Necessity is the Mother of ... a Great Kindle Gadget

Today I found myself out-of-town with my Kindle almost out of battery life, no Kindle charger, and a long plane flight tomorrow.  Passing a Radio Shack, I went in, with the intention of buying yet another charger for it  (I knew from a similar experience that I needed 5 volts with an "A" plug).  But I knew my charger was at home, and was hesitant to pay $20-30 for what would after today be an extra.

So I bought the following:

  • The cheapest USB cable I could find
  • An "A" plug
  • A short wire Radio Shack sells with a socket for the plug on one end and bare wires on the other (both the last two of these are located in the store near the replacement transformers)
  • A small roll of black electrical tape

I realized something key:  I already had a 5v power supply, in my computer, with a handy outlet, called "USB."  All I had to do was get all the plugs to match.

I borrowed some scissors and cut the USB  cable about 8 inches from the flat end, throwing the rest away.  I stripped off the insulation, and found the red and black wires - these are the 5V and ground wires (just search the Internet for USB pinouts if you want to be sure).  I then twisted one wire from the plug wire to the red and the other to the black, and taped the whole thing up (a bit of soldering would have been better, but I forgot my handy MacGyver construction kit). 

And what do you know, I now have a USB charger for my Kindle  (When I first plugged it in, the charge light did not go on, but I reversed the plug in its socket and that did the trick).  This will now charge my Kindle on the road from my laptop or when I am driving from my 12V car charger that has a USB connection.

I think this is a pretty handy accessory, and a quick Internet search did not show anyone currently selling one.

Update:  OK, someone else already thought of this, and has pictures of the procedure.  He notes that the supplied Kindle usb cable will not charge the device as well  (the Kindle cable goes from USB to a special miniature USB port, like the ones on a camera -- my cable goes from USB to the power inlet).  My homegrown version charged it very quickly.


  1. Anonymous:

    Could you please post a Kindle review one of these days.

  2. ErikTheRed:

    Just make sure that the Kindle doesn't draw more current than your USB port is spec'd for, otherwise you may damage your motherboard. This has happened before with devices that try to pull too much power through the USB port.

    According to Wikipedia (my first Google match): "Initially, a device is only allowed to draw 100 mA. It may request more current from the upstream device in units of 2 mA up to a maximum of 500 mA."

    The USB port may refuse to provide more power if it can't supply it - so anything over 100 mA is not guaranteed. Anything over 500 mA is playing with fire (or at least magic smoke).

  3. Kit:

    You could of just bought a paperback. ;)

  4. epobirs:

    Any Wal-mart or Target will have a combination USB data and charge cable for the Sony PSP portable game system. Typically close to $10 but often well below, closer to $5. This will work as a charger for many other items besides the PSP that were designed with USB in mind. A harder to find item is the cable that has two host plugs on the USB side, allowing almost a full amp for charging twice as fast. (The PSP AC adapter supplies 2 amps, so this isn't a problem.) These cable are also frequently found in USB enclosures for 2.5" hard drives

    The newer PSP models can charge directly off the USB port, going from USB host to the PSP USB port, rather than needing a two-headed data & power cable, so these items may start to get scarce. Others exist for other devices but the electrical specs will be the same thanks to the USB spec.

    So there are frequently generic sources for this stuff with no hacking needed.

  5. Zach:

    There's another problem with charging by USB (besides the electrical current problem mentioned by Erik). If the device is totally dead, the computer won't notice that it's been plugged in, and therefore won't send juice down the USB cable. I found this out when I tried to charge my very-dead cell phone one day at the office (I keep my AC charger at home, but have a USB-microUSB cable in my laptop bag).

  6. happyjuggler0:

    I can't help but wonder if airport security will let you through with a homemade looking electrical thingy.

    I guess we'll know soon enough. If you don't post about it then you surely got through no problem.

    By the way, I second the motion of just finding a paperback instead of playing super duper electrical geek.

  7. Doug:

    Just to establish my credentials, I'm a real live genuine electrical engineer who designs power supplies for a computer vendor whose name you would all recognize. My specialty is power supplies, so I know a thing or two about USB.

    The dirty little secret about USB hosts (aka "your PC") is that they seldom include the circuitry required to limit current between either 100 mA or 500 mA (USB spec limits for low-power and high-power ports, respectively). There is a very good and simple reason for it: it costs $$$ to "enforce" the current limits. You can't just wave a wand and read current. It requires silicon to read the current and react accordingly. PC manufacturers, almost to a man, are far more focused on margins than they are specs. The cheaper the machine, the more true this statement becomes. They are not going to pop for a $.50 part to perform this task. It subtracts from the bottom line. Period. The warning splash screens that you might see on a computer or "device" (what the PC host is connected to) are NOT a result of actually measuring current, but instead communication between host and device where the device says "I need 500 mA" and the host replies "no can do." There is no current measurement at all involved. Only one microprocessor talking to another. Not always, but usually, there is no current limiting circuitry between the two.

    I talk to a lot of the major semiconductor vendors as part of my job. They all tell me that the "enforcer" chips to limit USB current are not their best sellers, by any stretch. With 100 million+ PCs per year, and maybe 6 or more USB ports per PC, there would be a heck of a lot of potential sales here for this sort of silicon. They sell very little.

    There MAY be a self-resetting fuse at the host, but these things are so inaccurate that they need to bump up the typical fusing current to ensure the minimum current. For instance, to guarantee 100 mA of current, they may buy a 300 mA fuse because the slop and tolerance of the fuse is such that it cannot guarantee 100 mA (minimum) of continuous current unless its typical fuse current is substantially higher. Further, this fuse might cost the manufacturer another $.06, so he is again reluctant to spend the money. Those cheap PCs are cheap for a reason.

    UL requirements limit the current coming out of any computer port to a maximum of 8 amps, if I recall correctly. I've seen some cheap asian clone PCs that wire the VBUS supply directly to the PC's +5V power rail. God help you if you short this lead on the outside, as the PC's +5V rail is probably capable of supply a good 20-30 amps of current. This is a time when smoke might ensue.

    Laptops, which typically cost a lot more money than desktops, may attempt to enforce this current limit --- they've got the budget for it. But if you own a $500 laptop or desktop, I would bet good money that there's no current limit whatsoever in it.

    Specs are one thing. Dollars are another. Let's not forget what happened when Apple first proposed licensing its Firewire technology @ $1/ port. The entire PC industry howled in outrage. Apple then backed off to $.25/port, but that was still too high and the damage was done. In response, the Wintel conglomerate got together and came up with USB 2.0. The rest is history. Cost was the over-riding concern of the PC vendors then, as it it now.

  8. ErikTheRed:

    @Doug - Thanks for the inside info. Interesting, and it makes sense.

  9. Doug:

    Addendum: Erik --- it's pretty hard to damage a USB port, other than by a full short. Going from 100 mA up to 500 mA should never result in physical damage. It's hard to do, as the wattage simply isn't there to do harm. If you can concentrate this wattage in a small enough area ("energy density"), you CAN blow things up. But as a general rule, it's not likely. If the host has any sort of current limiting to it, then the current usually "folds back" harmlessly when you try to extract more current from it than this pre-set limit current, and the +5V voltage will gracefully sag to a lower voltage. In this fashion, the delivered wattage (voltage times current) is limited to some low value. As a designer, you test for this eventuality and ensure that no harm comes from it --- it's stupid to design a circuit that could destroy itself like this and create a customer return, don't you agree? At least that's the philosophy of both my employer and me.

    The USB spec says that the host should limit current drain, but few host PCs perform this task. Surprisingly, it's the device (the thing you plug into the PC) that often enforces the current limits, not the host. These devices, like an iPod, for instance, have the budget for this sort of circuitry, unlike the PC. (Just one more reason why an iPod might cost more than some other music players.) The device will then limit its current consumption from the host to the USB spec limit(s).

    The device will draw as much as, say, 500 mA, and any remaining current needs of the device are then "stolen" from the device's battery. This is how/why some poorly-designed USB devices can be put on a USB charger and still not charge. If the device needs more than 500 mA to charge AND operate, you have a net deficit of current, which has to come FROM the battery. Hence, the battery never charges, or takes a lot of time to do so. If possible, to help things along, turn off the backlight on your device while charging, as this leaves more current available to charge the device. Backlights are REAL power hogs! I note that when my iPhone is in its charger cradle, that it's backlight is dimmed (less power consumption) and doesn't stay on very long (NO power consumption).

    I routinely use USB power to power some of my small projects, just as Coyote has described. But don't expect to power a subwoofer, for instance. That sort of power simply is not there.

    Sorry to get overly detailed. It's one of those few times when my technical skills exceeds Coyote's, so I have to pipe up when I can!

  10. epobirs:

    The battery status of a portable device shouldn't be an impediment to initiate charging. How else do suppose USB flash drive, with no independent power at all, work so well? I've been able to charge up a completely dead PSP countless times. In fact, I've rarely used the unit's AC adapter since the launch purchase.

    Notably, the PSP needs considerably more than half an amp to operate. A PSP drawing power from a USB port will run down the battery. Just somewhat slower than without the power input. Going from fully drained battery to full charge takes around 3 hours compared to one hour on the 2 amp AC adapter.

  11. Suz King:

    You just saved my trip....and the return home. 12 hours between airport layovers and flights. My Kindle was down to 1/4 battery...and all I had was the *&$^*% USB because I was counting on a trickle charge...humpf! Some trickle DRAINED my battery from 1/2 to 1/4 in about 2 hours and that with the power switch and whispernet turned off on the kindle. THANKS for posting this!