Working Principle Of Cell Balance Circuit

The lithium battery protection board is different according to the battery protection IC, voltage and other different parameters. The protection board has two core components: a protection IC, which is more accurate to obtain reliable protection parameters; the other is the MOSFET string in the main It acts as a high-speed switch in the charge and discharge circuit to perform protection actions. Let's explain with DW01 with dual NMOS tube 8205A.

The circuit principle of the lithium battery balance circuit protection device is shown in the above figure. Generally speaking, it is mainly realized by the battery protection control ICDW01 and the external discharge switch M1 and the charge switch M2. The control IC is responsible for monitoring the battery voltage and loop current, and controlling the gates of the two MOSFETs. The MOSFETs act as switches in the circuit. When the P+/P- terminals are connected to the charger and the battery is charged normally, M1 and M2 are both in conduction. Status: When the control IC detects abnormal charging, it turns off M2 to terminate charging. When the P+/P- terminal is connected to the load and the battery is discharged normally, both M1 and M2 are turned on; when the control IC detects the abnormal discharge, the M1 is turned off to terminate the discharge.

The circuit has the functions of overcharge protection, overdischarge protection, overcurrent protection and short circuit protection.

The working principle of the battery balance circuit is analyzed as follows:

1) Normal state

In the normal state, the "CO" and "DO" pins of the DW01 output high voltage in the circuit. Both MOSFETs are in the on state, and the battery can be charged and discharged freely. Because the on-resistance of the MOSFET is small, usually less than 30 milliohms, so its on-resistance has little effect on the performance of the circuit.

In this state, the current consumption of the protection circuit is uA.

2) Overcharge protection

The charging method required for lithium-ion batteries is constant current/constant voltage. In the initial stage of charging, it is constant current charging. With the charging process, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require a constant voltage value of 4.1V ), switch to constant voltage charging until the current becomes smaller and smaller. When the battery is being charged, if the charger circuit loses control, the battery voltage will continue to be charged with constant current after the battery voltage exceeds 4.2V. At this time, the battery voltage will continue to rise. When the battery voltage is charged to more than 4.3V, the battery’s chemistry Side reactions will intensify, causing battery damage or safety issues.

In a battery with a protection circuit, when the control IC (DWO1) detects that the battery voltage reaches 4.3V (this value is determined by the control IC, different ICs have different values), its "CO" pin will change from high voltage to Zero voltage turns M2 from on to off, thereby cutting off the charging circuit, making the charger no longer able to charge the battery and playing an overcharge protection role. At this time, due to the existence of the body diode VD2 of the M2, the battery can discharge the external load through the diode. When the control IC detects that the battery voltage exceeds 4.05V and sends the signal to turn off the M2, the overcharge is released, and the M2 is turned on to start charging.

3. Over discharge protection

When the battery is discharging the external load, its voltage will gradually decrease with the discharge process. When the battery voltage drops to 2.5V, its capacity has been completely discharged. At this time, if the battery continues to discharge the load, it will cause battery damage. Permanent damage

In the battery discharge process, when the control IC detects that the battery voltage is lower than 2.5V (this value is determined by the control IC, different ICs have different values), its "DO" pin will change from high voltage to zero voltage, making M1 It turns from on to off, which cuts off the discharge circuit, so that the battery can no longer discharge the load, which plays a role of over-discharge protection. At this time, due to the existence of the body diode VD1 of M1, the charger can charge the battery through this diode.

Since the battery voltage cannot be lowered in the overdischarge protection state, the current consumption of the protection circuit is required to be extremely small. At this time, the control IC will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.1uA.

4. Overcurrent protection

When the battery discharges the load normally, when the discharge current passes through the two MOSFETs connected in series, due to the on-resistance of the MOSFETs, a voltage will be generated at both ends of the MOSFET. The voltage value U=I*RDS*2, RDS is a single MOSFET conduction resistance, the "CS" pin on the control IC detects the voltage value. If the load is abnormal for some reason, the loop current will increase. When the loop current is large enough to make U>0.15V (this value is controlled by IC decides that different ICs have different values), its “DO” pin will change from high voltage to zero voltage, turning M1 from on to off, which cuts off the discharge circuit and makes the current in the circuit zero. To over-current protection.

In the above control process, it can be seen that the overcurrent detection value depends not only on the control value of the control IC, but also on the on-resistance of the MOSFET. When the on-resistance of the MOSFET is larger, the overcurrent protection of the same control IC The smaller the value.

5. Short circuit protection

When the battery is discharging the load, if the loop current is so large that U>1V (this value is determined by the control IC, different ICs have different values), the control IC will judge that the load is short-circuited, and its "DO" pin will quickly Turning from high voltage to zero voltage, M1 is turned from on to off, thereby cutting off the discharge circuit and playing the role of short circuit protection. The delay time of short circuit protection is extremely short, usually less than 7 microseconds. Its working principle is similar to over current protection

The CS pin of DW01 is the current detection pin. When the output is short-circuited, the on-voltage drop of the charge and discharge control MOSFET increases sharply, and the voltage of the CS pin rises rapidly. The DW01 output signal makes the charge and discharge control MOSFET turn off quickly, thereby achieving overcurrent or Short circuit protection.