Schematic

Above is the final circuit for the four cell LiFePO4 charger. I tweaked a few parts of the original design for safety, testing, and power considerations. A diode has been added after the main voltage output to allow for the capability to parallel multiple battery packs. Another thing is that I’m using generic PNP and power resistors so that the maximum charging current of the design can be adjusted as necessary. Since the charging current of a battery is loosely based on its capacity (see C Rating), allowing for a wide range of currents makes it theoretically possible to use any battery size. I also plan on using a charge current of ~100mA for my first go at this. Doing so allows the batteries to charge slower and give me more resolution when testing the charging circuitry.

BOM

The components I’m using from this design are as follows:

• 4x 2N3906 PNP transistors – from the SparkFun Discrete Semiconductor Kit
• 1x power diode – from the SparkFun Discrete Semiconductor Kit
• 1x LM317 voltage regulator – from the SparkFun Discrete Semiconductor Kit
• 4x TL431 voltage references – from Sparkfun
• 4x 10 kOhm through hole resistors that were laying around my office
• 4x 100 Ohm through hole resistors that were laying around my office
• 4x trim potentiometers – from Sparkfun
• 6x screw terminals – from Sparkfun
• 6x assorted capacitors – from the Sparkfun Capacitor Kit
• 1x 8 Ohm power resistor – from the Sparkfun Power Resistor Kit
• 1x PCB – from OSH Park

Board

One last thing to note is that I purposefully chose to use through hole parts for the sake of debugging. The excess space and exposed connectors make it easier to use alligator clips to measure the voltages and signals, making sure that everything is operating as it’s supposed to.

Next Steps

Soldering, assembly and testing!