solar energy storage lifepo4 battery 24v 200ah.Introducing the balancing principle of lithium battery protection board

release time:2023-10-18 Hits: Popular:AG11 battery

This article focuses on the balancing principle of lithium battery protection boards used in power lithium batteries. Each lithium battery requires protection from charging overvoltage, discharge undervoltage, overcurrent, and short circuit. During the charging process, the issue of balanced charging of the entire battery set is introduced. This paper proposes a design scheme for a battery pack protection board with a balanced charging function that uses a single-cell lithium battery protection chip to protect any number of series-connected lithium batteries. Simulation results and industrial production applications have proven that the protection board has perfect protection functions, stable operation and high cost performance.

When charging a group of 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. Commonly used balancing charging technologies include constant shunt resistor balancing charging, on-off shunt resistor balancing charging, average battery voltage balancing charging, switched capacitor balancing charging, buck converter balancing charging, inductor balancing charging, etc. However, the existing single-cell lithium battery protection chips do not contain the balanced charge control function; the balanced charge control function of the multi-cell lithium battery protection chip requires an external CPU and is implemented through serial communication with the protection chip (such as I2C bus), which increases the This increases the complexity and design difficulty of the protection circuit, reduces the efficiency and reliability of the system, and increases power consumption.

This article focuses on the balancing principle of lithium battery protection boards used in power lithium batteries. Each lithium battery requires protection from charging overvoltage, discharge undervoltage, overcurrent, and short circuit. During the charging process, the issue of balanced charging of the entire battery set is introduced. This paper proposes a design scheme for a battery pack protection board with a balanced charging function that uses a single-cell lithium battery protection chip to protect any number of series-connected lithium batteries. Simulation results and industrial production applications have proven that the protection board has perfect protection functions, stable operation and high cost performance.

Lithium battery protection board balancing principle Commonly used balancing charging technologies include constant shunt resistance balancing charging, on-off shunt resistor balancing charging, average battery voltage balancing charging, switched capacitor balancing charging, buck converter balancing charging, inductor balancing charging, etc. When charging a group of 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. However, the existing single-cell lithium battery protection chips do not contain the balanced charge control function. The balanced charge control function of the multi-cell lithium battery protection chip requires an external CPU; it is realized through serial communication with the protection chip (such as I2C bus), which increases the This increases the complexity and design difficulty of the protection circuit, reduces the efficiency and reliability of the system, and increases power consumption.

This article is aimed at the use of power lithium batteries in groups. Each lithium battery requires protection from charging overvoltage, discharge undervoltage, overcurrent, and short circuit. During the charging process, the entire group of batteries must be balanced and charged. This paper introduces a method using a single cell. The lithium battery protection chip protects any number of series-connected lithium batteries in a battery pack protection board with a balanced charging function. Simulation results and industrial production applications prove that the protection board has perfect protection functions, stable operation, high cost performance, and the balanced charging error is less than 50mV.
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1. Balanced charging principle structure of lithium battery pack protection board

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

Balancing principle of lithium battery 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 MOS tube gate for charging control in the main circuit; the balancing principle 8 of the lithium battery protection board is that the discharge undervoltage, overcurrent, and short circuit protection signals are isolated by an optocoupler and formed in series. The relationship drives the MOS tube gate for discharge control in the main circuit; 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. Balancing principle of lithium battery protection board The number of single-cell lithium battery protection chips is determined according to the number of lithium battery cells, 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.

2Hardware design
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2.1 Charging circuit

When the lithium battery pack is charged according to the balancing principle of the lithium battery protection board, 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 and the single-cell lithium battery 1~ in the battery pack. N. Discharge control switching device, charging control switching device, battery pack negative electrode BAT-, the current flow direction is shown in Figure 2.

Balancing principle of lithium battery protection board

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 lithium batteries During the charging process, it first enters the over-voltage protection state. The balancing principle of the lithium battery protection board is controlled by the over-voltage protection signal to discharge the shunt discharge branch connected in parallel at the positive and negative ends of the single-cell lithium battery. At the same time, the shunt discharge branch connected in series in the charging circuit is discharged. The corresponding single lithium battery is disconnected from the charging circuit.

2.2 Main circuit and shunt discharge branch

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. Balancing principle of lithium battery protection board When all single-cell batteries are charged and enter the over-voltage protection state, the voltages of all single-cell lithium batteries are completely equal within the error range, and the charging protection control signals of each protection chip become low and cannot be used as the main circuit The charging control switching device in the device provides gate bias to turn it off, and the main circuit is disconnected, that is, balanced charging is achieved and the charging process is completed.

Balancing principle of lithium battery protection board

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. The general current size can be designed between 50~100mA.

2.3 Discharge circuit

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. Lithium battery protection board balancing principle 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 isolated by optocouplers and then output in series to provide a gate for the conduction of the discharge switching device in the main circuit. pole voltage; once the battery pack encounters special conditions such as undervoltage, overcurrent, and short circuit of a single lithium battery during the discharge process, the corresponding single-cell lithium battery discharge protection control signal becomes low, and the discharge control switching device in the main circuit cannot Provide gate bias to turn it off and disconnect the main circuit, which ends the discharge process.

Balancing principle of lithium battery protection board

Generally, lithium batteries 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 of the lithium battery protection board balancing principle control circuit can be selected according to the voltage level, protection delay time, etc. of the single-cell lithium battery to be protected. 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.

3 Balanced charging protection board circuit simulation

Based on the above balancing principle of the lithium battery protection board, 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 and 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 lithium batteries connected in series is given, as shown in Figure 5.

Balancing principle of lithium battery protection board

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 lithium battery protection board balancing principle 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 lithium battery protection plate equalization principle simulation model can have slight differences on the premise that the waveforms are generally consistent to represent the differences in individual battery charge and discharge. 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.

Balancing principle of lithium battery protection board

4 system experiments

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 manganese oxide power battery pack protection boards connected in parallel and 10 cells in series. The single-cell lithium battery protection chip uses S28241 of Japan's Seiko Company. The protection board is mainly composed of the main circuit, the control circuit, the shunt discharge branch, the filtering, the optocoupler isolation and the level adjustment circuit. The basic structure of the lithium battery protection board balancing principle 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.

Balancing principle of lithium battery 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, 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 performed 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 in a disconnected state when the battery pack is not working, the static loss is almost 0. 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, the resistor heat loss in the shunt branch is larger, but the time is shorter, and the overall dynamic loss is at an acceptable level during the normal working cycle of the battery pack.

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

According to the needs of the application, the balancing principle of lithium battery protection board can realize protection and equal charging of power lithium battery packs of any structure and voltage level after changing the protection chip model and series number, and the power level of switching devices and energy consumption components in the circuit. . For example, the FS361A single-cell lithium battery protection chip of Taiwan Fujing Company can realize the design of 3 groups of parallel and 12-series lithium iron phosphate battery pack protection boards.

The reason why lithium battery (rechargeable type) needs protection is determined by its own characteristics. Since the material of the lithium battery itself determines that it cannot be overcharged, overdischarged, overcurrent, short-circuited, or charged and discharged at ultra-high temperatures, the lithium battery components will always appear with a delicate protective board and a current fuse.

1. Normal state

Under normal conditions, the "CO" and "DO" pins of N1 in the circuit both output high voltage, and both MOSFETs are in the on state. The battery can charge and discharge freely. Since the on-resistance of the MOSFET is very small, usually less than 30 milliohms, so its on-resistance has little impact on the performance of the circuit..