Charging a Micro USB device without a charger

I kept hearing about people working on porting Android to the HP Touchpad, and I decided that I wanted to try to install the new release of Cyanogenmod on mine. I don’t really use the touchpad for anything other than browsing the internet so if it doesn’t work perfectly that will be ok.

I was so excited to begin, and started following a tutorial I found online. After step one I realized that my battery was at about 5% since I hadn’t charged in a while, and I would’t be able to continue without charging at least to 25%. All I had was the USB cable, but that doesn’t seem to do much more than trickle charge the device. Definitely not enough to charge in any useful amount of time. I looked everywhere to find a useful charger that would work a bit faster. The touchpad uses a micro USB plug to charge so I was excited to find my cell phone charger on my desk. Turns out it is only 1.0 amps and wasn’t much better than charging over USB.

I have access to an awesome Agilent power supply, and figured it must be possible to charge the touchpad with it. My original goal was to just cut a micro USB cable and attach the wires to the power supply with 5V 2Amp and it should work. It wasn’t quite so simple. Shorting D+ and D- and hooking it up directly only drew about .5Amps, which was just like being plugged into USB. After talking to a couple folks and learning a bit more about how the device knows to draw more power it made total sense.



I’m not an electrical engineer, so I turned to my smartest friend… Google. I found several forum posts about people trying to charge their touchpads in their cars, and have pretty much already solved the power problem using those cigarette lighter USB chargers. With a couple resistors you can convince the touchpad to draw more power.  A few minutes later I had my resistors all hooked up. It’s not pretty, but it matched the diagram, and I’m pretty sure it should work.


Not Pretty, but it works!

I was a bit scared to actually turn on power output, but I did it anyway. To my surprise nothing started smoking! Not only that, but the touchpad started charging without the warning about using the right power cable. Yay… It works!

After half an hour of charging, I had enough power to go through the Cyanogenmod install. The first time I did it, the touchpad got stuck in a strange state where I couldn’t get back into recovery mode, but after recovering the device and running it again everything worked.


Power Draw

So far I really like the Android interface, and it seems to run pretty smoothly on the device.