In this guide we discuss how to charge a Lithium Polymer (Li-Po) 3.7 V battery using a Li-Po charger chip.

You will need:

  • Li-Po 3.7V
  • Li-Po Charger Chip
  • Battery Holder
  • Prototyping Wires
  • USB to mini USB cable

Step by Step Instructions:

Step 1: Solder one end of the jumper cables each to BAT+ and BAT- terminals.

Step 2: Fix a Li-Po Battery in a battery holder.

Step 3: Using a breadboard, connect the positive and negative terminal from the battery holder to the BAT+ and BAT- terminals respectively.

Step 4: Use a Mini USB to USB cable to source power. Connect the mini USB end of the cable, to the Li-Po charger and the other end to a wall plug or a Laptop’s USB port.

Step 5: The Red LED on the Li-Po charger will start glowing steadily. Leave the setup for 4-6 hours.

Step 6: The red LED turns off once the Li-Po battery will be fully charged and the blue LED starts glowing steadily.

block diagram for LiPo charger working system

More Information:

There are two ways of sourcing the required power needed to charge the battery. Now, one way is to use a constant voltage source (4.5 to 5 V) and connect it between the two input terminals (IN+ and IN-). The second and the more efficient way is to use a USB cable and connect it to the Li-Po chip through a computer or a wall adaptor. Once the battery is fully charged, it gets charged up to 4.2 V.

There are two LEDs, on the chip; a blue LED and a red LED. When the chip is connected to a voltage source, for example through a mini-USB, the blue LED will glow steadily. Once a discharged battery is connected between the terminals BAT+ and BAT-, the blue LED will stop glowing and the red LED will start to glow steadily, indicating it has entered charging mode. But if the battery is removed before it is fully charged, the red LED will continue to glow steadily but the blue LED will start to flicker, indicating that the battery was removed before it was fully charged. It takes roughly 4 to 6 hours to charge a Li-Po battery cell. Once the battery is fully charged the red LED turns off and the blue LED once again starts to glow steadily.

Maximum charging current is set by the programmable resistor (Rprog shown in the figure) on the chip. By increasing the programmable resistor, we can decrease the charging current. The default resistance of 1.2 KΩ of the programmable resistor on the chip allows the maximum charging current to be 1A.

The 4056 ES IC on the Li-Po charger chip controls the four different charging stages of the battery. When a fully discharged Li-Po charger is connected to a Li-Po charger, it first enters a per charge stage. During this stage, the current is 10% of its maximum charging current. Next it enters a constant current charging stage. During this stage the voltage across the battery starts to increase sharply. Then the battery voltage becomes constant, this stage is called the constant voltage stage. Finally, the current starts to fall and when the current becomes 10% of its maximum charging current, it enters the termination of charge stage. When the charging stage hits this stage the battery is fully charged and the red LED turns off and the blue LED starts to glow steadily.