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 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 you unsure the charging procedure is complete and secure.
Circuit Working:
Parts List:
| Category | Component | Quantity |
|---|---|---|
| Resistors | 330Ω CFR | 1 |
| Rx (as given) CFR | 1 | |
| Preset 1k CFR | 1 | |
| Capacitors | PPC 100nF | 1 |
| Electrolytic 1000µF | 1 | |
| Semiconductors | Transistor BC547 | 1 |
| IC LM317 | 1 | |
| Miscellaneous | Fuse 1 Amp | 1 |
| Heatsink for LM317 | 1 |
For Li-ion and Li-Po accumulators the normal target voltage is 4.2V or 4.1V for some types which is different from the nominal 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.
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 0.6V 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 measured 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 is found using the following formula:
Vout = Vref (1 + R2/R1) + Iadj * 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.
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 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 right measured 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:
- To avoid short circuits make 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 your batteries stated voltage and current limitations.
- Stay cautious 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.
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