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Last updated: September 19, 2025

18650 Battery Charger Voltage Protector Circuit Using TL431

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TL431 Transistor

TL431 Adjustable Shunt Regulator

Precision programmable shunt regulator, works like an adjustable Zener diode (2.5Vโ€“36V)

BD140 Transistor

BD140 Transistor

PNP medium power transistor, collector current up to 1.5A, voltage up to 80V

BC548B Transistor

BC548B Transistor

NPN general-purpose low noise transistor, hFE range 200โ€“450, collector current up to 100mA

3V LED 5mm

3V LED 5mm

3V, 5mm round light-emitting diode

1K Fixed Resistor 1/4W

1K Fixed Resistor 1/4W

A 1,000-ohm resistor rated for 0.25 watts

4.7R Fixed Resistor 2W

4.7R Fixed Resistor 2W

A 4.7-ohm resistor rated for 2 watts

20K Fixed Resistor

20K Variable Resistor

Adjustable resistor with 20,000 ohm resistance

680-ohm Fixed Resistor 1/4W

680R Fixed Resistor 1/4W

A 680-ohm resistor rated for 0.25 watts

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18650ma Battery charger Voltage Protector Circuit

Build a 18650 battery charger with voltage protection using TL431. Ensures safe charging, prevents overcharge, and extends battery life.

TL431 voltage protector for 18650

The battery charger will have a 18650 battery with TL431 voltage protection to ensure safe charging that will terminate at 4.2 V and prevent overcharging to prolong the battery life.

overcharge protection using TL431

The 18650 Lithium-ion battery has a safe charging voltage of 4.2V/cell. The overcharging may destroy a cell or even become dangerous. To avoid this, a TL431-based voltage protection circuit can be developed. TL431 is a programmable shunt voltage reference IC that behaves as an adjustable Zener diode. TL431 used as a circuit watch in this circuit is set to sense the battery voltage. A TL431 is activated once the voltage across the battery has reached 4.2 V and forces the flow of charge across a transistor or a relay to avoid overcharging. The IC has its reference voltage based on a network of resistor dividers. It can also be supplemented with an LED indication to indicate charging (charging or cutoff). This is a simple but useful charger protection circuit, which is extensively utilized in DIY lithium-ion charging, battery banks, power banks, and small UPS systems. It also guarantees a long battery life, overheating is avoided, and it is safer and comfortable when charging batteries of 18650.

Work / Installation (Inputs โ†’ Outputs)

This circuit has a DC input (5V12V), then through a regulator (LM317 or 7805), and then to the battery (18650). The TL431 maintains a constant check on the output voltage. When battery voltage goes to 4.2 V, TL431 is activated, turning off a transistor/relay and breaking the charger circuit. This offers an auto-cutoff system, which is safe.

Testing & Final Adjustments

Once assembled, a discharged 18650 cell should be connected. DC supply to the regulator (5 V or 12 V). Charge with a multimeter to measure battery voltage. Tilt the resistor divider network to fine-tune the cutoff at 4.2V. Check whether it will automatically cease charging when the voltage is 4.2V or not. When the cutoff point is wrong, adjust the values of the resistors. Heat check TL431, and transistor install heatsinks where necessary. Include a charging/full indicator. In the case of a battery using multiple cells, a battery management system (BMS) should be used rather than a single TL431. The charger would guarantee long-lasting battery life and safety with proper adjustment.

Frequently Asked Questions - 18650ma Battery charger Voltage Protector Circuit:

Why use TL431 in a charger?

It provides precise voltage reference for cutoff at 4.2V.

What is the cutoff voltage for 18650?

4.2V per cell is the safe charging limit.

Can TL431 replace a BMS?

For single cells, yes. For multiple cells, use a BMS.

Does it protect from overcurrent?

No, it mainly protects against overvoltage.

What input voltage is required?

5V to 12V DC depending on regulator used.

Is it safe for lithium batteries?

Yes, it ensures safe charging by preventing overcharge.

Can I charge multiple 18650 cells?

Not directly. Each cell needs individual protection.

What is TL431?

It is a programmable shunt voltage reference IC.

How much current can it handle?

It handles small current; a transistor/relay is needed for higher loads.

Can I use LED indicator?

Yes, LED can be added for charging and full status.

Rojony Khatun

Rojony Khatun

CalculatorToolsProject

Rojony Khatun is the creator of a free online tools website designed to simplify everyday and engineering-related calculations for students, professionals, and hobbyists. The platform offers a wide variety of basic and advanced calculators across key categories, including Electrical Calculators, Electronic Calculators, and Time & Date Calculators, helping users solve problems quickly and accurately.