Computers & Mobile Craft & Design Technology

Hackmeister Benjamin J. Heckendorn discovers that a cheapo hand powered light will crank out just enough juice to charge his HTC EVO. With Sprint bringing 4G to more cities recently, the usefulness of this hack will only increase. [via geek.com]

14 thoughts on “Hand powered smartphone charger

  1. how long do you have to wind that thing to get usefull charge for actually doing anything?
    also you know you can buy these that are actually made for charging phones right?

  2. I wish more electro knowledgeable people would research this kind of thing, I’d love to rig up a hand crank with power buffered by a super capacitor or that would allow you to reasonably build up a hardy emergency usb charger for phones in emergencies/the back country. The capacitor of course allowing you to smooth out the charging and say.. rest your hand for a couple seconds. ; )

    Then I want the crank to be tough and modular so you could rig it to whatever rotational energy source you happen to find.. windmills, pedals, etc. It’d probably be really popular in Africa and other places where there are vendors that sell cellphone charging services because power is so scarce.

    1. I remember taking apart and characterizing a freeplay indigo – miles ahead of a ‘cheapo crank flashlight’ as far as quality and design goes. Inside is a brushless rare earth magnet generator that could put a max of 3W – averaged a tad under 1W under some extreme cranking.

      An EVO 4G has a 3.7V 1500mAhr battery according to ifixit – that’s 5.4 watt hours of energy.

      Lets take a 1W average… It will take 5.4 hours to recharge that phone assuming the charging circuit in the phone is 100% efficient (it’s not – 90% is probably pushing it). to get from dead to 10% will take at least 30 minutes IF the phone is off while charging. This leads to terrible user experience and in an emergency, 30 minutes just doesn’t cut it.

      You could add super/ultra caps – but that phone is going to suck down power faster than the crank generator can deliver….

      You’d need a bigger generator – these generators aren’t cheap to begin with :/

      1. Does anyone know the typical power consumption of a smart phone? The post above assumes that the power usage to talk on a phone is the same as the power output of the charger.

        Google didn’t seem to have any reliable sources.

        1. Hi David S,

          I’ve since purchased an EVO 4G…

          When dead, it will suck down about 5W of power at 5V – that doesn’t mean it can’t charge on less…

          Thanks for calling me out. Although, my post never said anything about talk time (just the time to go from 0% to 10% state of charge). As Alan pointed out, the usage time will vary and can be easily estimated if you know some of the phones rated specs.

          The rated talk time on the 5.4Whr batter is 6 hours for the EVO 4G. So, while using it as a phone, it’s consuming ~.9W IF it can actually deliver on it’s rated spec (I’m not about to try talking for 6 hours straight). There are some assumptions here (example: can’t bring the battery to completely dead), but it doesn’t change that value much.

          .9W leads me even further to believe that the small generator in this blog post just won’t cut it. Sure, the light will turn on – no, you’re not moving appreciable amounts of power. 1 minute of talk time will take more than 1 minute of cranking using this tiny generator.

          So, in an emergency – this isn’t something to rely on.
          In day to day use – it’s just not going to be practical (GPS, Wifi, etc. consume power faster than cell time – according to Android’s reporting).

          Don’t get me wrong, I’m not saying the charge light didn’t turn on… But, I work in the space of small scale (less than 100W) power generation… Things like this trivialize power generation and power usage which in turn sways publlic perception.

          Solution, if you really want hand crank power…. Bigger generator and heavily managed expectations. (probably around 7W peak so you can average around 4W – that will be 8 minutes of strenuous cranking to go from “0%” to 10%).

          Solution for emergencies when you want power now and may need to have free hands – backup batteries (even those cheap AA boost converters).

  3. David, you just need to know the capacity of the battery and how long it lasts under the conditions of interest.

    Tre3 mentions a 5.4 W-hr battery. If it takes, say, 10.8 hours to drain, then consumption is 0.500W.

    The issue is that there are too many things one can do with a smartphone to have a concrete number. It can take days for a phone to run out of juice if it is doing nothing but waiting for a call. Or you could fire up Netflix Streaming and watch the battery meter drop in near-real-time.

  4. Ben Heck MUST have tech folks backing him up. Anyone who pronounces “voltmeter” as “voltimeter” (similar to altimeter) needs to spend some time learning more. This comes from where the crank generator is attached to the Harbor Freight DVM.

    So, what gives here, Ben?????

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