Simple Li-ion Battery Charger Circuit using LM317 IC

Do any one wants to make their 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 how to build a simple charger using a common chip called the IC LM317.

We will also discuss 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 built correctly.

Only do this under adult or expert supervision.

What is a Li-ion Battery Charger Circuit:

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

These Li-ion batteries are often found in different kinds of electronic devices including computers, cellphones, cameras and electric vehicles because of their high energy density and rechargeable qualities.

For a Li-ion battery charger circuit an effective and carefully monitored charging process is implemented to extend a batteries life and prevent overcharging.

Charging Characteristics:

Li-ion and Li-Po cells are charged using two stage method called current mode and voltage mode.

When the charger is first turned ON it maintains a steady current while allowing the voltage to increase.

The charger switches into voltage mode where the voltage remains constant and the current decreases slowly to zero as soon as the right voltage Umax is achieved.

This helps to unsure the charging procedure is complete and safe.

Circuit Working:

Parts List:

FFor Li-ion and Li-Po devices the normal voltage range is 4.2V or 4.1V for some types which is different from the low voltage of about 3.7V or even 3.6V.

It is necessary to remember that charging the cell to 4.2V could reduce its lifespan.

A 10% decrease in capacity can happen when the goal voltage is lowered to 4.1V but the lifespan nearly gets doubled.

Cells must not be discharged below 3.4 to 3.3V and must be kept partially charged in order to maintain battery health.

Charger Schematic and Construction:

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

Reaching accurate charging voltage its 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 suitable for 90% capacity charging 4.1V.

Trimmer P1 sets the target voltage which is accurately controlled by the LM317 circuit.

An NPN transistor and a shunt resistor are used to regulate the current.

The transistor opens to lower the voltage on the adjust pin and regulate the current when the shunt Rxs voltage loss reaches around 0.6V.

Using the following formula the value of Rx is calculated by the kind of charged cell:

Rx = 0.6 / Imax.

For safety it is recommended to connect a properly adjusted fuse in series with the cell.

Formulas:

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

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

where,

• The internal reference voltage (Vref) of the LM317 usually is 1.25V.
• The output Vout and adjust Adj pins are separated by the fixed resistor R1.
• Ground  is connected to the variable resistor or potentiometer R2 and Adj is adjusted.
• Normally the adjust pins quiescent current or Iadj ranges from 50 to 100 uA.

Following 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 voltage disrupts its functionality.

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

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

How to Build:

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 adjust adj pin to the collector of Q1.
• Join one end of P1 to the base of BC547.
• Use the formula Rx = 0.6 / Imax where Imax is the required charging current to get the value of Rx.
• Connect Rx in series with the negative terminal of the battery.
• Connect an right measured fuse in series with the positive terminal of the battery for safety.
• Join 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:

• It is advised to connect the charger to a power source that is within the required range
• Use a multimeter and be sure the output voltage is equal to the required voltage level.
• One should be sure that the charging current is at the right level by keeping an eye on it.
• Check if there are any any odd heating or charging issues while the device is charging.

Safety Precautions:

• To avoid short circuits be sure all connections are secure and the insulation is sufficient.
• Do not forget to verify the connections polarity twice.
• For effective heat dissipation keep the heat sink sufficiently aired.
• Verify the batteries stated voltage and current limits.
• Stay safe of overcharging since it might shorten battery life and present safety risks.

Conclusion:

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.

For accurate and safe charging use specialized Li-ion charger integrated circuits (ICs) with inbuilt safety features especially for high power applications or costly batteries.

References:

Datasheet LM317

Lithium-ion battery