I made a LiPo Battery Charging Circuit as my final project for Circuit Design class. I made a circuit that can control charging, protect from over discharging and supply power to a load. I got the schematics from Kina Smith because he already had the schematics for making the circuit that I needed.

The schematic has 3 different parts that has 3 different functions.

This part is using MCP73831 microchip that controls charging and protects from overcharging. The resistor Rset determines the charging current. I am using 4.7k Ohm resistor so my charging current is 1000/Rset=1000/4700= 212 mAmps. It is not recommended to charge a battery with a current that is higher than 1/3rd of battery capacity. The smallest battery that i can charge is 638mAh.

Second part is the part that prevents over-discharging using S8241 chip. If battery is below 2.5 volts the system is locked out until battery reaches 2.9 volts.

And then there is power supply part that supplies 3.2 volts.

After studying the circuit, I ordered surface mount components from digikey.com.

I used etching method to create the board because Micro USB pads were too small to mill with the end mill bits we had. I had problems with transferring ink from blue transfer paper to PCB board, the paper wouldn’t melt or transfer the ink properly.

So I decided to use VOUGE magazine paper which worked perfect.

After transferring the ink successfully I etched the boards in acid.

As each board was etched I removed the ink off of the boards with scotch-brite and went through all of the routes under microscope to check if there were any short circuits. As the last step I coated all the boards to prevent oxidation.

After finishing etching it was time to solder surface mount components on the boards. Before doing that I drilled holes for header pins. I used press drill for this purpose. After drilling I started surface mounting small components.

Next step was to surface mount the big components. For MicroUSB I had to drill holes for supporting legs of the component and the holes had to be perfectly placed so the pads of the component would be perfectly placed. It was really hard to solder this component because it has really small pads and the SMD station wasn’t working at that time.

After surface mounting all the components I used soldering iron for soldering MicroUSB’s in place and for soldering the header pins.

I am happy to say that in the end I have 4 fully functional charging circuits.