CN101997328B - Lithium battery management system - Google Patents

Abstract

The invention relates to a lithium battery management system which comprises a constant-current and constant-voltage circuit using an LT1769 chip and a balance protection circuit using a BQ77PL900 chip, wherein the constant-current and constant-voltage circuit comprises a first switching element and a second switching element which are composed of P channel field-effect transistors and are connected with the LT1769 chip, the first switching element can keep the switching-on state in the charging process and can be fast switched off in case that an external power is subject to short-circuit reverse connection, and the second switching element can keep the switching-on state in the power supply process and can be immediately switched off as required when being connected with an external charger; and the balance protection circuit comprises a PIC18F458 singlechip and an external current expansion circuit, the PIC18F458 singlechip is connected with the BQ77PL900 chip and enables the BQ77PL900 chip to operate in an active control mode, the external current expansion circuit comprises an amplification circuit and a power discharge circuit, the amplification circuit comprises a PNP triode and is connected with the BQ77PL900 chip, and the power discharge circuit is connected with the amplification circuit and comprises power field-effect transistors connected with power discharge resistors. According to the invention, the optimized management on charging/discharging balance and protection of the lithium battery of a robot can be realized.

Description

Lithium battery management system

Technical field:

The invention relates to a power supply system of a robot, in particular to a lithium battery management system used for charging and discharging balance and protection of a group robot with an automatic charging function.

Background technique:

Since the swarm robot has powerful information processing capabilities, excellent motion performance, and maintains the characteristics of small size and compactness, the power supply of the robot is powered by 6 lithium iron phosphate batteries in series to meet the power consumption of the main processing unit and power unit. . According to the charging and discharging requirements of lithium batteries, the battery management system includes a constant current and constant voltage charging unit, a discharge undervoltage protection unit, a charging overvoltage protection power supply, and various short circuit and overload protection units. The use of lithium batteries in series has the problem of charging and discharging balance between batteries: although using lithium batteries in series can strictly ensure that the charging and discharging currents of each battery are equal, due to the differences in the energy storage characteristics of each battery, some The battery is fully charged ahead of time, while another part of the battery is discharged ahead of time. If the necessary charge and discharge balance management is lacking, over time some batteries will be undercharged for a long time, while others will be incompletely discharged, and the difference will accumulate more and more. Therefore, a management system using 6 lithium batteries in series needs a balanced charging unit to solve the above problems. What this patent narrates is exactly the balance protection circuit and constant current constant voltage circuit of lithium battery, see accompanying drawing 1.

With the gradual improvement of lithium battery performance, it can not only be used as a power source for traditional mobile electronic devices, but also can be used as a power source. However, the general power supply requires a relatively high voltage, so the charging balance problem when several or even dozens of batteries are used in series has become a technical difficulty. The existing charge balancing methods are:

(1) Passive balancing method: In the battery management system, each battery is connected to a load resistor through a switch, and this passive balancing circuit can discharge the selected battery cells individually. This method is only suitable for suppressing the voltage rise of the strongest cell in charging mode. In addition, to limit power consumption, such circuits generally only allow small current discharges, resulting in long charge balancing time, which can be as high as several hours. The passive balance charging circuit using discrete components and a general-purpose single-chip microcomputer uses power resistors to dissipate energy to increase the passive balance discharge speed, which can greatly reduce the balancing process time in the charging process, but because of the use of discrete components, the volume is too large, which is not suitable for compact requirements of swarm robots.

(2) Active balance method: The active balance method uses a storage element to transfer energy, one is to use a capacitor as a storage element, and the other is to store energy in a magnetic field, and then use a transformer to complete energy conversion. If the former is used, a huge switch array is required to connect the energy storage capacitors to all battery sections; if the latter is used, the switching sequence used for equalization must be strictly correct, otherwise the battery and its circuit will be short-circuited and burned. Danger, Therefore, its reliability is relatively low.

Comparing the above two schemes, if the capacitor active balancing method is adopted, it is difficult to realize the switch array, and if the transformer-based balancing method is adopted, a bulky transformer cannot be added to the internal space of the robot, and the switching timing problem also directly affects the reliability of the system. Therefore, by weighing the pros and cons, passive balance charging is a more feasible method, but it needs to solve the problems of power consumption and charging balance speed.

Figure 2 shows an existing passive balancing method circuit using Linear Technology and Texas Instrument chips. The advantages and disadvantages of this circuit are described below.

(1) Constant current and constant voltage circuit

Advantages - the circuit uses Linear Technology's LT1769 chip as a constant current and constant voltage charger. According to the requirements of 6-cell iron-lithium battery and taking into account the balance of charging speed and heat dissipation, the charging current is about 1.5A, and the maximum voltage is 6×3.6V =21.6V, the chip basically solves the charging problem.

Defects - the circuit generates a lot of heat during normal operation, which worsens the heat dissipation problem of the system in the compact and small space of the swarm robot.

(2) Balance protection circuit

Advantages - The circuit uses Texas Instrument's BQ77PL900 chip to complete the battery undervoltage, overvoltage, short circuit, overload and charge balance functions. Using this chip to complete the charge management function can greatly reduce the size of the battery management module.

Defects - First of all, this chip of Texas Instruments is designed for ordinary polylithium batteries. When the chip is running in automatic mode, the voltage threshold for starting battery balancing is 4.2V, and the voltage of a single-cell lithium iron battery exceeds 3.6V. There is a risk of overcharging and damage, so this chip is not suitable for iron-lithium batteries. The management of BQ77PL900 for iron-lithium batteries can only be realized through the chip's external MCU (MicroControl Unit) interface and flexible master control mode. Secondly, due to the power consumption limitation of the chip itself, the maximum current flowing through its own balance switch is 5mA, which causes the battery balance time to take several hours. If external current expansion measures are used to improve, it is necessary to use a 1V threshold Special MOS tube, this kind of MOS tube is not only very difficult to select, but also difficult to turn on under the condition of low dropout. In short, the length of battery balancing time is more difficult to solve.

Invention content:

The technical problem to be solved by the present invention is to overcome the defects of large power consumption, difficult heat dissipation and slow charging balance speed of the existing passive balance lithium battery management system of robots, and to provide a lithium battery management system, which adopts a reasonable main control mode and External current expansion measures reduce component power consumption, thereby achieving the purpose of increasing the charging balance speed of the system and improving heat dissipation conditions.

The technical scheme that the present invention solves its technical problem is as follows:

A lithium battery management system, including a constant current constant voltage circuit using the LT1769 chip and a balance protection circuit using the BQ77PL900 chip, is characterized in that: the constant current constant voltage circuit includes a first switch element connected to the LT1769 chip and a second Switching elements, the first switching element and the second switching element are P-channel field effect transistors, the first switching element can be kept on during charging and quickly turned off when the external power supply is short-circuited and reversed, and the second switching element It can be kept on during power supply and shut off immediately when the external connection is needed for charging; the balance protection circuit includes a PIC18F458 single-chip microcomputer and an external current expansion circuit connected to the BQ77PL900 chip, and the PIC18F458 single-chip microcomputer can control the charging balance of the battery , which is connected to the BQ77PL900 chip through the IIC bus and makes it run in the active control mode, the external current expansion circuit includes a first-stage amplifying circuit and a second-stage power discharge circuit, the first-stage amplifying circuit and the The above-mentioned BQ77PL900 chip is connected, which includes a PNP triode, and the second-stage power discharge circuit is connected with the first-stage amplifying circuit, which includes a power field effect transistor connected to a power discharge resistor.

The PIC18F458 single-chip microcomputer in the lithium battery management system of the present invention measures the voltage value of each battery and compares it with the threshold value to judge the balance of each battery. Unit; the PNP transistor of the first stage amplifier circuit is 9012, the power field effect transistor of the second stage power discharge circuit is SSM3K302T, and the power of the power discharge resistor is 1W.

Compared with the existing passive balance method circuit using LT1769 chip and BQ77PL900 chip, the lithium battery management system of the present invention adopts PIC18F458 single-chip microcomputer to realize the flexible master control mode of BQ77PL900 chip, and adopts one-stage amplification and one-stage power discharge at the same time The external current expansion circuit, thereby taking into account the balanced discharge speed and the circuit volume, has reached the effect of reducing the power consumption of the components and improving the charging balance speed; the present invention also uses a P-channel field effect transistor (MOSFET) to replace two Schottky Diodes to solve the problem of MOSFET gate voltage setting and fast turn-off, thus improving the heat dissipation of the entire circuit board and system.

In short, the lithium battery management system described in the present invention has the advantages of fast charging and balancing speed, low component power consumption, and good heat dissipation conditions, and can be used in group robots with automatic charging function to realize charging and discharging balance and protection of robot lithium batteries optimized management.

Description of drawings:

Figure 1 is a schematic diagram of the robot lithium battery power supply system.

FIG. 2 is a schematic circuit diagram of an existing passive balancing method lithium battery management system.

Fig. 3 is a schematic diagram of the circuit structure of the present invention.

Fig. 4 is a schematic diagram of the circuit structure of the constant current and constant voltage circuit of the present invention.

Fig. 5 is a schematic diagram of the circuit structure of the balanced protection circuit of the present invention.

Figure 6 is the general flow chart of the main program of the PIC18F458 microcontroller.

Fig. 7 is a flow chart of a subroutine for collecting lithium battery voltage and working state.

Fig. 8 is a state transition diagram of the lithium battery management main state machine.

9 to 14 are graphs of the voltage variation of each lithium battery obtained from the actual operation of the present invention.

Detailed ways:

The present invention will now be described in detail in conjunction with specific embodiments and accompanying drawings.

Please refer to FIG. 3 for a schematic circuit diagram of a specific embodiment of the present invention, which shows that the lithium battery management system is used for charge and discharge balance and protection management of the lithium battery of the group robot with automatic charging function. The lithium battery management system includes a constant current and constant voltage circuit using the LT1769 chip and a balance protection circuit using the BQ77PL900 chip. It is based on the circuit scheme of the existing passive balance method lithium battery management system shown in Figure 2. improvements:

1. The constant current and constant voltage circuit of the present invention includes a first switching element Q1 and a second switching element Q2 connected to the LT1769 chip. The first switching element Q1 and the second switching element Q2 are P-channel field effect transistors, as shown in FIG. 3 The dotted box (3) in the figure; the first switching element Q1 can be kept on during charging and quickly turned off when the external power supply is short-circuited and reversed, and the second switching element Q2 can be kept on during power supply and externally connected to the charging Immediate shutdown when required.

并且解决MOSFET控制极电压给定和快速关断的问题。Due to the limited internal space of the robot, and the charging chip LT1769 generates a lot of heat during normal operation, if other components also have serious power dissipation, it will aggravate the heat dissipation problem of the entire circuit board and system. Therefore, the present invention adopts P-channel MOSFET (Field Effect Transistor) to replace two Schottky diodes in the prior art scheme shown in Fig. 2,

And it solves the problem of MOSFET gate voltage setting and fast turn-off.

Firstly, the function of the second switching element Q2 is to keep on when the battery supplies power, and turn off immediately when the external connection is needed for charging. Please refer to Fig. 4 for a schematic diagram of the circuit structure of the constant current and constant voltage circuit of the present invention. The elements D1, R1 and DZ2 form the gate control circuit of the second switching element Q2. When the lithium battery management system is suddenly connected to the charging station ( See Fig. 1), the external voltage will directly pass through the element D1 to quickly turn off the second switching element Q2 to prevent the direct short circuit of the lithium battery from the external power supply. If the charging is disconnected at this time or the external circuit is short-circuited or reversed, because the first switching element Q1 is quickly turned off, the element D1 is blocked by the back pressure, and then the gate charge of the second switching element Q2 will be slowly released from the resistor R1. Until the element DZ2 breaks down and stabilizes the voltage; because the parasitic diode of the second switching element Q2 exists, the power supply will not be interrupted within a few milliseconds from the disconnection of the external power supply to the opening of the second switching element Q2. Therefore, it is entirely feasible to set the second switching element Q2 in the form of fast switching and slow switching.

Secondly, the function of the first switching element Q1 is to keep on during normal charging, and quickly turn off when the external power supply is short-circuited and reversed. Please refer to Fig. 4 again, elements R2, R3, R4, D2, D3, DZ1, C1 and PNP transistor 9012 form the gate drive circuit of the first switching element Q1, and usually the first switching element Q1 is in an off state when it is not charged , so the gate is connected to the battery terminal through R2. When charging, the low level of the PWM waveform, the peak hold circuit formed by the distributed capacitance between the element D2 and the gate source, and the protection circuit composed of the elements DZ1 and R3 enable the gate to maintain a voltage of -16V to the source. The function of the element C1 in the circuit is to accelerate the turn-on and turn-off of the first switch element Q1. When the PWM is just working, the components DZ1, C1 and D1 provide a circuit that quickly pulls down the gate. When the external power supply is short-circuited or reversed, the conduction of the transistor 9012 and the bootstrap function of the voltage of the component C1 itself, The gate of the first switching element Q1 is instantly raised, so that the first switching element Q1 is quickly turned off. Since the PN junction controlled by the triode 9012 can only withstand the reverse voltage of -5V, the function of the element D3 is to protect the control PN junction so that the maximum voltage can only be maintained at about -0.7V, and the element R4 is the element D3 And the current limiting resistor of the triode 9012 bears the main voltage drop during operation.

2. The balance protection circuit of the present invention includes a PIC18F458 single-chip microcomputer and an external current expansion circuit connected with the BQ77PL900 chip. The PIC18F458 single-chip microcomputer can control the charging balance of the lithium battery, and it is connected with the BQ77PL900 chip through the IIC bus and makes it run in the active control mode, see the dotted line box (1) in Fig. 3 .

The invention uses a PIC18F458 single-chip microcomputer to make the battery management chip BQ77PL900 run in the active control mode through the IIC bus, and the single-chip microcomputer PIC18F458 controls the charging balance of the lithium battery. The basic principle is to write the chip BQ77PL900 through the IIC bus command, so that the voltage of each cell measured by the voltage output terminal can be switched cyclically. In this way, the voltage value of each battery can be measured by the ADC of the single-chip microcomputer PIC18F458. After proper conversion, it can be compared with the 3.6V threshold to judge the balance of each battery. At the same time, the CAN bus transceiver module of the single-chip microcomputer PIC18F458 can transmit various operating status information of the power supply to the central processing unit of the robot.

3. The external current expansion circuit of the balanced protection circuit of the present invention includes a first-stage amplifying circuit and a second-stage power discharge circuit, and the first-stage amplifying circuit is connected with the BQ77PL900 chip, which includes a PNP transistor 9012, the second The first-stage power discharge circuit is connected to the first-stage amplifying circuit, which includes a power field effect transistor SSM3K302T, and the power field effect transistor SSM3K302T is connected to a power discharge resistor with a power of 1W, as shown in the dashed box (2) in Fig. 3 .

Due to the limitation of the power consumption of the BQ77PL900 chip itself, the maximum current flowing through the balancing switch of the BQ77PL900 chip is only 5mA, which causes the battery balancing time to take as long as several hours. power resistors for current expansion. Please refer to the schematic diagram of the circuit structure of the balance protection circuit shown in Figure 5. Since the internal FET is turned on at 3.6V, the voltage drop on the 700Ω resistor is only 0.8V. In this way, only one PNP transistor 9012 is turned on in the first stage of the amplifier circuit. Since the PNP base Pole current limit, in the actual measurement, the triode with a current amplification factor of 250 can only amplify the current to about 70mA at most. In order to discharge at a higher rate, a 100Ω resistor is selected to connect to the collector of the triode 9012. At this time, the triode 9012 is saturated and turned on. Discharge power MOSFET (field effect transistor) SSM3K302T forms a voltage difference of 3.6-0.6=3V between the GS electrodes, and SSM3K302T is a power MOSFET (power field effect transistor) with a conduction threshold of 1.8V and an on-resistance of 87mΩ, so the first The secondary power tube can be fully turned on. In order to balance the charging speed, power consumption and circuit size of the system, each battery uses two 1W power resistors to achieve a balanced discharge current greater than 300mA. At this time, the total power loss of the two resistors is 3.6V×0.3A=1.08W, For the upper limit of 2W and the control strategy of the over-voltage battery cell balance discharge in turn, the lithium battery management system can run stably and reliably in a compact space, which can not only take into account the speed and circuit volume of the balance stage, but also facilitate component selection. type and implementation.

The following is the workflow and state transition process of the lithium battery management system control software of the present invention.

Please refer to the general flowchart of the main program of PIC18F458 MCU shown in Figure 6. The main program flow of PIC18F458 MCU is mainly divided into two parts for lithium battery management. One part is to collect various information about the operation of lithium batteries, including the voltage of each battery and whether the chip triggers protection Action and current balance information; the other part is to judge the state jump of the lithium battery shutdown main state machine according to the collected information, and at the same time the state machine gives the corresponding command to write into the BQ77PL900 chip to make it operate correctly.

(1) The specific flow of the information collection program is shown in Figure 7. In order to make the collected voltage value have a certain accuracy, the charging and balancing of the lithium battery must be turned off during the collection. Therefore, before the shutdown action is executed, it is necessary to read the current state to recover after collecting the voltage. When the charging switch is turned off when collecting the voltage, the charging efficiency will be reduced. The collection time is mainly related to the transit time and the time constant of the switching capacitor inside the chip. The experimental verification is that all the collection work can be completed in 4ms, and the collection program is only run every 200ms. Once, so sacrificing 1/50 of the speed for accurate battery voltage unaffected by battery internal resistance is acceptable.

(2) Please refer to the state transition diagram of the battery management main state machine in Figure 8. Under normal circumstances, the charging and discharging of lithium batteries belong to the "normal" state in the state machine. Every time the program is in the "normal" state, it is only necessary to judge whether each battery exceeds 3.6V in order to jump to the "balanced" state in time. In addition, it also judges the status flag read from the chip register so as to jump to the "protection" state. After the judgment is completed, the program jumps out immediately until the next trigger of the timer. As long as there is an overvoltage of a lithium battery, the state machine will jump to the "balanced" state. In the balanced state, the program searches for the battery section higher than 3.57V. Due to the chip's withstand voltage, each battery can only be balanced separately. , until all battery cells are lower than 3.57V, the charging switch is turned on and continues to charge normally. Regardless of whether the state machine is in the "balanced" or "normal" state, once it reads that the abnormal state in the chip register is set, it immediately jumps to the "protection" state, waits for 25 protection cycles through the 200ms benchmark, and then writes to the chip Enter the recovery command to try whether the abnormal state is resolved. If the error is not resolved, the chip will automatically perform a new protection action.

In order to visually display the operating status and voltage of each battery, the RS232 serial port is used to transmit all information to the PIC18F458 microcontroller during debugging, and the graphical programming platform LabVIEW developed by National Instrument is used to make the battery monitoring program on the PIC18F458 microcontroller.

The following are test data and results of an actual operation of the system of the present invention.

Figures 9-14 record the respective voltage variation curves of the 6-cell lithium batteries. It can be seen from any of Figures 9-14 that the curve of the battery voltage during balance has been oscillating between 3.57V-3.61V, and the time of the balance stage gradually becomes longer, because at the last balance, all batteries Basically have reached above 3.57V. Look at the time of the last balance. The figure shows that the last complete balance process starts at about 600 sampling points and ends at about 950 sampling points. Calculated based on the sampling rate of 200ms: (950-600)×0.2=70s, That is to say, it takes about 1 minute and 10 seconds to balance the 6 batteries respectively, which is much shorter than the 30 minutes to 40 minutes in the prior art. It is conservatively estimated that if the charging process needs to go through 10 such extreme The balance time, the total balance time will not be higher than 15 minutes. Therefore, the lithium battery management system of the present invention greatly improves the charging speed of the battery; in addition, the on-resistance of the power tubes used for switching is less than 15mΩ, so the temperature rise cannot be felt when the switching power devices on the circuit are in normal operation. In a word, the present invention can be successfully used in group robots such as Swarm.

Claims (2)

1. A lithium battery management system, comprising a constant current and constant voltage circuit using the LT1769 chip and a balance protection circuit using the BQ77PL900 chip, characterized in that: the constant current and constant voltage circuit includes a first switching element connected to the LT1769 chip and The second switching element, the first switching element and the second switching element are P-channel field effect transistors, the first switching element can be kept on during charging and quickly turned off when the external power supply is short-circuited and reversed, and the second The switch element can be kept on during power supply and is turned off immediately when the external connection needs to be charged; the balance protection circuit includes a PIC18F458 single-chip microcomputer and an external current expansion circuit connected with the BQ77PL900 chip, and the PIC18F458 single-chip microcomputer can control the battery. Charge balance, which is connected to the BQ77PL900 chip through the IIC bus and makes it run in the active control mode. The external current expansion circuit includes a first-stage amplifying circuit and a second-stage power discharge circuit. The first-stage amplifying circuit Connected with the BQ77PL900 chip, it includes a PNP transistor, the PNP transistor is 9012, the second stage power discharge circuit is connected to the first stage amplifier circuit, it includes a power field effect tube connected to a power discharge resistor, the power field effect The tube is SSM3K302T, and the power of the power discharge resistor is 1W. 2. The lithium battery management system according to claim 1, characterized in that: the PIC18F458 single-chip microcomputer measures the voltage value of each battery and compares it with a threshold to judge the balance of each battery, and meanwhile, the PIC18F458 single-chip microcomputer The operating status information is transmitted to the central processing unit of the robot.

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