Battery protection board implementation solution for multiple lithium batteries in series

When charging lithium batteries in series, each battery should be charged evenly, otherwise the performance and life of the entire battery group will be affected during use. Common balanced charging technologies include constant shunt resistor balanced charging, on-off shunt resistor balanced charging, average battery voltage balanced charging, switch capacitor balanced charging, buck converter balanced charging, inductor balanced charging, etc. However, the existing single-cell lithium battery protection chips do not contain balanced charging control functions; the balanced charging control function of multi-cell lithium battery protection chips requires an external CPU to be realized through serial communication with the protection chip (such as I2C bus), which increases the complexity and design difficulty of the protection circuit, reduces the efficiency and reliability of the system, and increases power consumption.

This paper aims at the problem that each lithium battery needs to be protected from over-voltage, under-voltage, over-current and short-circuit when using power lithium batteries in groups. During the charging process, the whole group of batteries needs to be charged evenly. A battery pack protection board with balanced charging function is designed to protect any number of lithium batteries in series by using a single lithium battery protection chip. Simulation results and industrial production applications have proved that the protection board has perfect protection functions, stable operation, high cost performance, and balanced charging error is less than 50mV.

Basic working principle of balanced charging of lithium battery protection board

The schematic diagram of the lithium battery pack protection board with balanced charging capability designed with a single-cell lithium battery protection chip is shown in Figure 1. Among them: 1 is a single-cell lithium-ion battery; 2 is a charging overvoltage shunt discharge branch resistor; 3 is a shunt discharge branch control switch device; 4 is an overcurrent detection protection resistor; 5 is an omitted lithium battery protection chip and circuit connection part; 6 is a single-cell lithium battery protection chip (generally including a charging control pin CO, a discharge control pin DO, a discharge overcurrent and short circuit detection pin VM, a battery positive terminal VDD, a battery negative terminal VSS, etc.); 7 is a charging overvoltage protection signal that is isolated by an optical coupler to form a parallel relationship to drive the charging control MOS tube gate in the main circuit; 8 is a discharge undervoltage, overcurrent, and short circuit protection signal that is isolated by an optical coupler to form a series relationship to drive the discharge control MOS tube gate in the main circuit; 9 is a charging control switch device; 10 is a discharge control switch device; 11 is a control circuit; 12 is the main circuit; 13 is a shunt discharge branch. The number of single-cell lithium battery protection chips is determined according to the number of lithium battery pack batteries, and they are used in series to protect the corresponding single-cell lithium battery from charge and discharge, overcurrent, and short circuit. While providing charging protection, the system controls the on and off of the shunt discharge branch switch device through the protection chip to achieve balanced charging. This solution is different from the traditional method of achieving balanced charging at the charger end, and reduces the cost of lithium battery pack charger design and application.

Figure 1 Schematic diagram of a lithium battery pack protection board with balanced charging capability

When the lithium battery pack is charged, the positive and negative poles of the external power supply are connected to the positive and negative poles BAT+ and BAT- of the battery pack respectively, and the charging current flows through the positive pole BAT+ of the battery pack, the single lithium battery 1~N in the battery pack, the discharge control switch device, the charge control switch device, and the negative pole BAT- of the battery pack. The current flow direction is shown in Figure 2.

Figure 2 Charging process

The charging overvoltage protection control signal of the single-cell lithium battery protection chip in the control circuit part of the system is output in parallel after optical coupling isolation, providing the gate voltage for the conduction of the charging switch device in the main circuit; if one or several lithium batteries enter the overvoltage protection state first during the charging process, the overvoltage protection signal will control the shunt discharge branch connected in parallel at both ends of the positive and negative electrodes of the single-cell lithium battery to discharge, and at the same time, the corresponding single lithium battery connected in series in the charging circuit will be disconnected from the charging circuit.

When charging a lithium battery pack in series, the influence of the difference in the capacity of a single battery is ignored, and the battery with a smaller internal resistance is generally charged first. At this time, the corresponding overvoltage protection signal controls the switch device of the shunt discharge branch to close, and a shunt resistor is connected in parallel at both ends of the original battery. According to the PNGV equivalent circuit model of the battery, the shunt branch resistance is equivalent to the load of the first fully charged single lithium battery. The battery discharges through it to maintain the battery terminal voltage within a very small range near the full state. Assuming that the first lithium battery is charged first and enters the overvoltage protection state, the current flow in the main circuit and the shunt discharge branch is shown in Figure 3. When all single batteries are charged and enter the overvoltage protection state, the voltage of all single lithium batteries is completely equal within the error range, and the charging protection control signals of each protection chip become low, which cannot provide gate bias for the charging control switch device in the main circuit, causing it to shut down and the main circuit to be disconnected, that is, balanced charging is achieved, and the charging process is completed.

Figure 3. Traffic balancing process

When the battery pack is discharged, the external load is connected to the positive and negative terminals BAT+ and BAT- of the battery pack respectively, and the discharge current flows through the negative terminal BAT- of the battery pack, the charging control switch device, the discharge control switch device, the single lithium battery N~1 in the battery pack and the positive terminal BAT+ of the battery pack, and the current flow direction is shown in Figure 4. The discharge undervoltage protection, overcurrent and short-circuit protection control signals of the single lithium battery protection chip in the control circuit part of the system are output in series after optical coupling isolation, and provide gate voltage for the conduction of the discharge switch device in the main circuit; once the battery pack encounters special situations such as single lithium battery undervoltage, overcurrent and short circuit during the discharge process, the corresponding single lithium battery discharge protection control signal becomes low, and it is unable to provide gate bias for the discharge control switch device in the main circuit, so that it is turned off, and the main circuit is disconnected, that is, the discharge process ends.

Figure 4 Discharge process

Generally, lithium batteries use constant current-constant voltage (TAPER) charging control. When charging at constant voltage, the charging current decreases approximately exponentially. The switching devices of the main charging and discharging circuits in the system can be selected according to the maximum operating current and operating voltage required by the external circuit.

The single-cell lithium battery protection chip of the control circuit can be selected according to the voltage level, protection delay time, etc. of the single-cell lithium battery to be protected.

The resistance of the discharge branch connected at both ends of a single battery can be calculated based on the charging voltage of the lithium battery charger, the parameters of the lithium battery, and the size of the discharge current. The balancing current should be selected reasonably. If it is too small, the balancing effect is not obvious; if it is too large, the energy loss of the system is large, the balancing efficiency is low, and the thermal management requirements of the lithium battery pack are high. The general current size can be designed between 50 and 100 mA.

The shunt discharge branch resistance can be realized by a power resistor or a resistor network. It is more reasonable to use a resistor network to realize the shunt discharge branch resistance, which can effectively eliminate the influence of resistance deviation and also play a role in reducing thermal power consumption.

Balanced charging protection board circuit working simulation model

According to the basic working principle of the balanced charging protection board circuit mentioned above, a system simulation model was built in the Matlab/Simulink environment to simulate the working conditions of the protection board during the charging and discharging process of the lithium battery pack to verify the feasibility of the design scheme. For simplicity, a simulation model of a lithium battery pack consisting of only two lithium batteries in series is given, as shown in Figure 5.

Figure 5 Simulation model of equalized charging protection for two lithium batteries in series

In the model, a controlled voltage source is used to replace a single lithium battery to simulate the battery charging and discharging. In Figure 5, Rs is the total internal resistance of the battery pack in series, RL is the load resistance, and Rd is the shunt discharge branch resistance. The single lithium battery protection chip S28241 used is packaged as a subsystem, making the overall model expression more concise.

The protection chip subsystem model mainly uses logic operation modules, symbolic function modules, one-dimensional table lookup modules, integration modules, delay modules, switch modules, mathematical operation modules, etc. to simulate the timing and logic of protection actions. Due to the certain differences between the simulation environment and the real circuit, filtering and strong and weak electrical isolation are not required during simulation, and redundant modules are likely to lead to lengthy simulation time. Therefore, in the actual simulation process, the filtering, optocoupler isolation, level conditioning and other circuits are removed, and the resistor network designed for large current shunt is changed to a single resistor, which reduces the complexity of the simulation system. When establishing a complete system simulation model, it is necessary to pay attention to the possible differences in input and output data and signal types of different modules. The connection order of the modules must be arranged correctly, and data type conversion must be performed when necessary. The voltage detection module is used in the model to realize the conversion connection problem of strong and weak signals.

The given signal of the controlled voltage source in the simulation model can have slight differences under the premise of roughly consistent waveforms to represent the differences in individual battery charge and discharge. Figure 6 shows the simulation results of single-cell battery voltage detection in the battery pack. It can be seen that the circuit can work normally by adopting the method of equalizing charge in the overcurrent discharge branch.

Figure 6 Lithium battery voltage detection simulation results

System Experiment

In actual application, according to the needs of a certain brand of electric bicycle manufacturer, a 36V8A·h lithium manganese oxide power battery pack protection board with 2 groups in parallel and 10 in series was designed and implemented. The single-cell lithium battery protection chip uses S28241 from Seiko Corporation of Japan. The protection board is mainly composed of the main circuit, control circuit, shunt discharge branch, filtering, optocoupler isolation and level conditioning circuit, and its basic structure is shown in Figure 7. The discharge branch current is selected to be around 800mA, and a 510Ω resistor is used in series and parallel to form a resistor network.

Figure 7 Basic structure of lithium battery pack protection board

The debugging work is mainly divided into two parts: voltage test and current test. The voltage test includes two steps: charging performance detection overvoltage, equalization charge, and discharge performance detection undervoltage. You can choose to use a battery simulation power supply instead of the actual battery pack for testing. Due to the series connection of multiple batteries, the one-time test cost of this solution is relatively high. You can also use the assembled battery pack for direct testing, cycle the battery pack, observe whether the protection device works normally during overvoltage and undervoltage, record the real-time voltage of each battery during overcharge protection, and judge the performance of equalization charging. However, this solution takes a long time to test once. When testing the charging performance of the battery pack, a 3.5-digit precision voltmeter is used to monitor the charging voltage of 10 batteries. It can be seen that each battery is within the normal working voltage range, and the difference between the monomers is very small. The voltage deviation during charging is less than 100mV, the full charge voltage is 4.2V, and the voltage deviation is less than 50mV. The current test part includes two steps: overcurrent detection and short circuit detection. For overcurrent detection, an ammeter can be connected in series between the resistive load and the power supply circuit, and the load is slowly reduced. When the current increases to the overcurrent value, see if the ammeter indicates a current cutoff. Short circuit detection can directly short-circuit the positive and negative poles of the battery pack to observe the current meter status. On the premise of ensuring that the device is intact and the circuit is welded correctly, the current test can also be performed directly through the status of the power indicator light on the protection board.

In actual use, considering that external interference may cause unstable battery voltage, which will cause overvoltage or undervoltage in a very short time, resulting in incorrect judgment of the battery protection circuit, the protection chip is equipped with corresponding delay logic, and a delay circuit can be added on the protection board if necessary, which will effectively reduce the possibility of malfunction of the protection circuit caused by external interference. Since the switching devices on the protection board are in the off state when the battery pack is not working, the static loss is almost zero. When the system is working, the main loss is the conduction loss of the two MOS tubes in the main circuit. When the equalization circuit is working in the charging state, the resistance heat loss in the shunt branch is large, but the time is short. The overall dynamic loss is at an acceptable level within the normal working cycle of the battery pack.

After testing, the design of the protection circuit can meet the protection needs of the series lithium battery pack. It has complete protection functions and can reliably protect against overcharge and overdischarge, while realizing the balanced charging function.

According to the needs of the application, after changing the protection chip model and the number of series connections, the power level of the switch devices and energy consumption components in the circuit, the power lithium battery pack of any structure and voltage level can be protected and charged equally. For example, the FS361A single-cell lithium battery protection chip of Taiwan Fujing Company can realize the design of 3 parallel groups and 12 series lithium iron phosphate battery pack protection board. The final multiple industrial products are reasonably priced, and there are no returned products after 3 years of market testing.

in conclusion

This paper adopts a single-cell lithium battery protection chip design to realize a battery pack protection board with multiple lithium batteries in series. In addition to completing the necessary overvoltage, undervoltage, overcurrent and short circuit protection functions, it can also realize balanced charging function. The simulation and experimental results verify the feasibility of the scheme, and the market usage verifies the stability of the design.