Series Balanced LiFePO4 Charger
The Lithium Iron Phosphate (LiFePO4) Cells are gaining popularity as storage cells in power banks. The charger is designed to balanced charge three LiFePO4 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 Iron Phosphate cells are listed below:
1) High capacity to weight ratio (high specific energy storage)
2) Available in high capacity, like 6000mAH
3) More safe, while charging and discharging
4) Over voltage charge tolerant
5) Suitable for power banks
6) Low internal leakage
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 LiFePO4 (3 cells) Charger is shown below:
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 3.8VDC, 7.6VDC and 11.4VDC, 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 box (which holds 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 does not exceed 3.8VDC, the charging voltage for each cell also limited to 3.8V 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. Then, you may disconnect the charger from the battery box.
For Series Balanced Quad LiFePO4 (4 cells) Charger, refer circuit diagram below: