1S 18650 Lithium Cell Charger

Overcharging protection is crucial for lithium batteries as it keeps them safe, helps them last longer, ensures optimal performance, and prevents fires. This single-cell (1S) charger continuously monitors the voltage of the 18650 lithium cell during charging and ensures that the 3.7V cell does not exceed its safe charging limit.

Components List

The following components are required to make this project.

Designation	Parts	Quantity
Q1	BD140 PNP Bipolar Medium Power Transistor	X1
U1	TL431 Adjustable Precision Shunt Regulator	X1
R1, R2	22kΩ 1/4W Resistor	X2
R3	20kΩ Potentiometer	X1
R4	1kΩ 1/4W Resistor	X1
R5	330Ω 1/4W Resistor	X1
D1, D2, D3, D4	1N4007 Diode	X4
D5	3MM Red LED	X1

Circuit Diagram

The schematic of a 1S lithium cell BMS circuit using tl431 is shown below.

Circuit Explanation

The circuit uses a constant 5V, 500mA input and has a voltage divider made of two resistors (R1, R2) to set a reference voltage, which is then adjusted with a potentiometer (R3). The TL431 is an integrated circuit that internally compares this input reference voltage against a precise voltage reference. The resistor (R4) limits the current for the IC (U1) and also provides biasing for the transistor (Q1). When connected to a 3.7V lithium cell, which has a typical fully charged voltage of 4.2V, the TL431 monitors the cell voltage during charging.

As the cell voltage reaches 4.2V, the TL431 circuit activates a PNP transistor (Q1). This transistor effectively becomes a switch that allows excess charging voltage to be bypassed. The bypassed current flows through a series of diodes (D1-D4), which may be used for rectification or additional current path management.

The excess voltage that is bypassed is dissipated as heat in the PNP transistor and the diodes, preventing overcharging of the lithium cell. A LED (D6) is activated to indicate that the cell is fully charged, and it is connected in series with a current-limiting resistor (R5).