Design of three/four series lithium battery protection system

Lithium-ion rechargeable batteries are new high-energy batteries developed in the 20th century. Compared with traditional nickel-cadmium batteries and nickel-metal hydride batteries, they have the advantages of large capacity, high operating voltage, wide operating temperature range, long cycle life, low self-discharge rate, no memory effect, and no pollution. Since their introduction, they have been widely used in military and civilian small electrical appliances, such as mobile phones, portable computers, camcorders, cameras, etc., and have partially replaced traditional batteries. The voltage of a single lithium-ion battery is about 3.6V, and the capacity cannot be infinite. Therefore, single lithium-ion batteries are often connected in series or parallel to meet the requirements of different occasions. In order to ensure the safe and reliable use of lithium-ion batteries, this article introduces a strict and thorough design of a charging and discharging protection system. The scheme adopts a control method of charging and discharging separation, and has two-level single-cell overcharge protection, single-cell over-discharge protection, two-level discharge overcurrent protection, discharge short circuit protection, discharge temperature protection, charging temperature protection, charging reverse connection protection, and prohibition of discharge during charging. It can be applied to various occasions where three/four lithium-ion rechargeable batteries are used in series.

1 System Overview

The protection system uses Seiko Electronics' S-8254, a dedicated charge and discharge protection IC for three or four series-connected lithium-ion rechargeable batteries, to build a first-level protection. The S-8254 series has built-in high-precision voltage detection circuits and delay circuits, which perform high-precision voltage detection on each battery cell to achieve single-cell overcharge protection and single-cell over-discharge protection. It also has three-stage overcurrent detection functions, and the overcharge detection delay time, over-discharge detection delay time, and overcurrent detection delay time 1 can be set through external capacitors (overcurrent detection delay time 2 and overcurrent detection delay time 3 are fixed inside the chip). The system uses Seiko Electronics' S-8244 series, a dedicated lithium-ion rechargeable battery second-level protection IC with built-in high-precision voltage detection circuits and delay circuits, to achieve single-cell secondary charge protection for the battery, and its protection delay time can be set through the capacitance of the external capacitor.

The S-8254 can switch between three or four batteries in series through the SEL terminal; the S-8244 can be used as a secondary protection when three batteries are used in series by short-circuiting the fourth battery voltage detection terminals VCC3 and VSS through resistor R22.

2 Implementation of each protection function

The S-8254 series charge and discharge protection voltage and overcurrent detection voltage are in 50 mV increments, and the S-8244 series overcharge detection voltage is in 5 mV increments. The system can select the appropriate model according to the use requirements of different occasions. Taking the protection system in Figure 1 as an example, S-8254AAVFT and S-8244AAPFN are used as protection ICs to specifically explain the implementation process of each protection function.

Figure 1 is a schematic diagram of the protection system when four batteries are used in series.

2.1 Over-discharge protection

Under normal conditions, the discharge control terminal DOP of S-8254 is at VSS (negative voltage of battery 4) potential, the discharge MOS tubes QDIS1 and QDIS2 are in the on state, and the system can perform discharge work normally. When it is detected that the voltage of a battery is lower than 2.7 V (VDLn), and this state is maintained above TDL (TDL time is determined by the external capacitor CS of the over-discharge detection delay terminal CDT), the voltage of the DOP terminal becomes VDD (positive voltage of battery 1) potential, the discharge MOS tube is turned off, and the discharge stops. This state is called over-discharge state. After entering the over-discharge state, the voltage of the VMP terminal is pulled down by the load to below VDD/2 through resistor R3, and S-8254 turns to sleep state; after the load is disconnected, the voltage of the VMP terminal is pulled up by VDD to above VDD/2 and below VDD through resistor R9, charging MOS tubes QCHR1 and QCHR2, and S-8254 exits the sleep state. When all battery voltages are above 3.0 V (VDUn), the over-discharge state is released and the system resumes normal discharge work.

2.2 Overcurrent and short circuit protection

The system uses two parallel 20 mΩ power resistors RS1 and RS2 for overcurrent detection. When the discharge current is greater than 20 A, the voltage difference between the overcurrent 1, 2 detection terminals VINI and VSS is greater than the overcurrent detection potential 1 VI0V1 (0.2 V), and this state is maintained above TIOV1 (TIOV1 time is determined by the overcurrent 1 detection delay terminal CDT external capacitor C3), the voltage of the DOP terminal becomes the VDD potential, the discharge MOS tube is turned off, the discharge stops, and enters the overcurrent 1 protection state. In the overcurrent state, the VMP terminal voltage is pulled down to VSS by the load through the resistor R3; after the load is disconnected, the VMP terminal voltage is pulled up to the overcurrent detection potential 3 VIOV3 (the positive voltage of battery 1 VC1 ~ 1.2 V) through the internal RVMD resistor of the IC, the overcurrent state is released, and the system resumes normal discharge. When the discharge current is greater than 50 A, the voltage difference between VINI and VSS is greater than the overcurrent detection potential 2 VIOV2 (0.5 V), and this state is maintained for more than TIOV2 (1 ms), and the overcurrent 2 protection state is entered. When the load is short-circuited, the voltage of the overcurrent 3 detection terminal VMP is instantly pulled below VIOV3 (the detection delay time TI0V3 is 300μs), and the system enters the short-circuit protection (overcurrent 3 protection) state.

2.3 Overcharge protection

In order to ensure the safety of the battery, the system adopts two-level protection measures for the overcharge state. First, when the voltage of a battery is detected to be higher than 4.05 V (VCU2n), and this state is maintained above TCU2 (TCU2 time is determined by the S-8244 overcharge detection delay terminal ICT external capacitor C16), the S-8244 charging control terminal CO outputs a dynamic "H", the secondary charging MOS tube QCHR2 is turned off, and charging stops. This state is called the overcharge state; after entering the overcharge state, when all battery voltages are below 3.80 V (VCL2n), the overcharge state is released. If the S-8244 protection fails for some reason, the S-8254 overcharge protection takes effect. When it is detected that the voltage of a battery is higher than 4.25 V (VCUln), and this state remains above TCUl (TCUl time is determined by the S-8254 overcharge detection delay terminal CCT external capacitor C2), the S-8254 charging control terminal COP becomes high impedance, the G pole of the first-level charging MOS tube QCHRl is pulled up by the external resistor R2, QCHRl is turned off, and enters the overcharge state; when all battery voltages are below 4.15 V (VCLln), the overcharge state is released.

2.4 Charging temperature protection

In order to ensure the safety during charging and extend the service life of the battery, the charging temperature of the battery should be controlled between 0 and 45°C. The system uses a negative temperature coefficient NTC temperature sensor RES and a two-way comparator LM393 to realize charging temperature protection. Its schematic diagram is shown in Figure 2: When the charging temperature is between 0 and 45°C, the outputs of the two comparators of LM393 are both high-impedance, and the PNP transistor Q1 is turned off, which has no effect on the charging circuit; as the temperature rises, the resistance of RES gradually decreases. When the temperature is greater than 45°C, the comparator below LM393 is reversed and outputs a low level. The B pole of Q1 is pulled down through the diode D6, Q1 is turned on, and the G pole C_QCHR of the charging MOS tube QCHRl is forced to be pulled high, QCHRl is turned off, and charging stops; similarly, as the temperature decreases, the resistance of RES gradually increases. When the temperature is less than 0°C, the comparator above LM393 outputs a low level, Q1 is turned on through the diode D5, thereby turning off QCHRl and stopping charging.

2.5 Other protection functions

The system cleverly achieves certain required protection functions through some simple and effective circuit designs.

2.5.1 Discharge temperature protection

In order to ensure the safety of battery use, the discharge temperature of the battery needs to be limited. The system connects a normally open recoverable temperature fuse F1 between the G pole C_QDIS and VDD of the discharge MOS tubes QDIS1 and QDIS2. Under normal conditions, F1 remains open. It does not affect normal discharge; when the battery temperature is higher than 75°C, F1 is closed, C_QDIS and VDD are connected. The discharge MOS tube is closed and the discharge stops, thereby realizing the discharge temperature protection function.

2.5.2 Charging reverse connection protection

If the positive and negative poles of the charger are connected to the system in reverse, a large current loop will be formed by the charger and the battery, causing damage to components and even greater safety hazards. This system connects a reverse connection protection diode D1 in series in the charging loop, so that even if the charger is reversely connected, the potential of CHR1 will be higher than CHR+, and the system will not form a loop due to the presence of diode D1, thus protecting it.

2.5.3 Discharging is prohibited during charging

If the system is allowed to discharge while connected to a charger for charging, it may bring unnecessary safety hazards. Therefore, the system connects a diode D4 between the positive input terminal CHR+ and C_QDIS of the charger. When the charger is not connected, CHR+ is suspended and has no effect on the discharge work; when the charger is connected for charging, C_QDIS is forced to be pulled high by CHR+ through D4, QDIS1 and QDIS2 are closed, and discharge is prohibited.

3 Conclusion

Lithium-ion rechargeable batteries have been widely used in many fields due to their unique performance advantages, and it is expected that they will become one of the main power sources in the 21st century. With the development of the lithium-ion rechargeable battery industry, the protection system as an integral part will play an increasingly important role.