Replacing the current sense resistor in Portable Chargers/Power Banks for powering low power DIY projects.

Mobile Phone/Tablet charging Power Banks can be used to power so many DIY projects. But if your project is low power and draws very little current, the power bank will auto turn off after a few seconds thinking that it isn't being used to charge gadget (because the current draw is very low). I need to figure out a way to hack one and locate and replace the current sense resistor with one of a higher value. Here is how I did it.

1. This told me that the resistor would be located on the return path (ground path) of the output USB connector. I promptly located the R100 SMD resistor in my iBall portable charger. The fact that this resistor was fatter (higher power dissipation rating) and that it was 0.1 ohms in value confirmed that this had to be the current sense resistor

   

   Original 0.1 ohms current sense resistor

2. I measured the voltage across it when nothing was connected. It was zero volts and the portable charger turned off in a few seconds when nothing was being charged.
3. I then turned on the charger back again and this time measured the voltage across it while charging my galaxy tab. The voltage across the resistor was 0.07 volts
4. I now disconnected the galaxy tab and tried powering my arduino circuit which I knew for sure was drawing very very less current because the charger used to keep turning off after a few seconds. The low current drawn by Arduino didn't cause appreciable voltage drop across the current sense resistor and so the charger decided to turn off. When I measured the voltage across the 0.1 ohm current sense resistor while powering the arduino circuit, the multimeter didnt even register a reading - even on the lowest range.
5. On a hunch I replaced the 0.1 ohm SMD resistor with a 1/4 watt 10 ohm thru hole resistor. I figured, that USB ports provide upto 500 mA of current and so the phone must be drawing that much current when charging while my arduino circuit would only be drawing a few mA of current (maybe 5 mA coz it had a 7 segment display on it). So i increased the value of the current sense 100 times. Also 10 ohms was the least value I had in my resistor collection.

   

   Replaced the 0.1 ohm resistor with a 10 ohm one

6. After replacing the resistor, I measured the voltage across it while powering the Arduino. It was now 0.07 volts and now the power bank wouldn't turn off!!!
7. Awesomeness!
   
   
   

   

   OMG it's alive! and doesn't auto turn off now!

Update 2014-10-26
The latest portable chargers like the ones in the picture below now longer have any kind of switch on them. They just have a two USB connectors on them - one to charge the cell phones (output port) and other one to charge the charger themselves (input port). The chargers automatically detect if something is plugged into their output connector and automatically turn on. These chargers don't care about how much current you are drawing - any amount is okay - as long as there is a cable connected from their output port to something that can draw some current from them.

Left: Minix S5 10,000 mAh (INR 1999 at flipkart)
Right Portronics Pico II (INR 704 at flipkart)