Series Balanced Li-Ion Charger
The Lithium Ion (Li-Ion) Cells are highly used as storage cells in power banks and electronic gadgets. The charger is designed to balance charge three Li-Ion cells in series using 12V transformer and LM338 adjustable voltage regulators. The circuit may be modified to charge 4 cells in series, using 15V transformer, if required.
Some of the advantages of Lithium Ion cells are listed below:
1) High capacity to weight ratio (high specific energy storage)
2) Available in high capacity, 1800mAH or more.
3) Easily available and affordable
4) Suitable for power banks
and so on....
The circuit diagram presented here is simple (without any micro-controller) and easy make-at-home project. The current circuit diagram can charge THREE cells in series, but can be modified to FOUR (or more) as required.
The full circuit diagram of Series Balanced Li-Ion (3 cells) Charger is shown below:
WORKING PRINCIPLE:
The mains power supply is stepped down to 12VAC using a transformer (12-0-12V 3A) and rectified to DC volts using 2 nos of 1N5408 diodes. A 4700 micro-Farad capacitor is used as filter capacitor. Then the Raw-DC is stepped down to 4.2VDC, 8.4VDC and 12.6VDC, using three numbers of LM338 voltage regulator ICs. Three trimpots are provided to get required output voltages by adjustment. The three outputs and ground connections are used for charging the three cells in series as shown. Using a four pin connector to battery pack (or separate three cells in series) is useful to quickly connect and disconnect for charging.
A 7812 fixed voltage regulator IC, is used to get 12VDC, which is useful to connect 12V cooling fan for cooling of three LM338 ICs. Heat sinks are to be provided for the LM338 ICs, which may be get heated up, while charging of cells.
As the difference between the two successive outputs of the four connections should not exceed 4.2VDC, the charging voltage for each cell also limited to 4.2V only. A combined Digital Voltmeter-cum-Ammeter is connected to know the voltage across the three cells and charging current value at any moment. Once the charging current shows 0.00A, then it is an indication of full charge of the cells. It is also observed that, if the charging current is less than 0.02A, then it indicates, the cells are charged more than 95 percent.