Simple Li-ion Battery Charger Circuit using LM317 IC

Want to make your own charger for lithium ion (Li-ion) batteries, like the ones in phones and laptops?

It can be a fun way to learn how electronics work.

This article will show you how to build a basic charger using a common chip called the IC LM317.

We will also talk about how batteries like this charge best and how to keep them safe so they last a long time.

WARNING: Building chargers for lithium ion batteries can be dangerous if not done correctly.

Only do this with adult supervision.

What is a Li-ion Battery Charger Circuit:

A Li-ion (Lithium-ion) battery charger circuit is an electronic circuit designed to charge Li-ion batteries.

Li-ion batteries are widely used in various electronic devices, including smartphones, laptops, cameras, and electric vehicles, due to their high energy density and rechargeable nature.

A Li-ion battery charger circuit needs to provide a controlled and safe charging process to maximize battery life and prevent overcharging.

Charging Characteristics:

The charging process for Li-ion and Li-Po cells involves a two stage approach: current mode and voltage mode.

Initially, the charger operates in the current mode maintaining a constant current while allowing the voltage to rise.

Once the target voltage Umax is reached, the charger transitions into voltage mode where the voltage is constant and the current gradually approaches zero.

This ensures a thorough and safe charging process.

Circuit Working:

Parts List:

Target Voltage Considerations:

The typical target voltage for Li-ion and Li-Po accumulators is 4.2V or 4.1V for specific types, which differs from the nominal voltage of around 3.7V(sometimes 3.6V.

It is important to note that charging the cell to the full 4.2V may decrease its lifetime.

Reducing the target voltage to 4.1V can result in a 10% capacity drop but nearly doubles the lifetime.

To preserve battery health, it is crucial not to discharge cells below 3.4 to 3.3V and store them partially charged.

Charger Schematic and Construction:

The Li-ion charger schematic utilizes the LM317 integrated circuit as a voltage stabilizer.

Achieving accurate charging voltage is critical especially for Li-ion and Li-Po cells.

Adjusting the voltage with +/- 1% accuracy is necessary for full voltage charging 4.2V while a slightly lower accuracy around 3% is acceptable for 90% capacity charging 4.1V.

The LM317 circuit provides precise voltage stabilization with the target voltage set by trimmer P1.

Stabilizing current is achieved through a shunt resistor and an NPN transistor.

When the shunt Rxs voltage drop reaches approximately 0.6 V, the transistor opens reducing the voltage on the adjust pin and stabilizing the current.

The value of Rx depends on the type of charged cell, with a formula for calculation:

Rx = 0.6 / Imax.

It is advisable to include a properly dimensioned fuse in series with the cell for safety.

Formulas:

The output voltage Vout of a circuit using an IC LM317 voltage regulator and adjustable resistors can be found using the following formula:

V_out = V_ref (1 + R₂/R₁) + I_adj * R₂

where,

• The LM317s internal reference voltage Vref is normally 1.25V.
• The fixed resistor R1 is positioned between the adjust Adj and output Vout pins.
• The variable resistor, or potentiometer, R2 is linked to ground GND and adjust Adj.
• The adjust pins quiescent current, or Iadj is normally between 50 and 100 uA.

Operating Parameters and Safety:

The charger operates with a supply voltage in the range of 9 to 24V balancing power loss and proper circuit operation.

Too high a voltage increases LM317 circuit power loss, while too low a voltage disrupts its functionality.

Adequate heat dissipation is ensured by placing the LM317 circuit on a sufficiently large heat sink.

The charger is designed to be resistant to output short circuits.

Circuit Construction:

Below mentioned are the steps to build a Simple Li-ion Battery Charger Circuit using LM317 IC.

• Connect the LM317 to the prototyping board.
• Connect 330 ohm between the output and the adjust adj pin.
• Connect adjust adj and the input Vin pin to 6 to 12V DC.
• Connect a capacitors 10μF and 1000μF between the adjust adj pin and ground.
• Connect the potentiometer between the adjust adj and ground.
• Adjust P1 to set the desired target voltage.
• This should be done without connecting the battery ensuring the target voltage corresponds to the output voltage with no load.
• Connect the emitter of Q1 to ground.
• Connect the collector of Q1 to the adjust adj pin.
• Connect the base of BC547 to one end of P1.
• Calculate the value of Rx using the formula: Rx = 0.6 / Imax, where Imax is the desired charging current.
• Connect Rx in series with the negative terminal of the battery.
• Connect an appropriately dimensioned fuse in series with the positive terminal of the battery for safety.
• Attach the LM317 to a sufficiently large heat sink to dissipate heat generated during charging.
• Connect the power supply within the range of 9 to 24V.

Testing:

• Connect the charger to a power supply within the specified range.
• Use a multimeter to verify that the output voltage matches the set target voltage.
• Monitor the charging current to ensure it aligns with the desired value.
• Check for any unusual heating or issues during the charging process.

Safety Precautions:

• Ensure proper insulation and secure connections to prevent short circuits.
• Double check the polarity of connections.
• Keep the heat sink well ventilated for efficient heat dissipation.
• Adhere to the specified voltage and current limits for your batteries.
• Avoid overcharging as it can lead to reduced battery life and safety hazards.

Conclusion:

To conclude, this simple li-ion battery charger circuit is important to recognize the limitations of a basic LM317 circuit, even though it can produce a working charger.

Use specialized Li-ion charger ICs with built in safety features for dependable and safe charging, particularly for expensive batteries or high power applications.

References

Datasheet LM317

Lithium-ion battery