Super Power LiFePO4 Cell Charger
LiFePO4 ( Lithium Iron Phospate ) is a powerful cell, may consider as improved Lithium-Ion cell, which has following advantages compared to other Secondary cells:
1) Occupies less volume for same capacity (mAH)
2) Low weight w.r.t. capacity in mAH
3) Available in higher capacity (up to 6000 mAH or more)
4) Can deliver higher power / current as per demand.
5) High charging voltage tolerant
6) Does not catch fire easily, while charging
7) Useful to make high capacity power banks
8) Longer life
. . . . . .
LiFePO4 Charging has to under go two types of charging processes:
1) Constant Current Process up to 3.6 volts
2) Constant Voltage Process above 3.5 volts
To avoid quick relay on and off in between 3.5V and 3.6V, 0.1V overlap is maintained in the code.
If the Voltage is more than 3.9V, then constant voltage process continues up to 4.1V.
So, the circuit diagram presented here, charges each cell, up to FOUR cells in parallel (individually). The Charging process of each cell is controlled by the ARDUINO-NANO micro controller by operating concerned relay connected to Voltage Regulator IC LM317. By operating relay, the LM317 IC may be either configured as Constant Current (CC) Process or Constant Voltage (CV) Process.
The Voltage of each cell is read by the four ADC channels of ARDUINO-NANO, and charge the cell accordingly. The Voltages of the four cells and status (CC / CV / CF) are always displayed on the OLED.
Status codes displayed on the OLED:
CC = Constant Current (may consider as fast charging). LM317 may get heated up during this period, so heat sink must be provided.
CV = Constant Voltage (may consider as slow charging).
CF = Charging Full ( or Cell Absent )
Refer full circuit diagram:
The above circuit, can charge 4 LifePO4 cells in parallel. As the 4 charging circuits are shown in small size, a typical Charging circuit is shown here for clarity.
The charging current in CC mode may be about 1.2A per cell. Heat sinks should be used for LM338, to dissipate the heat generated by the LM338 ICs. In case of charging 3 LifePO4 cells, then the transformer 12-0-12V 5A is suffieceint. Similarly for charging 2 LifePO4 cells, then the transformer 12-0-12V 4A is sufficient.
Once the circuit is ready, then load the sketch "Super_Power_LifePO4_Cell_Charger.ino" to arduino IDE. Then select Arudino-Nano board and port before compiling and uploading to Arduino-Nano micro controller. Initially, the OLED screen displays 0 to 9 numbers as self test. Then, the Voltage and Status of each cell, in four lines, is displayed on OLED.
Now, connect LiFePO4 cell in charging circuit-1 (out of 4) and read the voltage across the cell using multi-meter (in Voltmeter mode). Now adjust the Trimpot, VR1 (1K near to the cell positive terminal), until the displayed voltage value (first line) on OLED matches the voltage value on multi-meter. Repeat the above process for remaining three charging circuits also. i.e., VR2 for second line, VR3 for third line and VR4 for forth line.
Now, the circuit is ready. You may connect (or insert in the box) all the 4 cells and observe the voltage change and status on OLED. To quickly dissipate the temperature of LM338 ICs from heat sinks, a 12V cooling fan may be connected across the Relay power supply (Red line) and ground.