A protection board solution for balanced charging battery packs of series connected 3.7V 18650 lifepo4 battery

　　Basic working principle of balanced charging of lithium battery pack protection board

　　Among them: 1 is a single-cell lithium-ion battery; 2 is a charge overvoltage shunt discharge branch resistor; 3 is a switching device for shunt discharge branch control; 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 charge control pin CO, discharge control pin DO, discharge overcurrent and short circuit detection pin VM, battery positive terminal VDD, battery negative terminal VSS, etc.); 7 is The charging overvoltage protection signal is isolated by an optocoupler and forms a parallel relationship to drive the gate of the charge control MOS tube in the main circuit; 8 is the discharge undervoltage, overcurrent and short circuit protection signal, which is isolated by an optocoupler and forms a series relationship to drive the discharge in the main circuit. Control MOS tube gate; 9 is the charge control switching device; 10 is the discharge control switching device; 11 is the control circuit; 12 is the main circuit; 13 is the shunt discharge branch. The number of single-cell lithium battery protection chips is determined based on the number of cells in the lithium battery pack, and they are used in series to protect the corresponding single-cell lithium battery from the charge and discharge, overcurrent, and short-circuit conditions. While charging protection, the system uses the protection chip to control the on and off of the shunt discharge branch switching device to achieve balanced charging. This solution is different from the traditional approach of achieving balanced charging at the charger end and reduces the design of lithium battery pack chargers. Application cost.

　　When the lithium battery pack is charging, 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. The charging current flows through the positive pole BAT+ of the battery pack, the single 3.7V 18650 lifepo4 battery 1~N in the battery pack, and the discharge control switching device. , charging control switch device, battery pack negative electrode BAT-, the current flow direction is shown in Figure 2.

　　The charging overvoltage protection control signal of the single-cell lithium battery protection chip in the control circuit part of the system is isolated by an optocoupler and output in parallel to provide the gate voltage for the conduction of the charging switch device in the main circuit; such as one or several 3.7V 18650 lifepo4 battery During the charging process, the overvoltage protection state is entered first, and the overvoltage protection signal controls the discharge of the shunt discharge branch connected in parallel at the positive and negative ends of the single-cell lithium battery. At the same time, the corresponding single lithium battery connected in series in the charging circuit is disconnected. Leave the charging circuit.

　　When charging lithium battery packs in series, the impact of the difference in capacity of single cells is ignored. Generally, the battery with smaller internal resistance is fully charged first. At this time, the corresponding overvoltage protection signal controls the switching device of the shunt discharge branch to close, and a shunt resistor is connected in parallel to both ends of the original battery. According to the pNGV equivalent circuit model of the battery, the shunt branch resistance at this time is equivalent to the load of the single-cell lithium battery that is fully charged first. The battery is discharged 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 as shown in Figure 3. When all single-cell batteries are charged and enter the over-voltage protection state, the voltages of all single-cell 3.7V 18650 lifepo4 battery are completely equal within the error range. The charge protection control signals of each protection chip become low, and the charge control switching device in the main circuit cannot be used. The gate bias is provided to turn it off and the main circuit is disconnected, that is, balanced charging is achieved and the charging process is completed.

　　When the battery pack is discharging, 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 electrode BAT- of the battery pack, the charge control switch device, the discharge control switch device, and the single-cell lithium battery N in the battery pack. ~1 and the positive electrode BAT+ of the battery pack, the current flow direction is shown in Figure 4. In the control circuit part of the system, the discharge under-voltage protection, over-current and short-circuit protection control signals of the single-cell lithium battery protection chip are output in series after being isolated by optocoupler to provide the gate voltage for the conduction of the discharge switching device in the main circuit; once the battery pack When a single-cell lithium battery encounters special conditions such as undervoltage, overcurrent, and short-circuit during the discharge process, the corresponding single-cell lithium battery discharge protection control signal becomes low and cannot provide gate bias for the discharge control switching device in the main circuit. It is turned off and the main circuit is disconnected, which ends the discharge process.

　　Generally, 3.7V 18650 lifepo4 battery adopt constant current-constant voltage (TApER) charging control. During constant voltage charging, the charging current decreases approximately exponentially. The switching devices of the main charging and discharging circuit in the system can be selected according to the maximum operating current and operating voltage that the external circuit requirements meet.

　　The single-cell lithium battery protection chip for 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 discharge branch resistance connected in parallel 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 discharge current. The balancing current should be selected reasonably. If it is too small, the balancing effect will not be obvious; if it is too large, the energy loss of the system will be large, the balancing efficiency will be low, and the thermal management requirements of the lithium battery pack will be high. Generally, the current size can be designed between 50 and 100mA.

　　The shunt discharge branch resistance can be implemented using 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. In addition, it can also reduce thermal power consumption. Balanced charging protection board circuit working simulation model

　　Based on the basic working principles 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 the sake of simplicity, a simulation model in which the lithium battery pack consists of only 2 3.7V 18650 lifepo4 battery connected in series is given, as shown in Figure 5.

　　In the model, a controlled voltage source is used instead of a single-cell lithium battery to simulate the battery charging and discharging conditions. In Figure 5, Rs is the total internal resistance of the battery in the series battery pack, RL is the load resistance, and Rd is the shunt discharge branch resistance. The single-cell 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. Since there are 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 can easily lead to lengthy simulation time. Therefore, during the actual simulation process, circuits such as filtering, optocoupler isolation, and level adjustment were removed, and the resistor network designed for large current shunting was changed to a single resistor, which reduced 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 sequence of the modules must be correctly arranged, and the data type must be converted when necessary. The voltage detection module is used in the model to realize strong and weak signals. Conversion connection problem.

　　The given signal of the controlled voltage source in the simulation model can have slight differences on the premise that the waveform is generally consistent to represent the differences in the charge and discharge of individual batteries. Figure 6 shows the simulation results of voltage detection of a single cell in the battery pack. It can be seen that the circuit can work normally by using the overcurrent discharge branch equalization method.

　　System experiment

　　In practical applications, in response to the needs of a certain brand of electric bicycle manufacturer, we designed and implemented 2 sets of 36V8A·h lithium manganate power battery pack protection boards connected in parallel and 10 cells in series. The single-cell lithium battery protection chip adopts S28241 of Japan's Seiko Company. , The protection board mainly consists of the main circuit, control circuit, shunt discharge branch, filtering, optocoupler isolation and level adjustment circuit. Its basic structure is shown in Figure 7. The discharge branch current is selected to be around 800mA, and 510Ω resistors are connected in series and parallel to form a resistor network.

　　The debugging work is mainly divided into two parts: voltage test and current test. The voltage test includes two steps: charging performance detection overvoltage, equalizing charge and discharge performance detection undervoltage. You can choose to use a battery simulation power supply instead of an actual battery pack for testing. Since multiple batteries are connected in series, the test cost of this solution is relatively high. You can also use the assembled battery pack for direct testing, cycle 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 solution takes a long time to test once. When testing the charging performance of the battery pack, a 3-digit half-precision voltmeter was used to monitor the charging voltage of 10 batteries. It can be seen that each battery is within the normal operating voltage range, and the difference between the cells is very small. During the charging process 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: overcurrent detection and short circuit detection. For overcurrent detection, an ammeter can be connected in series between the resistive load and the power circuit, and the load can be slowly reduced. When the current increases to the overcurrent value, see whether the ammeter indicates a current cutoff. Short circuit detection can directly short-circuit the positive and negative terminals of the battery pack to observe the ammeter status. On the premise that the device is in good condition and the circuit welding is correct, the current test can also be conducted directly through the status of the power indicator light on the protection board.

　　In actual use, considering that external interference may cause the battery voltage to become unstable, which will cause overvoltage or undervoltage in a very short period of time, leading to misjudgment by the battery protection circuit, so the protection chip is equipped with a corresponding delay. Logic, if necessary, a delay circuit can be added to the protection board, 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 disconnected when the battery pack is not working, the static loss is almost zero. When the system is working, the main loss is the on-state loss on the two MOS tubes in the main circuit. When the balancing circuit is working in the charging state, the resistor heat loss in the shunt branch is larger, but the time is shorter, and the overall dynamic loss is in the battery. It is at an acceptable level within the normal working cycle of the group.

　　After testing, the design of the protection circuit can meet the protection needs of series-connected lithium battery packs. It has complete protection functions and can reliably protect overcharge and over-discharge, while achieving a balanced charging function.

　　According to the needs of the application, after changing the protection chip model and series number, and the power level of the switching devices and energy consumption components in the circuit, power lithium battery packs of any structure and voltage level can be protected and evenly charged. For example, the FS361A single-cell lithium battery protection chip of Taiwan Fujing Company can realize the design of 3 groups of parallel connection and 12 series of lithium iron phosphate battery pack protection boards. The final product is a variety of industrial products with reasonable prices, and there are no products that have been repaired after three years of market testing.

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