The working principle of lithium battery protection board and introduction of over-discharge, over-charge and short-circuit protection
The circuit and parameters of the lithium battery protection board are different according to the use of IC, voltage, etc. The following is an explanation with DW01 with MOS tube 8205A:
1. The normal working process of the lithium battery protection board is:
When the cell voltage is between 2.5V and 4.3V, the first pin and the third pin of DW01 both output high level (equal to the supply voltage), and the second pin voltage is 0V. At this time, the voltage of pin 1 and pin 3 of DW01 will be applied to pins 5 and 4 of 8205A respectively. The two electronic switches in 8205A are in the conducting state because their G poles are connected to the voltage from DW01, that is Both electronic switches are in the open state. At this time, the negative pole of the battery is directly connected to the P- terminal of the protection board, and the protection board has a voltage output.
2. The over-discharge protection control principle of the protection board:
When the battery cell is discharged through an external load, the cell voltage will slowly decrease, and at the same time, DW01 will monitor the cell voltage in real time through the R1 resistor. When the cell voltage drops to about 2.3V, DW01 will consider that the cell voltage has been In the over-discharge voltage state, immediately disconnect the output voltage of the first pin, so that the voltage of the first pin becomes 0V, and the switch tube in the 8205A is closed because there is no voltage on the fifth pin. At this time, the B- of the battery cell and the P- of the protection board are in a disconnected state. That is, the discharge circuit of the battery cell is cut off, and the battery cell will stop discharging. The protection board is in an over-discharge state and has been maintained. After the P and P- of the protection board have indirect charging voltages, DW01 will immediately stop the over-discharge state after B- detects the charging voltage, and output a high voltage at pin 1 again to turn on the over-discharge control tube in the 8205A. That is, the B- of the battery cell and the P- of the protection board are reconnected, and the battery is directly charged by the charger.
3. Protection board overcharge protection control principle:
When the battery is normally charged by the charger, as the charging time increases, the voltage of the cell will become higher and higher. When the cell voltage rises to 4.4V, DW01 will consider the cell voltage to be in an overcharge voltage state. It immediately disconnects the output voltage of the third pin, so that the voltage of the third pin becomes 0V, and the switch tube in the 8205A is closed because there is no voltage on the fourth pin. At this time, the B- of the battery cell and the P- of the protection board are in a disconnected state. That is, the charging circuit of the battery cell is cut off, and the battery cell will stop charging. The protection board is in an overcharged state and has been maintained. After the P and P- of the protection board discharge the load indirectly, although the overcharge control switch is turned off, the forward direction of the diode inside is the same as the direction of the discharge circuit, so the discharge circuit can discharge. When the voltage of the battery cell When the voltage is lower than 4.3V, DW01 stops the overcharge protection state and outputs high voltage on pin 3 again, so that the overcharge control tube in 8205A is turned on, that is, the B- of the battery and the protection board P- are reconnected , The battery cell can be charged and discharged normally.
4. Protection board short circuit protection control principle:
As shown in the figure, in the process of external discharge of the protection board, the two electronic switches in the 8205A are not completely equivalent to two mechanical switches, but are equivalent to two resistors with very small resistance, and are called 8205A. Turn-on internal resistance, the turn-on internal resistance of each switch is about 30m\U03a9 and a total of about 60m\U03a9. The voltage applied to the G pole actually directly controls the on-resistance of each switch tube. When the G pole When the voltage is greater than 1V, the internal resistance of the switch tube is very small (tens of milliohms), which is equivalent to the switch being closed. When the voltage of the G pole is less than 0.7V, the internal resistance of the switch tube is very large (several MΩ). Equivalent to switch off. The voltage UA is the voltage generated by the on-resistance of 8205A and the discharge current. When the load current increases, UA will inevitably increase. Because UA0.006L? IUA is also called the tube voltage drop of 8205A, UA can be abbreviated to indicate the size of the discharge current. . When it rises to 0.2V, it is considered that the load current has reached the limit value, so the output voltage of pin 1 is stopped, so that the voltage of pin 1 becomes 0V, the discharge control tube in 8205A is closed, and the discharge circuit of the battery is cut off. Discharge control tube. In other words, the maximum allowable output current of DW01 is 3.3A, which realizes over-current protection.
5. Short-circuit protection control process:
Short-circuit protection is a limit form of over-current protection. Its control process and principle are the same as over-current protection. Short-circuit is only equivalent to adding a small resistance (about 0Ω) between PP- to protect the load of the board. When the current reaches more than 10A instantaneously, the protection board immediately performs over-current protection.