How does the lithium battery protection board work?

The lithium battery protection board has different circuits and parameters depending on the IC used, voltage, etc. The protection board has two 

core components: a protection IC, which uses an accurate comparator to obtain reliable protection parameters; the other is a MOSFET string in 

It acts as a high-speed switch in the main charge and discharge circuit and performs protective actions. The following is an explanation using 

DW01 with MOS tube 8205A:

Method to activate the protection board: When the protection board P+ and P- have no output in the protection state, you can short-circuit B- and 

P- to activate the protection board. At this time, Dout and Cout will both be at low level (these two ports of the protection IC are High level 

protection, low level normal) state opens two MOS switches.

1. The normal working process of the lithium battery protection board is:

When the cell voltage is between 2.5V and 4.3V, the 1st and 3rd pins of DW01 both output high level (equal to the supply voltage), and the voltage 

of the second pin is OV. At this time, the voltage of pins 1 and 3 of DWO1 will be added to pins 5 and 4 of 8205A respectively. The two electronic 

switches in 8205A are in a conductive state because their G pole is connected to the voltage from DWO1, that is, Both electronic switches are on. 

At this time, the negative electrode of the battery core is directly connected to the P- terminal of the protective plate, and the protective plate has a 

voltage output.

2. Protection board over-discharge protection control principle:

When the battery core is discharged through the external load, the voltage of the battery core will slowly decrease. At the same time, the DW01 

will internally monitor the battery core voltage in real time through the R1 resistor. When the battery core voltage drops to about 2.3V, DW01 will 

consider that the battery core voltage has expired. In the over-discharge voltage state, the output voltage of pin 1 is immediately disconnected, 

so that the voltage of pin 1 becomes OV, and the switch tube in 8205A is turned off because pin 5 has no voltage. At this time, the B- of the 

battery core and the P- of the protective plate are disconnected. That is, the discharge circuit of the battery core is cut off and the battery core will 

stop discharging. The protection board is in an over-discharge state and remains so. After the P and P- of the protection board are indirectly 

connected to the charging voltage, DW01 immediately stops the over-discharge state after detecting the charging voltage through B-, and outputs 

a high voltage on pin 1 again, turning on the over-discharge control tube in the 8205A. That is, the B-- of the battery core and the P- of the 

protection board are reconnected, and the battery core is charged directly by the charger. 

3. Protection board overcharge protection control principle:

When the battery is charged normally through 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, DWO1 will consider that the cell voltage is in an overcharge voltage state. The output voltage of pin 3 is 

immediately disconnected, so that the voltage of pin 3 becomes OV, and the switch tube in 8205A is turned off because pin 4 has no voltage. 

At this time, the B- of the battery core and the P- of the protective plate are disconnected. That is, the charging circuit of the battery core is cut 

off and the battery core will stop charging. The protection board is in an overcharged state and remains so. After the P and P- of the protection 

board are indirectly connected to the discharge load, although the overcharge control switch is turned off at this time, 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 cell When it is 

placed below 4.3V, DWO1 stops the overcharge protection state and outputs high voltage at pin 3 again, causing the overcharge control tube in 

the 8205A to conduct, that is, the B- of the battery core and the protection board P- are reconnected. , the battery core can charge and discharge 

normally.

4. Protection board short circuit protection control principle:

During the process of external discharge of the protection board, the two electronic switches in the 8205A are not completely equivalent to the two 

mechanical switches, but are equivalent to two resistors with very small resistance, and are called the conduction internal resistance of the 8205A. 

The on-resistance of each switch is about 30mU 03a9 and the total is about 60mU O3a9. The voltage applied to the G electrode actually directly 

controls the on-resistance of each switch tube. When the G electrode voltage is greater than 1V, The on-on internal resistance of the switch tube is 

very small (tens of milliohms), which is equivalent to the switch being closed. When the G electrode voltage is less than 0.7V, the on-on internal 

resistance of the switch tube is very large (several MQ), which is equivalent to the switch being off. . Voltage UA is the voltage generated by the 

on-resistance of 8205A and the discharge current. As the load current increases, UA will inevitably increase. Because UA0.006L×IUA is also 

called the tube voltage drop of 8205A, UA can simply 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 OV. The discharge control 

tube in 8205A is closed, cutting off the discharge circuit of the battery core, and shutting down Discharge control tube. In other words, the maximum 

output current allowed by DWO1 is 3.3A, realizing over-current protection.