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Battery powered USB charger help?

Former Member

I'm trying to build a small, portable battery powered USB charger for my android phone. I don't want to use 9V batteries because of their internal resistance and short lifespan. My thoughts were just to use 3V from 2 NiMH or Lithium Ion batteries so a 7805 regulator is out which greatly complicates things (for me at least). What would be the best approach for me to build this while keeping it as simple as possible...and within 20 or so dollars.

  • Karak,

     

    You should maybe consider some safety for your android phone which probably costs in excess of a few hundred pound. Your battery charger should be designed to work with the internal circuitry of the android, which you can't obtain. You should also maybe consider what would happen if say for example you short the pins of your charger, or what your charger will do when the battery is full?

     

    Saying that you can probably use a charge pump to bring up the power from 3v to 5v and then clamp the voltage with a zener diode, your batteries will probably lose charge very quickly tho. Not sure if you are better off just using a resistor and 5v zener diode combo with a 9V PP3. I'm not convinced the above regulator will work on a 3 volt supply.

  • in reply to smsbeam

    Drop in 3 volts it might just take longer.

  • in reply to Former Member

    That wouldn't work, most mobile phone batteries are in the region of 3.6V especially say for example a popular swede/american android phone. You really need something in the region of 5V @ 500ma which is what the USB standard states. Perhaps even 4 AA batteries.  I would imagine that the device would also draw power whilst in use and as I said previously will have some sort of charger input circuit and may see 3V as too low to even begin charging.

  • in reply to smsbeam

    My battery is a 1500mAH 3.6V Samsung, I've made a few of the 9V chargers using etched boards with great success. I was wanted to use some sort of step up converter like the Linerar Tech LT1302CN8-5#PBFLT1302CN8-5#PBF I found here on Element14. My whole reasoning behind using 2AAs or 2AAAs is they're cheaper than 9Vs ,there are more recharable AA readily avaliable than 9Vs, and some of my friends requested that I make them a charger but my 9V models go through batteries like theres no tomorrow (they'll charge an iPhone or my Samsung about twice before they're no good).

  • in reply to Former Member

    The linear one looks about right. You might find you have better success with 2AA's they have about 3 Ah by todays standards of rechargables which is a lot. However your battery is a 1.5Ah so... you'll probably get two charges out of them. If you want a shotgun approach use 3AA's a 5V zener and a .5A current limiting resistor you might just get 3 goes out of that. You'll probably find that the samsung has got a zener onboard to limit the voltage to 5V's. I would have thought two charges out of a battery that size is actually pretty good.