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How to charge a mobile phone (5v500mAh) from 6v 1300mAh Ni-Mh battery

  1. Jan 31, 2014 #1
    i need to use as a portable charger(POWER BANK) for my mobile
    can i use LM7805 to deliver my rated output !
    please guide me with circuits !
     
    Last edited by a moderator: Feb 1, 2014
  2. jcsd
  3. Feb 1, 2014 #2

    Simon Bridge

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    Looks possible - one approach is just to get the battery to supply what the phone's AC adaptor socket expects.
    There are other approaches - so you want to read around and pick one.
    If you are unfamiliar with electric circuits then the advise is: don't do it.

    Also see:
    https://www.physicsforums.com/showthread.php?t=575310

    Don't know how good these are - only skimmed them.
    http://www.newton.dep.anl.gov/askasci/gen06/gen06628.htm
    http://www.instructables.com/id/Easy-How-to-charge-a-cellphone-battery-from-other-/
     
  4. Feb 1, 2014 #3

    meBigGuy

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    A 6V NiMH battery is a 5 cell battery pack. At peak charge it can be close to 7.5V.

    Most cell phones have a 5V charger, and you need to be able to supply the current supplied by the charger.

    You need to calculate the maximum power dissipation in the regulator and use a proper heatsink.

    http://www.jameco.com/Jameco/Product...DS/1731278.pdf [Broken] is the wrong voltage, but has a nice section (page 10 and 11) on thermal considerations.

    Looking at a 7805 spec, if your phone draws 500ma, then the regulator will dissipate 1.25 watts. (2.5V X 0.5A). A TO-220 package with no heatsink would heat to 65 x 1.25 = 81.25 degrees C above ambient. (which means you should probably use a small heatsink)

    You should read some application notes.
     
    Last edited by a moderator: May 6, 2017
  5. Feb 1, 2014 #4
    IC-7805-as-current-source.jpg
    Will this help me to produce constant current from LM7805 !
    also tell me what is input should be provided for this circuit !
     
  6. Feb 1, 2014 #5

    meBigGuy

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    Why do you want a constant current?

    You edited your original post after people responded.

    I no longer have any idea what you are trying to do.
     
  7. Feb 1, 2014 #6

    Borek

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    I edited his original post to get rid of fancy formatting - wording has been not changed.
     
  8. Feb 1, 2014 #7

    meBigGuy

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    ahh --- the information I thought dissapeared was in the title.

    Sorry, When I say "supply the current" I don't mean you need to build a current source.

    When I said "supply the current", I meant you need to create a 5V Voltage source that is capable of supplying the required current without overheating. Carefully analyze my 7805 example and read the Thermal section on the other data sheet I linked to.
     
    Last edited: Feb 1, 2014
  9. Feb 2, 2014 #8
    No you don't want constant current, you just need a somewhat stable voltage around 5 volts. That is, you need to go from the somewhat stable voltage of around 6 to 7 volts that your pack provides to a somewhat stable voltage around 5 volts that the phone's internal charging circuit requires.

    A simple linear regulator will probably give the best results, but if you want something really quick and nasty then two series silicon diodes would probably get the job done (eg 1 or 2 amp general purpose diodes).

    If you do go for a linear regulator then don't use the 7805, there may not be enough input to output voltage differential for it to work properly. Get a low dropout type like the LM2940 for example.
     
  10. Feb 2, 2014 #9

    meBigGuy

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    Regardless of what regulator you use (chemnoob is right, 7805 is marginal to bad, LDO is better) make sure you understand the power dissapation as I described.
     
  11. Feb 2, 2014 #10

    sophiecentaur

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    Why not just look at the spec for what's available from a USB power out? Phones can cope with that.
     
  12. Feb 3, 2014 #11

    meBigGuy

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    Knowing the USB spec is going to help him, how? Read the title to the OP.
    BTW, Do *you* understand USB charging? Not a simple topic.
     
  13. Feb 3, 2014 #12

    sophiecentaur

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    My phone does all the necessary clever stuff. Any USB will serve it.
     
  14. Feb 3, 2014 #13

    sophiecentaur

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    Provide your phone with 5V and a few 100mA and it will be more than happy to make the best of it.

    @MeBigGuy The OP asks how to charge a mobile phone. I read that much to know he doesn't wasn't to get inside the phone battery - hence he just wants the simplest of voltage sources. The phone's charger is smarter than you think.
     
    Last edited: Feb 3, 2014
  15. Feb 3, 2014 #14
    Phones can definitely deal with chargers that don't meet the USB specification; in USB there's actual communication that goes on where the slave device requests an amount of power from the master end. Many chargers don't care and are happy to cram 2A into the phone(way more than USB allows), with the phone handling it just fine. IPhones seem to come with a 2A charger standard, but most Android phone chargers seem to be the 500mA type, but the phones themselves take 2A no problem(might degrade the battery slightly, but I haven't had any issues charging that way). TL;DR: The USB specification isn't very relevant here, and supplying more than 500mA shouldn't be an issue at all.
     
  16. Feb 3, 2014 #15

    meBigGuy

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    I see, somehow knowing the USB spec will help him charge a phone from a 5 cell NiMH battery (read the title).

    Actually, all he needs to know is in the title (again, read the title). Build a 5V 500ma capable regulator.

    BTW, the USB will only provide 100ma (at as low as 4.4V) unless you enumerate and ask for 500ma. Phones are not "smarter than I think" in that I build (among other things) LI-ion battery chargers, USB interfaces and chargers, and USB Audio devices for a living.
     
  17. Feb 3, 2014 #16
    Sounds like you know the specification off the top of your head. I know that the device requesting the power has to request it from the source through the data connection, but is there anything special that the receiving device needs to know about how much power it's receiving? I've seen chargers that don't appear to even implement USB signalling and the phone couldn't care less.

    Or put more simply, is there any reason that he would need to worry about the data component, or will any 5V source capable of providing the requisite current do?
     
  18. Feb 3, 2014 #17

    meBigGuy

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    From the phone's viewpoint (most phones), it just wants 5V 500ma (or whatever it actually uses). "Legal" USB compatible wall chargers short the USB D+ D- pins to indicate they are a charger. Apple iphone/ipad/etc chargers use a proprietary signalling method. Some phones might care (I know some USB products care and will not charge if The USB is not active or shorted), most do not. I don't know what *all* the phones actually do. Some vendors might only work with their charger. For this thread I'm assuming that simply supplying 5V with 500ma capability will work. If it doesn't, then I'd short DP and DM. If that doesn't work then I'd research the specific phone.

    As far as the USB port is concerned, all will supply at least 4.4V at 100ma by default for a while. If you try to draw more, it might shut down (some hubs care, some don't). If you enumerate and request 500ma, you can draw 500ma. If you do not enumerate at all, it might also shut down (probably will, eventually).
     
  19. Feb 3, 2014 #18

    sophiecentaur

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    You seem to be suggesting that the cheap and cheerful car, solar and mains chargers are more complicated than 5V voltage regulators. Are they? I cannot imagine phone manufacturers risking a charger input that couldn't deal with the very crudest of inputs.
    What do I need to know about USB charging, in this context (and what, exactly does that phrase mean, aamof?)?

    You clearly know about charging Lion cells but is that necessarily relevant if you are not planning to go inside the phone to modify its internal charging circuit? You keep referring us to the title and the title is about supplying a phone not battery charging.
     
  20. Feb 4, 2014 #19

    meBigGuy

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    That comment has no relevancy given that the OP wanted to know how to charge a cell phone from a 6V 1300mAh battery. The answer was: use a regulator to create 5V. Be carefull about voltage drop and power dissipation. Then you chimed in with the above. How does knowing anything about USB going to help him with that?

    I'm done. You are much too difficult to communicate with.
     
  21. Feb 4, 2014 #20

    sophiecentaur

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    It's a shame you started to interpret my comments as some sort of criticism of you. You get abrasive rather quickly.
    My comments about USB were intended to put the difficulty of the problem in some context. A good reason for the OP to believe the opinions he read on PF is to connect them with the fact that USB works fine as a power source. Is that being contentious?
    You should try viewing what you read here a bit more generously. It will make you a happier person.
     
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