The normal output voltage of a single-cell lithium-ion battery is about 3.7V, which can be directly used as the power supply for mobile phones, MP3/MP4 and some small-screen tablet computers. For electrical appliances that require higher voltage, such as mobile DVD/EVD or large-screen tablet computers, multiple lithium-ion batteries can be connected in series to obtain the required voltage. For a tablet computer that requires 11.1V power supply, a battery pack is required It is three lithium-ion batteries connected in series. The protection circuits of single-cell lithium-ion batteries and multi-series lithium-ion batteries are different. The following examples are analyzed.

1. Single-cell lithium-ion battery protection circuit

There are many specific composition methods of single-cell lithium-ion battery charge and discharge protection circuits, but the working principles are not much different. The following is an example of a circuit that is widely used in mobile phones for reference.

The control chip of this circuit is DW01 (or 312F), and the MOS switch tube is 8205A. As shown in Figure 6, B+ and B- are the positive and negative electrodes of the battery cell respectively; P+ and P- are the positive and negative electrodes output by the protection board, respectively. Negative pole; T is the temperature resistance (NTC) port, which generally needs to cooperate with the CPU of the electrical appliance to perform protection control.
DWO1 or 312F is a lithium-ion battery protection chip with built-in high-precision voltage detection and time delay circuit. The important parameters are as follows: the overcharge detection voltage is 3V, the overcharge release voltage is 4.05V; the overdischarge detection voltage is 2.5V V, the over-discharge release voltage is 3.0V; the over-current detection voltage is 5V, and the short-circuit current detection voltage is 1.0V; the maximum current that DW01 allows the battery to output is 3.3A. The pin functions of the chip are shown in Table 1.
battery management system
(1) normal work
In the circuit of the protection board, when the cell voltage is between 2.5V and 4.3V, the ① and ③ pins of DW01 both output a high level (equal to the power supply voltage), and the ② pin voltage is 0V. At this time, the two N-channel field effect transistors Q1 and Q2 in the 8205A are in the conducting state. Since the on-resistance of the 8205A is very small, it is equivalent to the direct connection between the D and S poles. At this time, the negative pole of the cell and the P of the protection circuit The - terminal is equivalent to direct connection, the protection circuit has voltage output, and its current loop is as follows: B+→P+→ load. P-→ ②, ③ feet of 8205A → ① feet of 8205A → ⑧ feet of 8205A → ⑥, ⑦ feet of 8205A → B-.

In this circuit, the 8205A internal field effect transistors Q1 and Q2 can be equivalent to two switches. When the G pole voltage of Q1 or Q2 is greater than 1V, the switch is turned on, and the internal resistance between D and S is very small (tens of milliseconds). Ohm), which is equivalent to closing the switch; when the voltage of the G pole is less than 0.7V, the switch is turned off, and the on-resistance between the D and S poles is very large (several megaohms), which is equivalent to the switch being turned off.

(2) Over-discharge protection
When the cell is discharged through an external load, the voltage across the cell will gradually decrease, and the DW01 will monitor the cell voltage in real time through the resistor R1. When the cell voltage drops to 2.3V (usually called the over-discharge protection voltage) ), DWO1 thinks that the cell is in an over-discharge state, the voltage of its ① pin becomes 0, and Q1 in 8205A is turned off. At this time, the B- and - of the cell are in a disconnected state, that is, the discharge circuit of the cell is cut off. , the cell will stop discharging.

After entering the over-discharge protection state, the cell voltage will rise. If it can rise to the threshold voltage of the IC (usually 3.1V, usually called the over-discharge protection recovery voltage), the ① pin of DW0 will return to output a high level, and the 8205A Q1 is turned on again.
(3) Battery charging

No matter whether the protection circuit enters the over-discharge state or not, as long as the charging voltage is applied between the P+ and P- terminals of the protection circuit, after the DW0 detects the charging voltage through the B terminal, it will immediately output a high level from the ③ pin, and the Q2 lead in the 8205A The current loop is as follows: the positive pole of the charger →p+→B+→B-, the ⑥ and ⑦ feet of 8205A → ⑧ of 8205A Feet → ① feet of 8205A → ②, ③ feet of 8205A → P-→ negative pole of the charger.

(4) Overcharge protection
When charging, when the battery is normally charged by the charger, the voltage across the cell will gradually increase as the charging time increases. When the cell voltage rises to 4.4V (usually called the overcharge protection voltage), DW01 will judge that the battery cell is in overcharged state, and immediately drop the voltage of pin ③ to 0V. Q2 in 8205A is turned off because pin ④ is low level. At this time, the B pole of the battery cell and the P- terminal of the protection circuit It is in a disconnected state and maintained, that is, the charging circuit of the battery cell is cut off and charging is stopped.

When the P+ and P- terminals of the protection circuit are connected to the discharge load, although Q2 is turned off, the forward direction of the internal diode is the same as the current direction of the discharge circuit, so the load can still be discharged. When the voltage at both ends of the cell is lower than 4.3V (usually called the overcharge protection recovery voltage), DW01 will exit the overcharge protection state, pin 3 will output a high level again, and Q2 will be turned on, that is, the B- terminal of the cell is connected to the The P- terminal of the protection circuit is reconnected, and the battery can be charged and discharged normally.

(5) Overcurrent protection
Since the MOs switch tube also has internal resistance when it is saturated and turned on, there will be a voltage drop between the D and S poles of the MOs switch tube when a current flows. The protection control IC will detect the voltage of the D and S poles of the MOs switch tube in real time. When the voltage rises to the IC protection threshold (usually 0.15V, called the discharge overcurrent detection voltage), the discharge protection execution terminal immediately outputs a low level, the discharge control MOs switch is turned off, and the discharge loop is disconnected.

DW01 detects the voltage drop on the MOs switch tube in real time through the resistor R2 connected between the V- end and the VSS end. When the load current increases, the voltage drop on Q1 or Q2 must also increase. When the voltage drop reaches 0.2V, DWO1 judges that the load current has reached the limit value, so the voltage drop of its ① pin is 0V, and the internal voltage of 8205A is 0V. The discharge control tube Q1 is turned off to cut off the discharge circuit of the cell. Realize overcurrent protection.

(6) Over temperature protection
The T port on the protection board is the over-temperature protection terminal, which is connected to the CPU of the electrical appliance. The common over-temperature protection circuit is relatively simple, that is, an NTC resistor (see R4 in Figure 7) is connected to the T terminal and the P- terminal, and the resistor is installed close to the cell. When the electrical appliance is in high-power working state for a long time (such as the mobile phone is in the state of talking for a long time), the temperature of the cell will rise, and the NTC resistance value will gradually decrease. The CPU of the electrical appliance detects the NTC resistance value. When the resistance value drops When the threshold is set by the CPU, the CPU immediately sends a shutdown command to stop the battery from supplying power to it, and only maintains a small standby current, so as to achieve the purpose of protecting the battery.