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Single Universal 3.7 V Li-Ion Battery Charger

The single universal battery charger for 3.7 V 18650 16340 14500 li-ion AA AAA rechargeable batteries, which is designed to charge your batteries quickly and safely with an auto-stop charging feature.

This Chinese mini li-ion charger has an EU/US plug, but the input mains supply is universal AC 100-240V, 47-63Hz.

The nominal charging voltage is 4.20V, and the current is 400mA.

It has a 3mm RG (red-green bi-colors) LED, red light lighting when charging mode, automatic stop and turns green when the battery is full charged.

Charger Specification

  • Model: US Plug / EU Plug
  • Input: AC 100-240V / 47-63Hz
  • Output: DC 4.2V / 400mA
  • Termination voltage: 4.20V +-1%
  • Charger size: 10x4x3cm / 3.93"x1.57"x1.18" (Approx.)
  • Compatible batteries: Size AA, AAA, C, D, RCR123, Gumstick 10440, 14500, 16340, 17335, 17500, 17670. 18500, 18650

Internal Circuit Board of Charger

Photo of single universal 3.7 V li-ion battery charger internal circuit board surface mount (SMD), and through hole (THT) component side.

Single Universal 3.7 V Li-Ion Battery Charger Circuit Diagram

Schematic of single universal 3.7V li-ion battery charger circuit with no chip (no charging control integrated circuit) shown below.

Working Principle of Charger Circuit

The circuit board of US/EU plug single universal 3.7V li-ion battery charger is a combination of three different circuits namely switched-mode power supply circuit, voltage reference circuit, and auto-stop charging circuit.

First, the charging circuit gets the power from the mains supply.

An inbuilt switched-mode power supply circuit contains two discrete transistors (13003H, 1HC) that step-down the high ac voltage to low dc voltage.

After rectifying, the voltage could be as high as 7.1V under no-load but very quickly drops 5.3V when the Li-ion battery is charging.

Next, a voltage reference 431 (TL431) circuit produces a fixed (constant) 4.20V to charge the battery safely.

To monitoring this process, an auto-stop charging circuit is used.

The monitoring circuit contains a PNP BC857 transistor, NPN BC847 transistor, few resistors, and connected with an RG LED.

When the voltage of a Li-ion battery is ≥ 3.7V, the red light will go high, indicating that the battery is being charged.

After reaching the target 4.20 volts, it will stop charging automatically, then the red light will turn Off, and the green light will turn On to indicate the completion.

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