Little LiFePO4 Battery Charger

The Little LiFePO4 Battery Charger is an analog, discrete charging circuit designed to safely replenish a single-cell lithium iron phosphate battery using a standard USB power source connected via CON1. At the heart of the system is an LM358 operational amplifier, which functions as a precision comparator to monitor the battery's terminal voltage relative to a stable reference. When charging is required, the op-amp controls the biasing of an S8550 PNP transistor, which acts as a pass element to regulate the charging current flowing into the battery terminals at CON2. A 1N4007 silicon diode is integrated into the path to prevent reverse current flow from the battery back to the USB source when power is removed, protecting the upstream circuitry. Resistors, including a 1.2k ohm resistor, are utilized across the circuit for current-limiting and setting appropriate voltage divider networks for the op-amp inputs. Indicator LEDs, such as a green LED, provide visual feedback regarding the operational status of the charger, lighting up to signal charging states. The entire discrete schematic is constructed on a perforated circuit board or custom PCB, offering a transparent, hardware-level look at linear power management principles without relying on complex microcontrollers or specialized integrated charger ICs.