Universal Single 3.7V Battery Charger Circuit

The Chinese universal single 3.7V battery charger is designed to safely and efficiently charge rechargeable Li-Ion (18650, 16340, 14500), AA, and AAA batteries with a voltage rating of 3.7V. It charges them to their nominal voltage of 4.2V while providing a steady current of 400mA.

The charger incorporates a Bi-color LED that indicates the charging status, with the LED emitting a red light during the charging process and automatically switching to green when the battery is fully charged.

Here, we will discuss and explain the workings of this 3V3 battery charger along with its circuit diagram.

Circuit Diagram

Schematic of universal single 3.7v battery charger circuit is shown below.

Working Explanation

The working principle of the Universal single 3.7V battery charger circuit is simple, and it works in four steps - AC to DC conversion, Self-Oscillation, another AC to DC conversion, and charging monitoring.

The input mains 220V AC passes through a single diode (D1), and half-rectifying the voltage to 220V DC, which is smoothed by the capacitor (C1).

The DC voltage turns on the transistor (Q1) by the Base resistor (R2) and conduct the current trough the primary winding (L1) of the SMPS transformer (T1). But at the same time, an opposite (but low) voltage starts rising across the auxiliary feedback winding (L3) of the transformer, and the capacitor (C3) stores voltage through the resistor (R6).

Additionally, the capacitor (C2) stores voltage through resistors (R1, R5) and completes the path through the diode (D2) to neutral.

After a few milliseconds, the primary winding reaches its maximum inductance capacity, stops the current flow through the transistor and slowly dicrising the inductance by the resistor (R4).

At this moment, the voltage of capacitor (C3) discharging through the Zener diode (D3) and providing the sufficient voltage to increas the biasing process of the transistor (Q2), which is helps to discharge the capacitor (C2) faster.

A self-oscillation begins itself very fast (around 10-50kHz) due to the charging-discharging process, depending on various parameters.

Therefore, a step-down voltage is generated in the secondary winding (L2) of the transformer with same frequency, which is then rectified using a single Schottky diode (D4) and smoothed by a capacitor (C4).

A bi-color LED (red and green) is connected in parallel to the capacitor (C4) between resistors (R7 & R8). Initially, it indicates a red light during no-load or battery charging.

The rectified DC voltage is approximately 7.1V without a load, and 5.3V with the load. A series resistor (R9) and diode (D6) drop the voltage to charging voltage +4.2V.

When a fully discharged battery positive terminal is connected to the charging voltage and the negative terminal to ground through a resistor (R10), the charging process begins.

Initially, the charger operates in current mode, keeping the current constant while gradually increasing the voltage. Once the target voltage of 4.2V is reached, the charger transitions into voltage mode, where the voltage is held constant, and the 400mA current gradually decreases to zero.

During this stage, the precision shunt voltage regulator TL431 IC (U1) with the use of an external voltage divider formed by resistors (R11 & R12) active it at 4.2V.

Now, the voltage is sufficient to turn on the transistor (Q4), and the current bypasses through the transistor (Q3) emitter-base, which emits the red light of LED (D5). This indicates that the cell is fully charged.