Design and Implementation of a Balanced Charging of Lithium Battery Pack Protection Board

Commonly used equalization charging technologies include constant shunt resistor equalization charging, on-off shunt resistor equalization charging, average battery voltage equalization charging, switched capacitor equalization charging, buck converter equalization charging, and inductive equalization charging. When groups of lithium batteries are charged in series, each battery should be balanced and charged, otherwise the performance and life of the entire battery will be affected during use. The existing single-cell Lithium Battery protection chip does not include the balanced charging control function, and the multi-cell lithium battery protection chip equalization charging control function needs an external CPU; through serial communication with the protection chip ( such as I2C bus ) , the The complexity and design difficulty of the protection circuit, the efficiency and reliability of the system are reduced, and the power consumption is increased.

In this paper, the power lithium battery is used in groups. The lithium batteries of each section require the protection of charging over voltage, discharge under voltage, over current and short circuit. In the charging process, the problem of equalizing the charging of the whole battery should be realized. Lithium battery protection chip Design of a battery protection board with balanced charging function for protecting any series of lithium batteries in series. The simulation results and industrial production application prove that the protection board has perfect protection function, stable operation, high cost performance, and the balanced charging error is less than 50mV.

1 Lithium Battery Pack protection board equalization charging principle structure

The block diagram of the lithium battery pack protection board with balanced charging capability designed by a single-cell lithium battery protection chip is shown in Figure 1 .

Figure 1 lithium battery pack protection board structure block diagram

Among them: 1 is a single-cell lithium-ion battery ; 2 is a charging over-voltage shunt discharge branch resistor ; 3 is a shunt discharge branch control switching 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 charging control pin CO, discharge control pin DO, discharge overcurrent and short-circuit detection pin VM, battery positive terminal VDD, battery negative terminal VSS, etc. ); The charging overvoltage protection signal is isolated by the optocoupler to form a parallel relationship driving the gate of the MOS transistor for charging control in the main circuit ; 8 is a discharge undervoltage, overcurrent, short circuit protection signal is isolated by the optocoupler to form a series relationship driving the main circuit discharge Control MOS tube gate ; 9 is charge control switching device ; 10 is discharge control switching device ; 11 is control circuit ; 12 is main circuit ; 13 is shunt discharge branch. The number of single-cell lithium battery protection chips is determined according to the number of lithium battery cells, and is used in series to protect the charge, discharge, overcurrent and short circuit conditions of the corresponding single-cell lithium battery. The system realizes balanced charging by controlling the on/off of the shunt discharge branch switching device while protecting the chip. This scheme is different from the traditional method of achieving balanced charging at the charger end, and reduces the design of the lithium battery pack charger. The cost of the application.

2 hardware design

2.1 charging circuit

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

Figure 2 lithium battery pack charging circuit

In the control circuit part of the control circuit, the single-cell lithium battery protection chip's charging over-voltage protection control signal is isolated by optocoupler and then connected in parallel to provide the gate voltage for the conduction of the charging switch device in the main circuit ; such as a certain section or several lithium batteries When the overvoltage protection state is first entered during the charging process, the overcurrent protection signal is controlled to discharge the shunt discharge branch connected in parallel between the positive and negative terminals of the single-cell lithium battery, and the corresponding single lithium battery connected in series in the charging circuit is broken. Leaving the charging circuit.

2.2 main circuit and shunt discharge branch

When the lithium battery pack is charged in series, the influence of the difference in the capacity of the single battery is ignored, and the battery with a small internal resistance is first filled. At this time, the corresponding overvoltage protection signal controls the switching device of the shunt discharge branch to be closed, and a shunt resistor is connected in parallel across the primary battery. According to the PNGV equivalent circuit model of the battery, the shunt branch resistance is equivalent to the load of the first full-cell lithium battery, and the battery is discharged through it to maintain the battery terminal voltage within a very small range near the full state. First assume that the first charging a lithium battery is completed, into the overvoltage protection state, the main circuit and the shunt branch discharge current flows as shown in FIG. When all the single-cell batteries are charged into the over-voltage protection state, the voltages of all single-cell lithium batteries are completely equal within the error range, and the protection protection signals of the protection chips of each section become low, and the charging control switching devices in the main circuit cannot be obtained. The gate bias is provided to turn off, and the main circuit is disconnected, that is, balanced charging is achieved, and the charging process is completed.

Figure 3 main circuit and shunt discharge branch

The discharge branch resistance of the single-cell battery connected in parallel can be calculated according to the charging voltage of the lithium battery charger and the parameters of the lithium battery and the discharge current. The equalization current should be reasonably selected. If it is too small, the equalization effect is not obvious ; if it is too large, the system has large energy loss and low equalization efficiency. The lithium battery pack has high thermal management requirements, and the general current can be designed between 50 and 100 mA .

2.3 discharge circuit

When the battery pack is discharged, the external load is respectively connected to the positive and negative terminals BAT+ and BAT- of the battery pack, and the discharge current flows through the battery pack negative BAT- , the charge control switch device, the discharge control switch device, and the single-cell lithium battery N in the battery pack. ~1 and battery pack positive BAT+, current flow as shown in Figure 4 . In the control circuit part of the control circuit, the single-cell lithium battery protection chip discharge undervoltage protection, overcurrent and short circuit protection control signals are isolated by optocoupler and then connected in series to provide the gate voltage for the conduction of the discharge switch components in the main circuit ; once the battery During the discharge process, the group encounters a special condition such as undervoltage or overcurrent and short circuit of a single lithium battery. The corresponding single-cell lithium battery discharge protection control signal becomes low, and the gate bias voltage cannot be provided to the discharge control switching device in the main circuit. , so that it is turned off, the main circuit is disconnected, that is, the discharge process is terminated.

Figure 4 battery pack discharge circuit

Generally, the lithium battery adopts a constant current - constant voltage (TAPER) type charging control, and the charging current is approximately exponentially reduced when the constant voltage is charged. The switching device of the main circuit of the charging and discharging system in the system can be selected according to the maximum working current and working voltage that the external circuit requires.

The single-cell lithium battery protection chip of the control circuit can be selected according to the voltage level of the single-cell lithium battery to be protected, the protection delay time, and the like. The shunt discharge branch resistor can be implemented by a power resistor or a resistor network. Here, the resistor network is used to realize the shunt discharge branch resistance, which can effectively eliminate the influence of the resistance deviation, and can also reduce the thermal power consumption.

3 balanced charging protection board circuit simulation

According to the basic principle of the above-mentioned balanced charging protection board circuit operation, the system simulation model is built in Matlab/Simulink environment to simulate the working condition of the protection board during the charging and discharging process of the lithium battery pack, and the feasibility of the design scheme is verified. For simplicity, the simulation model is given only by lithium batteries lithium series Section 2, as shown in FIG.

Figure 5 2- cell lithium battery series equalization protection simulation model

In the model, a controlled voltage source is used instead of a single-cell lithium battery to simulate the charge and discharge of the battery. In Figure 5 , Rs is the total internal resistance of the battery in series, RL is the load resistance, and Rd is the shunt discharge branch resistance. The single-cell lithium battery protection chip S28241 is packaged as a subsystem, which makes the overall model expression more concise.

The protection chip subsystem model mainly uses the logic operation module, the symbol function module, the one-dimensional table lookup module, the integral module, the delay module, the switch module, the mathematical operation module, etc. to simulate the timing and logic of the protection action. Because there is a certain difference between the simulation environment and the real circuit, no filtering and strong and weak electrical isolation are needed for the simulation, and the redundant modules are easy 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 high current shunting is changed to a single resistor, which reduces the complexity of the simulation system. When establishing a complete system simulation model, it should be noted that the input and output data and signal types of different modules may be different. The connection order of the modules must be correctly arranged. If necessary, the data type is converted. The voltage detection module is used to implement the strong and weak signals. The conversion connection problem.

The given signal of the controlled voltage source in the simulation model may have a slight difference under the premise that the waveform is substantially uniform, to represent the difference in charge and discharge of the battery individual. Figure 6 shows the simulation results of the single-cell battery voltage detection in the battery pack. It can be seen that the circuit can work normally by using the over-current discharge branch equalization method.

Figure 6 lithium battery voltage detection simulation results

4 system experiment

In practical application, for the demand of a brand of electric bicycle production plant, two sets of parallel and 10 series series of 36V8A · h- manganese lithium battery pack protection boards were designed and implemented. The single-cell lithium battery protection chip adopts S28241 of Seiko Co., Ltd. The protection board is mainly composed of a main circuit, a control circuit, a shunt discharge branch, and a filter, an optocoupler isolation and a level conditioning circuit. The basic structure is shown in FIG. 7 . The discharge branch current is selected to be around 800 mA, and a 510 Ω resistor is connected in series to form a resistor network.

Figure 7 lithium battery pack protection board debugging

The debugging work is mainly divided into two parts: voltage test and current test. The voltage test includes two steps of charging performance detection overvoltage, equalization, and discharge performance detection undervoltage. You can choose to use a battery analog power supply instead of the actual battery pack for testing. Because of the multi-cell battery connection, the cost of testing this solution is high. You can also use the assembled battery pack to directly test, charge and discharge the battery pack, observe whether the protection device operates normally during overvoltage and undervoltage, record the real-time voltage of each battery during overcharge protection, and judge the performance of balanced charging. However, this program takes a long time to test. When the battery pack is tested for charging performance, the charging voltage of 10 batteries is monitored by a 3 -digit half-precision voltmeter. It can be seen that the batteries in each section are within the normal working voltage range, and the difference between the monomers is small, during charging. The voltage deviation 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 of overcurrent detection and short circuit detection. Overcurrent detection can connect a current meter in series between the resistive load and the power supply circuit to slowly reduce the load. When the current increases to the overcurrent value, see if the ammeter indicates a current interruption. Short-circuit detection can directly short the positive and negative of the battery pack to observe the status of the ammeter. Under the premise of determining that the device is intact and the circuit is soldered correctly, the current test can also be performed directly through the state of the power indicator on the protection board.

In actual use, considering the external interference may cause the battery voltage to be unstable, which will cause overvoltage or undervoltage of the voltage for a very short time, resulting in incorrect judgment of the battery protection circuit, so the protection chip is equipped with a corresponding delay. Logic, if necessary, add a delay circuit on the protection board, which will effectively reduce the possibility of external circuit interference causing the protection circuit to malfunction. Since the battery pack does not work, the protective plate of each switching device in the OFF state, so that the static loss of nearly 0. When the system works, the main loss in the main circuit 2 on-state losses in the MOS transistor, when the state of charge When the equalization circuit works, the resistance heat loss in the shunt branch is large, but the time is short, and the overall dynamic loss is at an acceptable level during the normal working period of the battery pack.

After testing, the protection circuit is designed to meet the needs of series lithium battery pack protection, complete protection functions, reliable overcharge and overdischarge protection, and balanced charging.

According to the needs of the application, after changing the protection chip type and series number, the power level of the switching device and the energy-consuming component in the circuit, the power lithium battery pack of any structure and voltage level can be protected and equalized. For example, the FS361A single-cell lithium battery protection chip from Taiwan Fujing Company can realize the design of three sets of parallel and 12- string lithium iron phosphate battery pack protection boards. The final batch of industrial products is reasonably priced, and there is no repair product after three years of market inspection.

5 conclusions

In this paper, a single-cell lithium battery protection chip design is used to realize a multi-cell lithium battery series battery pack protection board. In addition to the necessary over-voltage, under-voltage, over-current and short-circuit protection functions, the balanced charging function can also be realized. The simulation and experimental results verify the feasibility of the scheme, and the market usage test the stability of the design.