I bought a cheap solar power bank from China to power my temperature sensor. Unfortunately it was shutting itself down once fully charged, requiring me to power on manually again. In this post I will show you my reverse-engineering notes, images and what I did to make it always powered on.
Originally entitled Free Shipping Power Bank 5000mAh Waterproof Solar Charger Dual USB Powerbank External Battery for Mobiles. Note: It is NOT waterproof!
Here is the image of the board
Pictures of the top and the bottom side of the PCB with my notes added to it
Here is how it works. It senses a device connected to the USB connector by that NPN transistor. During this time, n-channel mosfet at the bottom is turned off. When the microcontroller senses a device, it turns on the mosfet. On the other side is a dc-dc converter which is normally off until it needs to power the connected device. It turns it on by applying high to the “en” pin of the booster.
So to prevent it from automatically turning off the output when the battery is fully charged I had to do these things:
- Remove the n-channel mosfet and put a wire between s (source) and d (drain) of the mosfet. The PNP transistor is useless now but I kept it there.
- Solder “en” pin of the dc-dc booster with the VCC pin just next to the “en”
- Cut the original connection from the “en” pin to the microcontroller
Here is a complete schematic of the solar power bank. Please note that I made this schematic by just following the lines on the photos of the PCB, reading some datasheets and doing some googling so I can’t guarantee the accuracy of it. I made it to just understand how it all works.