Saturday, October 4, 2014

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)


mohammad tahir said...

Thanks for this post.
I tried to use the power bank to power my Arduino, but it turns off, so I wonder, wouldn't it be easier for me and others who don't wish to open the power bank and lose the warranty to just add another load besides the Arduino to draw more power and stop the power bank from sleeping (turning off).
For example, some lights.
Or even a rechargeable battery in some configuration so that when the battery is full, it will power the Arduino, and when it needs charging, then it will draw power from the power bank resulting in supplying power to itself and the Arduino as well.
This was just a thought, what do think .... ?

Anurag Chugh said...

@mohammad, I began with the same idea, but then decided that since my power bank was old anyway and out of warranty, it wouldn't really matter if I hacked it.

Say your arduino is drawing only a few milliamperes, you will need to add a load big enough to draw atleast 100mA if you want the power bank to not turn off. Besides that load generating heat, the power bank itself will run out of charge soon - not a good idea if you want long operating times.

If you still don't want to hack your power bank, go for the newer power banks which don't have any buttons, and only get turned on and off depending on if something is connected to their USB port or not. They dont really care about home much current is drawn - check out the Minix and Portronics chargers in the photo above. There is an eveready one without the switch too which sells for Rs. 450.

ierodotorg said...


Thanks for your article. I bought a Tecknet Powerbank 9000mAh and after opened it I found a big R050 resistor near 1A output plug.

I replaced it with 6 ohms, 10, 20 and 30 ohms resistors, but nothing works, it still power off.

An idea of what I can test ?



mtam7777 said...

could this be cause insted by the cable having slipt a tad?

mtam7777 said...

could this be cause insted by the cable having slipt a tad?

sayantan hajra said...

THANKS for sharing your experience..but a question is also iritated me in your picture smd resistance is R100 thats mean 100 ohm and the colour coded resistance which is BROWN BLACK BLACK thats mean it's also a 100 ohm resistance bt it's a 1/4 watt my question is what is important resistance value or watt.

-------- Rgu--------- said...

You can build a PJRC circuit for external solutions, this won't void warranty.

Sowpath das said...

nice post

Alice Taylor said...

Thanks for the post, it's so awesome how you were able to come up with such a project. I need to show this to my brother who's good with tinkering on almost anything. For now, I think the best option for me is to look around for a suitable powerbank for my phone. I'm going camping this summer and would like a reliable and affordable one. While searching online, I came to this site and read some great reviews. You've got to see this:

Circuit Designer said...

R100=10ohms and not 100ohms. Thanks

Anurag Chugh said...

R100 = 0.1 ohms

Look at table on second page here:

Post a Comment