The little board was built to charge a single lipo battery using the MCP73831 from Microchip. I plan to use it along with several other boards featured on this blog in a larger project, but I like to break things up and make sure they’re working individually before making them work together.
When I went to choose a chip for this board I used the MCP73831 (datasheet here) because that is the same chip Adafruit and Sparkfun used in their charger. Below is the most typical circuit used with this chip. Keep in mind that the pin placement on this image is not the actually pin placement on the part. Actual pin placement is shown in the image below that.
Below is the board design, which can be downloaded from here. One one side is a jst connector for the battery and on the other is a mini usb jack to connect to a computer or wall plug. A capacitor of at least 4.7uF should be used on both the input and output (I used 10uF for both of mine). Finally, an LED shows the state of charging. If the led is on, it is charging and if the led if off, it is done charging. (It is possible to modify the design to illuminate a different led when the charging is done by connecting an LED to the stat pin in the opposite direction… I think.)
This chip charges at 4.2V and cannot be changed. However, the charge current can be changed by altering the value of the resister labeled Res1 on the board. The formula for determining this is: Charge current in mA = 1000 / resistance in Kohm. You can also use the graph below. I use 2k for 500mA, 10k for 100mA, and 20k for 50mA charging. The key thing to remember is that you should not charge small lipos faster than 2C – meaning 2 times the mAh rating of the battery. So don’t charge at 500mA unless the battery is at least a 250mAh battery.
If you see anything I missed or have any questions, feel free to put them in the comments below.