Designing a DIY Lithium Polymer Battery Charger Circuit without Off-the-Shelf ICs

Designing a lithium polymer battery charger from scratch, without using any pre-made charging ICs, demands careful attention to safety and specific charging requirements.
Here’s a list of components you’ll need:

1. Voltage Regulator (LM317): This will regulate the charging voltage.
2. N-channel MOSFET (e.g., IRF540): It controls the charging current.
3. Resistor: Required to set the charging current limit.
4. Diode (e.g., 1N4007): To protect against reverse polarity.
5. Resistors and Capacitors: For voltage and current sensing during charging.
6. LED and Resistor: For indicating the charge status.
7. Potentiometer: To adjust the voltage regulation for your battery.

Let’s outline the circuit design:

1. Connect the positive terminal of the lithium polymer battery to the input of the LM317 voltage regulator.

2. The LM317’s output connects to the drain of the N-channel MOSFET.

3. Set the charging current limit by placing a resistor between the source of the MOSFET and the ground. To calculate the current limit (ILIM), use ILIM = 1.25V / R, where R is the resistor value in ohms.

4. Place a diode in series with the battery to prevent reverse current flow.

5. Use resistors and capacitors to monitor the battery’s voltage and current during the charging process.

6. Add an LED, along with a resistor, to indicate the charge status. For example, you can use a green LED while charging and a red one to show the battery is fully charged.

7. To regulate the charging voltage, include a potentiometer in the feedback path of the LM317. This allows you to set the desired voltage for your specific lithium polymer battery.

8. Connect a suitable power source (AC-DC adapter or DC power supply) to power the circuit and initiate the charging process.

Safety Precautions:

– Make sure the voltage and current limits are correctly set according to your lithium polymer battery’s specifications (e.g., 3.7V per cell and 350mA charging current for a 1C rate).

– Provide adequate heat sinking for the LM317 and MOSFET to dissipate any heat generated during charging.

– Carefully check the battery’s polarity and connections to avoid any reverse polarity issues.

– For added safety, consider incorporating features like temperature monitoring and overcurrent protection.

– During testing, regularly monitor the charging process and battery temperature to ensure the circuit operates within safe limits.