charging a phone with AAs

Discussion in 'The Projects Forum' started by DeeGore, Sep 10, 2010.

  1. DeeGore

    Thread Starter New Member

    Sep 10, 2010
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    Hi everybody, I just registered here. I'm putting together a phone charger for camping and was hoping someone could let me know if I'm on the right track.
    My plan is to use 4 enegizer e2 batteries, to a lvboost, to a usb plug for my cell phone.
    The e2s put out 1.2 volts, so I thought I should put 2 sets in series to get 2.4 volts. The lvboost pdf says at 2.4 volts the max current output would be about 500ma at 5volts. It also says that the max continuous input current is 1.5amps. The e2 batteries are rated at 2300mah. How do I know what the current to/from the lvboost will be?
    I don't know much about this stuff so any input is appreciated. thanks
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    The charging circuit should be in the phone itself, and USB standards say that 500ma should the max current drawn by any USB device.

    Therefore, the phones charging circuit should not draw any more than 500ma, regardless of what you do.

    So make the 5v available, and the phone charging circuit should handle the rest.

    As for the boost circuit, be sure to download the datasheet(s) for any and all ICs used by the circuit.

    They will be your guide to what you can and cannot do.
     
    Last edited: Sep 10, 2010
  3. DumboFixer

    Active Member

    Feb 10, 2009
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    Be careful though as not all USB charging phones will accept the charging setup. Some of them require an "intelligent" charger. For example I can charge my SatNav from a USB charger but the same charger will not charge my N97 or N8 phones.

    I'm assuming there is some sort of USB controller or something in the charger.

    Even the 2.5mm -> USB converter lead has a small box which I assume contains the USB bits.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    Some devices use different pins for charging.

    For instance, my previous phone "knew" the difference between the wall charger and the travel charger oddly enough the travel charger had voltage on the Tx pin. I have not scoped it to see if it was a particular "code" being transmitted or not.

    The IPhone or IPod has had similar problems, if I recall. It has a nonstandard plug on the phone side that used more than just the 5v pin and GND for charging.

    BUT typically, 5v on the 5v line and 0v on GND will do the trick on most phones.
     
  5. LTX71CM

    New Member

    May 23, 2010
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  6. DeeGore

    Thread Starter New Member

    Sep 10, 2010
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    Thanks for the answers. My phone is a nokia n900. It knows the difference between the wall charger and the usb charger even though they plug into the same spot. The wall charger outputs 1200ma at 5volts. Should I use some kind of current regulator or do you think the phone will only draw 500ma?

    I've heard the n900 will charge from a dummy charger if the data pins are shorted so I will try that if it doesn't work right away.
    Should I use a blocking diode to stop the phone batteries from draining or do you think the lvboost would take care of that?
     
  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    I see those chargers for sale all over the place at what I'd assume was a reasonable price. They seem to come with several adapters I'd also assume take into account the proper charging setup. Some only take one AA battery.

    Might want to grab one at Wal Mart or Home Depot so you can ohm out the little adapters - or just use it.
     
  8. DeeGore

    Thread Starter New Member

    Sep 10, 2010
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    I'm thinking I should limit the current before the lvboost to not exceed the over-current condition. I would appreciate if someone could double check my math.
    Putting 2 1.2v batteries in series would give me 2.4v * .5a = 1.2w. coming out of the lvboost. Efficiency should be about 85 percent so 1.2w / .85 = 1.412w going into the lvboost. watts / volts = amps so 1.412w / 2.4v = .588a. So to get 500ma at 5volts, I need to limit current to 588ma at 2.4volts going from the battery pack to the lvboost. Does that sound right? Is there any easy way to do that? thanks
     
  9. DeeGore

    Thread Starter New Member

    Sep 10, 2010
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    Not sure how to edit posts but there is a mistake in my math problem. There will be 5volts coming out of the lvboost so I will need to regulate current to 1.225amps. Whats the best way to go about doing that?
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    Your rechargeable AA cells start at a total of 2,8V then their voltage slowly drops to only 1.8V. Will the boost circuit regulate its output voltage?

    The AA cells will be dead after charging your phone for less than 2 hours. Will you carry many AA cells?
     
  11. DeeGore

    Thread Starter New Member

    Sep 10, 2010
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    The lvboost will take .5 to 5 volts and convert it to a steady 5 volts. I'm not sure how to make sure the input does not exceed the 1.5 amp max.
    I was thinking about putting 4 batteries in series so the current could be higher, but if the batteries start out at 2.8v it would be way over the limit of the lvboost.
    The E2 batteries say 1.2v on them. Will they put out 2.8 volts fully charged?
    I will charge the batteries with a solar panel or a teg.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Simply look at the datasheet of Ni-MH AA cells and see their Applications Manual on Energizer's website to see that the cells are fully charged at 1.4V to 1.5V each then slowly drop to about 1.25V for most of a discharge before dropping more.
    Here is a clipping from the Applications Manual:
     
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