The only way to cause the phone to take less current is to lower the voltage. But this will most likely cause it to shut down as well. I would try for 4.5V to see if that helps.


    Bob
     
  4. moeburn

    moeburn

    41
    0
    Jun 25, 2012
    The phone has two charging modes: AC and USB, and it will draw less current if it thinks it is connected to a computer USB port. Is there any simple way to simulate that?
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    Check the "magic" used by Apple chargers to communicate their capabilities to the phones. Perhaps your phone uses something similar.

    Apple place various voltages on the data pins to signal this.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Even if you can convince the phone to charge at a lower current, if you use two alkaline AA cells, you would need to replace them twice during a single recharge!
     
  7. eKretz

    eKretz

    251
    27
    Apr 8, 2013
    Yep, I built one of these but I use Li-ion batteries and am able to get a couple of charges out of (2) 2200mAH batteries. The one I made uses resistors as a voltage divider to send the proper voltages to the data pins. Found the schematic with Google.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,411
    2,779
    Jan 21, 2010
    Is it actually designed to charge from USB, or does the charger just use a USB style connector?

    If it is designed to charge from USB, does this work? Or does it overload the USB port too?
     
  9. moeburn

    moeburn

    41
    0
    Jun 25, 2012
    I tested it with a DMM and a spliced open USB cable. Plugged into the official Motorola 0.8A charger, it draws between 0.45A and 0.80A, depending on what you're doing on the phone. Plugged into the USB port on my computer, it starts out drawing only 0.10A, until it finishes unmounting the internal SD card for computer access, then it draws exactly 0.40A, and never budges from 0.40A, no matter what you're doing.

    On the Mintyboost, it starts out drawing only 0.30A, but it slowly climbs up to 0.56A, and maxes out there. I believe this is because the Atrix is actually trying to draw its max 0.8A, and the Mintyboost converter chip is just "warming up" before it finally reaches its max rated 600mA.

    So yes, my phone is capable of switching from 800mA for a wall charger, 400mA for a fast USB port, and 100mA for a slow usb port.

    But the only reason I was limited by 600mA was because that's the highest current DC-5vDC boost converter that exists on digikey, in a DIP package that was solderable. But apparently there are higher current ones in surface mount form, because I found this:

    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=370860441411

    [​IMG]

    It's a prebuilt 2-5v to 5v DC to DC boost converter, just like the Mintyboost, except it has twice the current capacity, at 1200mA, more than enough for my atrix, and possibly even enough for a tablet, which the minty boost could never do! Would this work as a replacement for the circuit in the mintyboost? It would certainly fit well! :D
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-