A lead-acid battery charging system and method for low-voltage DC charging

Abstract

The present invention discloses a lead-acid battery charging system and method for low-voltage DC charging, belonging to the technical field of lead-acid battery charging control. It includes a main control unit, a digital boost module, and a charging control module. The digital boost module includes a digital boost circuit and an output voltage sampling circuit for sampling the output voltage of the digital boost circuit. The charging control module includes a digitally controllable constant current circuit and a charging voltage sampling circuit for sampling the charging voltage of the lead-acid battery. Both the output voltage sampling circuit and the charging voltage sampling circuit include an ADC acquisition unit. The digitally controllable constant current circuit includes a DAC unit. The main control unit is connected to the digital boost circuit through a PWM output unit. The present invention adopts a low-voltage charging method, boosts from a relatively low DC voltage, and its boost circuit is digitally adjustable. Then, it realizes the charging of the lead-acid battery through a three-stage digitally controllable charging method, which is simple and reliable and improves the charging efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of lead-acid battery charging control, and particularly relates to a lead-acid battery charging system and method for low-voltage DC charging. Background Art

[0002] A valve-regulated lead-acid (VRLA) battery is a storage battery whose electrodes are mainly made of lead and its oxides, and the electrolyte is a sulfuric acid solution. In the discharged state of the lead-acid battery, the main component of the positive electrode is lead dioxide, and the main component of the negative electrode is lead; in the charged state, the main components of both the positive and negative electrodes are lead sulfate.

[0003] The nominal voltage of a single-cell lead-acid battery is 2.0V, it can discharge to 1.5V, and can be charged to 2.4V; in applications, 6 single-cell lead-acid batteries are often connected in series to form a lead-acid battery with a nominal voltage of 12V, and there are also 24V, 36V, 48V, etc.

[0004] Currently, the charging of lead-acid batteries on the market is all through rectifying the mains power by a transformer and then stepping down for charging, and there are few low-voltage charging methods. For this reason, a lead-acid battery charging system and method for low-voltage DC charging are proposed. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: how to solve the problem of low-voltage DC charging of lead-acid batteries, and further improve the charging efficiency, and a lead-acid battery charging system for low-voltage DC charging is provided.

[0006] The present invention solves the above technical problem through the following technical solutions. The present invention includes a main control unit, a digital boost module, and a charging control module; the digital boost module includes a digital boost circuit and an output voltage sampling circuit for sampling the output voltage of the digital boost circuit, the charging control module includes a digital controllable constant current circuit and a charging voltage sampling circuit for sampling the charging voltage of the lead-acid battery, both the output voltage sampling circuit and the charging voltage sampling circuit include an ADC acquisition unit, the digital controllable constant current circuit includes a DAC unit, the main control unit is connected to the digital boost circuit through a PWM output unit, the main control unit is respectively connected to the output voltage sampling circuit and the charging voltage sampling circuit through the ADC acquisition unit, and the main control unit is connected to the digital controllable constant current circuit through the DAC unit.

[0007] Furthermore, the main control unit is a microcontroller U1, the ADC acquisition unit is an ADC sampling module or an ADC sampling chip inside the microcontroller U1, the DAC unit is a DAC output module or a DAC chip inside the microcontroller U1, and the PWM output unit is a PWM module inside the microcontroller U1, and the output voltage of the digital boost circuit is digitally controlled by adjusting the duty cycle.

[0008] Further, the digital boost circuit includes a MOS transistor driver chip U2, an N-channel MOS transistor Q1, a storage inductor L1, an output diode D1, and a storage capacitor C1. One end of the MOS transistor driver chip U2 is connected to the microcontroller U1, and the other end is connected to the gate of the N-channel MOS transistor Q1. The source of the N-channel MOS transistor Q1 is grounded. The drain of the N-channel MOS transistor Q1 is respectively connected to one end of the storage inductor L1 and the anode of the output diode D1. The other end of the storage inductor L1 is connected to the low-voltage DC power input terminal. The cathode of the output diode D1 is connected to the digital controllable constant current circuit. One end of the storage capacitor C1 is connected to the cathode of the output diode D1, and the other end is grounded.

[0009] Further, the output voltage sampling circuit further includes an operational amplifier U4, feedback resistors R2 and R3. After the feedback resistors R2 and R3 are connected in series, one end is connected to the cathode of the output diode D1, and the other end is grounded. The non-inverting input terminal of the operational amplifier U4 is connected between the feedback resistors R2 and R3, the output terminal is connected to the ADC acquisition unit, and the inverting input terminal is connected to its own output terminal to form a follower.

[0010] Further, the digital controllable constant current circuit further includes an error amplifier U5, an N-channel MOS transistor Q3, a P-channel MOS transistor Q2, a current sampling resistor R6, and a feedback capacitor C2. The feedback capacitor C2 is provided between the output terminal and the inverting input terminal of the error amplifier U5. The non-inverting input terminal of the error amplifier U5 is connected to the DAC unit. One end of the current sampling resistor R6 is grounded, and the other end is connected to the negative electrode of the lead-acid battery. The inverting input terminal of the error amplifier U5 is connected between the current sampling resistor R6 and the negative electrode of the lead-acid battery. The gate of the N-channel MOS transistor Q3 is connected to the output terminal of the error amplifier U5, the source is grounded, and the drain is connected to the gate of the P-channel MOS transistor Q2. The source of the P-channel MOS transistor Q2 is connected to the cathode of the output diode D1, and the drain is connected to the positive electrode of the lead-acid battery.

[0011] Further, the digital controllable constant current circuit further includes MOS transistor driving resistors R4 and R5. The MOS transistor driving resistor R5 is provided between the drain of the N-channel MOS transistor Q3 and the gate of the P-channel MOS transistor Q2. One end of the driving resistor R4 is connected to the source of the P-channel MOS transistor Q2, and the other end is connected between the MOS transistor driving resistor R5 and the gate of the P-channel MOS transistor Q2.

[0012] Furthermore, the digitally controllable constant current circuit further includes DAC output voltage dividing resistors R9 and R10. One end of the DAC output voltage dividing resistor R9 is connected to the non-inverting input terminal of the error amplifier U5, and the other end is connected to the DAC unit. One end of the DAC output voltage dividing resistor R10 is grounded, and the other end is connected between the DAC output voltage dividing resistor R9 and the non-inverting input terminal of the error amplifier U5.

[0013] Furthermore, the charging voltage sampling circuit further includes an operational amplifier U6, sampling resistors R7 and R8. After the sampling resistors R7 and R8 are connected in series, one end is connected to the positive electrode of the lead-acid battery, and the other end is grounded. The non-inverting input terminal of the operational amplifier U6 is connected between the sampling resistors R7 and R8, the output terminal is connected to the ADC acquisition unit, and the inverting input terminal is connected to its own output terminal to form a follower.

[0014] The present invention also provides a method for charging a lead-acid battery with low-voltage direct current charging. The above system is used to charge the lead-acid battery, including the following steps:

[0015] S1: Activate the battery charging with a small current

[0016] When the lead-acid battery is in a state of depleted power, a small current charging method is used to slowly activate the lead-acid battery, the voltage of the lead-acid battery slowly rises, and the charging current is controlled to slowly increase from a small current;

[0017] S2: Constant current charging with a large current

[0018] When the charging voltage of the lead-acid battery rises to the constant current charging threshold, it enters the constant current charging mode with a large current, and the maximum constant current charging current of the lead-acid battery is maintained. During the charging process, as the charging continues, the charging voltage of the lead-acid battery also continues to rise;

[0019] S3: Constant voltage charging

[0020] When the charging voltage of the lead-acid battery enters the constant voltage charging threshold, it switches to the constant voltage charging mode, the charging voltage of the lead-acid battery is kept unchanged, the charging current of the lead-acid battery gradually decreases, and the main control unit samples the charging voltage of the lead-acid battery to control the DAC unit to adjust the target current of the lead-acid battery charging in real time.

[0021] Furthermore, in the step S2, controlling the DAC unit to give a fixed level can achieve constant current charging.

[0022] The present invention has the following advantages compared with the prior art: The lead-acid battery charging system with low-voltage direct current charging adopts a low-voltage charging method, boosts from a lower direct current voltage, its boost circuit is digitally adjustable, and then charges the lead-acid battery through a three-stage digitally controllable charging method, which is simple and reliable, improves the charging efficiency, and is worthy of being popularized and used. Brief Description of the Drawings

[0023] Figure 1 is the block diagram of the lead-acid battery charging system for low-voltage DC charging in the embodiment of the present invention;

[0024] Figure 2 is the circuit schematic diagram of the digital boost module in the embodiment of the present invention;

[0025] Figure 3 is the circuit schematic diagram of the charging control module in the embodiment of the present invention;

[0026] Figure 4 is the circuit schematic diagram of the lead-acid battery charging system for low-voltage DC charging in the embodiment of the present invention. Detailed Embodiments

[0027] The embodiments of the present invention will be described in detail below. These embodiments are implemented on the premise of the technical solution of the present invention, and detailed implementation manners and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0028] As Figure 1 shown, this embodiment provides a technical solution: a lead-acid battery charging system for low-voltage DC charging, which uses a digitally controllable boost circuit to achieve DC-DC Boost conversion, and then realizes the charging of a three-stage lead-acid battery through a charging control circuit; the charging system mainly includes three modules: a main control unit 1, a digital boost module 2, and a charging control module 3.

[0029] In this embodiment, the main control unit, implemented by a microcontroller U1, can perform operations such as ADC acquisition, DAC output, PWM output, overcurrent protection processing, and battery floating charge control;

[0030] Among them, ADC sampling can be implemented using the internal ADC sampling module of U1 or the U3 chip, and is mainly used to collect the boost voltage VCC_H and the battery charging voltage VBAT; VCC_H is used to implement closed-loop feedback control of the digitally controllable boost circuit, and VBAT is used to implement constant-voltage charging closed-loop control of the lead-acid battery charging circuit;

[0031] DAC output can be implemented using the internal DAC output module of U1 or the U7 chip, and is used to make the target constant current value for the constant-current charging of the lead-acid battery digitally controllable.

[0032] PWM output (implemented through the PWM module inside the microcontroller U1) digitally controls the output voltage of the digital boost module by adjusting the duty cycle;

[0033] Overcurrent protection processing is to prevent the circuit of the digital boost module from being damaged in case of accidental short - circuit overcurrent. It realizes the protection of the digital boost module circuit by controlling the microsecond - level turn - off of the PWM output signal of the PWM module.

[0034] Battery floating charge control is the constant - voltage charging mode. Since the circuit realizes constant - current charging, when constant - voltage charging needs to be achieved, the microcontroller U1 needs to implement closed - loop control through the sampled charging voltage of the lead - acid battery and the target constant - current value.

[0035] In this embodiment, the digital boost module is a standard Boost boost topology structure, including a MOS - tube driver chip U2, an N - channel MOS tube Q1, a storage inductor L1, an output diode D1, and a storage capacitor C1. To achieve the digital adjustable function, feedback resistors R2 and R3, a feedback voltage driver chip U4, and an ADC sampling chip U3 form a voltage sampling circuit to realize closed - loop control; to ensure the safety of the circuit, an over - current protection function is realized by a current sampling resistor R1 and an over - current protection circuit.

[0036] It should be noted that the over - current protection circuit is a safety protection module. On the premise that the reliability of the boost circuit meets the requirements, it can also be omitted. The over - current protection circuit can sample the voltage of the sampling resistor R1, filter it through an RC filter, and then compare it with a fixed reference voltage through a comparator. The amplitude of this reference voltage is the current threshold of the over - current protection. The output signal of this comparator is directly given to the external interrupt of the MCU (microcontroller U1) or the protection input interrupt of the PWM module of the MCU (microcontroller U1) to quickly turn off the output of the PWM module.

[0037] Among them, the MOS - tube driver chip U2 realizes the driving function of the N - channel MOS tube Q1, and realizes the function of increasing the drive by increasing the VGS voltage of the N - channel MOS tube Q1 and increasing the drive current of the MOS tube Q1;

[0038] The N - channel MOS tube Q1 is the switching device of the Boost boost topology. By controlling the on - off of the N - channel MOS tube Q1, the energy storage and transfer of the storage inductor L1 are realized. The breakdown voltage value of the N - channel MOS tube Q1 should be greater than the sum of the maximum output voltage VCC_H after boosting and the conduction voltage drop Vd of the output diode D1. The maximum current value of the N - channel MOS tube Q1 should be greater than the peak current of the storage inductor L1. Since the switching frequency of the N - channel MOS tube Q1 needs to be set between 50kHz and 200kHz, when the load current is large, both the switching loss and the conduction loss are relatively high, so good heat dissipation for the N - channel MOS tube Q1 is required.

[0039] The storage inductor L1 is the energy - storage device of the Boost boost topology. In each switching cycle, the storage inductor L1 stores energy and transfers it to the output end to realize energy transfer.

[0040] The output diode D1 utilizes the switching characteristics of the diode. When the N-channel MOS transistor Q1 is turned on, the inductor stores energy, and the diode D1 is in the off state. The output energy is provided by the energy storage capacitor C1. When Q1 is turned off, due to the fact that the current in the energy storage inductor L1 cannot change abruptly, the voltage at the positive terminal of the output diode D1 increases, causing D1 to turn on. The energy of the energy storage inductor L1 is transferred by D1 to the energy storage capacitor C1 for storage. To reduce the conduction loss of the output diode D1, a Schottky diode with a low conduction voltage drop is selected.

[0041] The energy storage capacitor C1 is mainly used for energy storage. Since the current change rate of the energy storage capacitor C1 is relatively large, a capacitor with a low impedance (ERS) needs to be used to reduce heat generation. The withstand voltage value of C1 must be higher than 30% of the maximum output voltage VCC_H.

[0042] The feedback resistors R2 and R3 achieve voltage division of the output voltage VCC_H. To make the monitored output voltage more accurate, precision resistors with a low temperature drift (25 ppm or less) need to be used. The resistance values of the feedback resistors R2 and R3 need to be calculated based on the full-scale reference voltage of the ADC. If the ADC reference voltage is Vref and the maximum output voltage is VCC_Hmax, then Through this relationship, by specifying the resistance value of one resistor, the resistance value of the other resistor can be calculated. It is required that the power on the feedback resistors R2 and R3 does not exceed 20% of the rated power of the resistor to avoid changes in the resistor value caused by self-heating of the resistor, which affects the feedback accuracy.

[0043] The feedback drive chip U4 is an operational amplifier. To make the data sampled by the ADC more accurate, the source impedance to be sampled needs to be very low. The feedback drive chip U4 forms a follower to achieve impedance conversion of the sampled voltage.

[0044] The ADC sampling chip U3 is a multi-channel sampling device that realizes sampling of the output voltage VCC_H and the VBAT voltage. If there is an ADC sampling module inside the U1 chip and the user has low requirements for sampling accuracy, the U3 chip can be omitted and the ADC sampling module inside U1 can be directly used.

[0045] The charging control module, a digitally controllable constant current circuit, realizes constant current charging of the lead-acid battery, including the DAC chip U7, the error amplifier U5, the feedback capacitor C2, the N-channel MOS transistor Q3, the P-channel MOS transistor Q2, the MOS transistor drive resistors R4 and R5, the DAC output voltage division resistors R9 and R10, and the current sampling resistor R6; To realize the constant voltage charging function of the lead-acid battery, a closed-loop sampling circuit for constant voltage charging is composed of the voltage sampling resistors R7 and R8, the battery voltage feedback drive chip U6, and the ADC sampling chip U3.

[0046] Generally speaking, the charging control module in this embodiment includes: a digitally controllable constant current circuit and a charging voltage sampling circuit;

[0047] In the digitally controllable constant current circuit:

[0048] The DAC chip U7 realizes the setting of the target constant current value of the digitally controllable constant current circuit. On the premise that the user has low requirements for constant current accuracy, the U7 chip can be omitted and the DAC output module inside U1 can be directly used.

[0049] U5 is an error amplifier in the digitally controllable constant current circuit. The voltage on the sampling resistor R6 reflects the magnitude of the charging current, and the voltage output by the DAC chip U7 reflects the magnitude of the target charging current. After these two voltages are differentially amplified, they drive the conduction degree of the N-channel MOS transistor Q3 to adjust the conduction degree of the P-channel MOS transistor Q2. The conduction degree of the P-channel MOS transistor Q2 controls the magnitude of the charging current Icharge of the lead-acid battery. The magnitude of the charging current Icharge determines the sampling voltage of the resistor R6, thus realizing closed-loop control to ensure the constancy of the charging current of the lead-acid battery.

[0050] The feedback capacitor C2 is to make the negative feedback of the error amplifier U5 more stable and prevent the occurrence of feedback oscillation;

[0051] The N-channel MOS transistor Q3 and the driving resistors R4 and R5 together realize the driving of the P-channel MOS transistor Q2, ensuring that the change in the output voltage of the error amplifier U5 can control the conduction degree of the P-channel MOS transistor Q2.

[0052] The P-channel MSO transistor Q2 can be understood as a variable resistor. By changing the conduction degree of Q2, the change of the VBAT voltage is realized, and then the charging current Icharge and the charging voltage VBAT of the lead-acid battery are adjusted. When the charging current Icharge is too large, a large conduction loss will occur on the P-channel MSO transistor Q2, so good heat dissipation treatment is required for Q2. In order to reduce the power generated on the P-channel MSO transistor Q2, the voltage of VCC_H can be dynamically adjusted to reduce the voltage difference between VCC_H and VBAT and reduce the power on the P-channel MSO transistor Q2.

[0053] The main purpose of the DAC voltage dividing resistors R9 and R10 is to proportionally reduce the output voltage of U7. Because the resistance value of the sampling resistor R6 cannot be set too large, otherwise there will be a large amount of power on R6. When R6 is relatively small, the maximum voltage generated on R6 will be relatively small, and the voltage at the positive end of the error amplifier U5 also requires a relatively small voltage value. However, the maximum output voltage of the DAC is generally greater than 1V, so the DAC output voltage needs to be proportionally reduced to match the voltage on the sampling resistor R6.

[0054] The sampling resistor R6 realizes the sampling of the battery charging current. The sampling resistor R6 needs to use a precision power resistor. If the maximum charging current reaches 2A and the resistance value of the sampling resistor R6 is 0.1 ohm, the power on the sampling resistor R6 will reach 0.4W. Therefore, under such conditions, at least a resistor with a power of more than 1W needs to be used.

[0055] In the charging voltage sampling circuit:

[0056] The sampling resistors R7 and R8 realize the voltage division of the charging voltage VBAT of the lead-acid battery. In order to make the monitored output voltage more accurate, precision resistors with low temperature drift (25 ppm and below) need to be used. The resistance values of the sampling resistors R7 and R8 need to be calculated according to the full-scale reference voltage of the ADC. If the ADC reference voltage is Vref and the maximum output voltage is VBATmax, then Through this relational expression, by specifying the resistance value of one resistor, the resistance value of the other resistor can be calculated. It is required that the power on the sampling resistors R7 and R8 does not exceed 20% of the rated power of the resistor to avoid the change of the resistor value caused by the self-heating of the resistor and affect the feedback accuracy.

[0057] The feedback driver chip U6 is an operational amplifier, and its function is the same as that of U4. In order to make the data sampled by the ADC more accurate, the source impedance to be sampled needs to be very low. A follower is formed through the feedback driver chip U6 to realize the impedance conversion of the sampling voltage.

[0058] The three-stage charging method of the lead-acid battery includes: small-current activation battery charging, large-current constant-current charging, and constant-voltage charging;

[0059] When the lead-acid battery is in a state of depleted power, a weak current needs to be used to slowly charge and activate the battery. At this time, a small-current charging method needs to be used to slowly activate the lead-acid battery from a severely discharged state. The voltage of the lead-acid battery slowly rises. At this time, the digital control charging current needs to slowly increase from a small current

[0060] When the charging voltage of the lead-acid battery rises to the constant-current charging threshold, it enters the large-current constant-current charging mode, maintaining the maximum constant-current charging current of the lead-acid battery (in order to improve the charging speed, this maximum constant-current charging current can be the maximum charging current required by the battery manual). During the charging process, as the charging continues, the charging voltage of the lead-acid battery also continues to rise. At this time, only a fixed level needs to be given by the DAC chip U7 to achieve constant-current charging.

[0061] When the charging voltage of the lead-acid battery enters the constant voltage charging threshold, it is necessary to switch to the floating charge mode (i.e., the constant voltage charging mode). At this time, the charging voltage of the lead-acid battery needs to be kept unchanged, and the charging current of the lead-acid battery will gradually decrease. Since the circuit is a constant current circuit, the microcontroller U1 needs to sample the charging voltage VBAT of the lead-acid battery and control the DAC chip U7 to adjust the target current of the lead-acid battery charging in real time. At this time, this function can be realized through the PID algorithm or the robust algorithm.

[0062] In order to reduce the heat loss generated by Q2 during charging, the voltage difference between VCC_H and VBAT is monitored in real time. When the voltage difference exceeds 1V, the switching duty ratio of Q1 is adjusted to reduce the voltage of VCC_H. Keep the voltage difference of Q2 always below the threshold (the threshold is 1V in this embodiment), thereby reducing the conduction loss of Q2 and improving the charging efficiency.

[0063] In summary, the lead-acid battery charging system for low-voltage DC charging in the above embodiments adopts a low-voltage charging method, boosts the voltage from a relatively low DC voltage, and its boost circuit is digitally adjustable. Then, the charging of the lead-acid battery is realized through a three-stage digitally controllable charging method, which is simple and reliable, improves the charging efficiency, and is worthy of being popularized and used.

[0064] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A lead-acid battery charging system for low-voltage DC charging, characterized in that, Comprising: A main control unit, a digital boost module, and a charging control module; the digital boost module includes a digital boost circuit and an output voltage sampling circuit for sampling the output voltage of the digital boost circuit, the charging control module includes a digitally controlled constant current circuit and a charging voltage sampling circuit for sampling the charging voltage of the lead-acid battery, the output voltage sampling circuit and the charging voltage sampling circuit both include an ADC acquisition unit, the digitally controlled constant current circuit includes a DAC unit, the main control unit is connected to the digital boost circuit through a PWM output unit, the main control unit is respectively connected to the output voltage sampling circuit and the charging voltage sampling circuit through the ADC acquisition unit, and the main control unit is connected to the digitally controlled constant current circuit through the DAC unit; The main control unit is a microcontroller U1, the ADC acquisition unit is an ADC sampling chip, the DAC unit is a DAC chip, and the PWM output unit is a PWM module inside the microcontroller U1, which digitally controls the output voltage of the digital boost circuit by adjusting the duty cycle; The digital boost circuit includes a MOS tube driver chip U2, an N-channel MOS tube Q1, a storage inductor L1, an output diode D1, and a storage capacitor C1; one end of the MOS tube driver chip U2 is connected to the microcontroller U1, and the other end is connected to the gate of the N-channel MOS tube Q1. The source of the N-channel MOS tube Q1 is grounded, and the drain of the N-channel MOS tube Q1 is respectively connected to one end of the storage inductor L1 and the positive pole of the output diode D1. The other end of the storage inductor L1 is connected to the low-voltage DC power input terminal, the negative pole of the output diode D1 is connected to the digitally controlled constant current circuit, and one end of the storage capacitor C1 is connected to the negative pole of the output diode D1, and the other end is grounded; The output voltage sampling circuit further includes an operational amplifier U4, feedback resistors R2 and R3. After the feedback resistors R2 and R3 are connected in series, one end is connected to the negative pole of the output diode D1, and the other end is grounded. The non-inverting input terminal of the operational amplifier U4 is connected between the feedback resistors R2 and R3, the output terminal is connected to the ADC acquisition unit, and the inverting input terminal is connected to its own output terminal to form a follower; The digitally controlled constant current circuit further includes an error amplifier U5, an N-channel MOS tube Q3, a P-channel MOS tube Q2, a current sampling resistor R6, and a feedback capacitor C2. The feedback capacitor C2 is set between the output terminal and the inverting input terminal of the error amplifier U5. The non-inverting input terminal of the error amplifier U5 is connected to the DAC unit. One end of the current sampling resistor R6 is grounded, and the other end is connected to the negative pole of the lead-acid battery. The inverting input terminal of the error amplifier U5 is connected between the current sampling resistor R6 and the negative pole of the lead-acid battery. The gate of the N-channel MOS tube Q3 is connected to the output terminal of the error amplifier U5, the source is grounded, and the drain is connected to the gate of the P-channel MOS tube Q2. The source of the P-channel MOS tube Q2 is connected to the negative pole of the output diode D1, and the drain is connected to the positive pole of the lead-acid battery; The digital controllable constant current circuit further includes MOS transistor driving resistors R4 and R5. The MOS transistor driving resistor R5 is arranged between the drain of the N-channel MOS transistor Q3 and the gate of the P-channel MOS transistor Q2. One end of the driving resistor R4 is connected to the source of the P-channel MOS transistor Q2, and the other end is connected between the MOS transistor driving resistor R5 and the gate of the P-channel MOS transistor Q2; The digital controllable constant current circuit further includes DAC output voltage dividing resistors R9 and R10. One end of the DAC output voltage dividing resistor R9 is connected to the non-inverting input terminal of the error amplifier U5, and the other end is connected to the DAC unit. One end of the DAC output voltage dividing resistor R10 is grounded, and the other end is connected between the DAC output voltage dividing resistor R9 and the non-inverting input terminal of the error amplifier U5; The charging voltage sampling circuit further includes an operational amplifier U6, sampling resistors R7 and R8. After the sampling resistors R7 and R8 are connected in series, one end is connected to the positive electrode of the lead-acid battery, and the other end is grounded. The non-inverting input terminal of the operational amplifier U6 is connected between the sampling resistors R7 and R8, the output terminal is connected to the ADC acquisition unit, and the inverting input terminal is connected to its own output terminal to form a follower.

2. A method for charging a lead-acid battery with low-voltage direct current charging, using the system as described in claim 1 to charge the lead-acid battery, including the following steps: S1: Activate the battery charging with a small current When the lead-acid battery is in a state of depleted power, use the small current charging method to slowly activate the lead-acid battery. The voltage of the lead-acid battery slowly rises, and the charging current is controlled to slowly increase from a small current; S2: Constant current charging with a large current When the charging voltage of the lead-acid battery rises to the constant current charging threshold, enter the constant current charging mode with a large current, and maintain the maximum constant current charging current of the lead-acid battery. During the charging process, as the charging continues, the charging voltage of the lead-acid battery also continues to rise; S3: Constant voltage charging When the charging voltage of the lead-acid battery enters the constant voltage charging threshold, switch to the constant voltage charging mode, keep the charging voltage of the lead-acid battery unchanged, and the charging current of the lead-acid battery gradually decreases. The main control unit samples the charging voltage of the lead-acid battery and controls the DAC unit to adjust the target current of the lead-acid battery charging in real time.

3. A lead-acid battery charging method for low-voltage DC charging according to claim 2, characterized in that: In the step S2, controlling the DAC unit to give a fixed level can achieve constant current charging.

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