CN107733007B - Dual-target direct equalization circuit and equalization method for battery pack - Google Patents

Abstract

The invention discloses a battery pack double-target direct equalization circuit. The series battery pack comprises at least three single batteries, and is divided into an upper part and a lower part, each single battery is connected with an equalization sub-circuit, the equalization sub-circuit consists of two MOSFETs and an energy storage inductor, the series battery pack and the equalization sub-circuit are connected between VCC and GND in a bridging way, and each equalization sub-circuit is connected with a control circuit to realize direct equalization of an equalization target. The control circuit takes the terminal voltage and the SOC of the single battery as balance indexes at the same time, balances the terminal voltage and the SOC of the single battery in stages in one balance period, and realizes double-target balance of the terminal voltage and the SOC of the single battery by controlling the on-off of two MOSFETs of the balance sub-circuit and utilizing the energy storage effect of the energy storage inductor. The equalization circuit and the equalization strategy are suitable for an equalization management system of an energy storage device in a hybrid electric vehicle, a pure electric vehicle or an energy storage power station.

Description

A battery pack dual-target direct balancing circuit and balancing method

Technical field

The invention relates to a dual-objective direct balancing technology and balancing method for a series battery pack, which is suitable for battery management systems of hybrid electric vehicles, pure electric vehicles or energy storage devices in energy storage power stations.

Background technique

Since the capacity of a single battery is limited and the voltage of a single battery is low, a power battery pack is generally composed of multiple single batteries connected in series and parallel to meet usage requirements. Due to the inevitable inconsistency between single cells of the same model, the service life of the battery pack will be seriously affected, and overcharge and overdischarge may easily occur. After a series battery pack has undergone multiple charge and discharge cycles, the distribution of the remaining capacity of each single cell will generally appear in three situations: the remaining capacity of individual single cells is high; the remaining capacity of individual single cells is low; the remaining capacity of individual single cells is low; The remaining capacity of the battery is high and the remaining capacity of individual cells is low.

In response to the above situation, domestic and foreign scholars have proposed their own solutions. For example, in response to the high remaining capacity of individual single cells, some researchers have proposed a parallel resistor shunt method, which controls the corresponding switching device to consume the energy of the battery module with high remaining capacity through the resistor. This method reduces the energy It is wasted and a large amount of heat is generated during the equalization process, which increases the thermal management load of the battery. Some researchers have also proposed balancing circuits such as the bidirectional DC-DC balancing method and the coaxial transformer balancing method. These circuits all use transformers, which increases the cost of the balancing circuit.

The current balancing control methods of lithium-ion battery packs can be divided into two categories: energy dissipative type and energy non-dissipating type according to the energy consumption of the circuit during the balancing process; according to the balancing function classification, it can be divided into charge balancing and discharge Equilibrium and dynamic equilibrium. Charge balancing refers to balancing during the charging process. Generally, balancing begins when the battery pack cell voltage reaches the set value, and overcharging is prevented by reducing the charging current. Discharging balancing refers to balancing during the discharging process, by charging the remaining Low-energy single cells replenish energy to prevent over-discharge; the dynamic balancing method combines the advantages of charge balancing and discharge balancing, which refers to balancing the battery pack during the entire charge and discharge process. A large number of balancing topologies and control strategies have been proposed. Regarding the research on balancing circuit control strategies, Kobzev, Tae-hoon Kim, etc. all use the battery terminal voltage as the balancing index to balance the battery pack. However, the performance of the battery cannot be measured solely by the voltage. The capacity of the battery pack is low. When or after charging, the terminal voltage of a battery may be higher than that of other batteries. If this balancing method is adopted, the result of balancing is that the low-capacity battery replenishes the energy of the high-capacity battery, which increases the size of each battery in the battery pack. capacity gap. Danielson, Huang W and others believe that the advantage of using SOC as the balancing variable is that sudden changes in current under different working conditions will not cause the battery state of charge to fluctuate, making the changes in the balancing target relatively stable, which is beneficial to reducing the impact of balancing oscillations on the battery. However, This balancing method can only solve the problem of performance degradation of batteries with larger capacity in the battery pack due to long-term insufficient charging, but cannot reduce or eliminate the gap in the actual capacity of each battery. Generally speaking, current research on balance control strategies mostly uses a single terminal voltage or a single SOC as the balance indicator.

Contents of the invention

The purpose of the present invention is to use a balancing circuit in a battery management system of a series battery pack to ensure that the single cells in the battery pack do not overcharge or overdischarge during the charging and discharging processes, and to improve the imbalance of the series battery pack. , increase the available capacity of the battery pack, reduce the maintenance and replacement cycle of the series battery pack, extend the service life of the battery pack, and reduce the operating costs of hybrid vehicles, electric vehicles and energy storage power stations. During the charging process, when the energy of any single cell in the battery pack is too high, the energy of this single cell can be balanced to all other remaining cells in the battery pack; during the discharging process, when the energy of any single cell in the battery pack is too high, When it is low, the energy of all other remaining cells in the battery pack can be balanced to the cell whose energy is too low. And the balance control strategy is formulated using SOC and terminal voltage as balance indicators at the same time. By balancing SOC and terminal voltage in stages, the consistency of the single cells of the power battery pack is essentially improved.

In order to achieve the above objects, the present invention is implemented through the following technical solutions.

The battery pack dual-target direct balancing circuit consists of a series battery pack and a balancing subcircuit. Among them, the series battery pack is divided into upper and lower parts. The upper part of the single cell is the upper battery, and the lower part of the single cell is the lower battery. When the total number of single cells n is an even number, the number of upper and lower single cells is (n/2), when the total number of single cells n is an odd number, the number of upper single cells is [(n+1)/2], and the number of lower single cells is [(n-1)/2]; The batteries are named B ₁ , B ₂ , B ₃ ,...B _n from top to bottom. The positive electrode of B ₁ is connected to VCC, and the negative electrode of B _n is connected to GND. Each single cell is connected to a balancing subcircuit.

Single batteries can be lead-acid batteries, lithium-ion batteries, nickel-metal hydride batteries, supercapacitors and other secondary batteries.

As a preferred solution, each balancing subcircuit consists of two MOSFETs with freewheeling diodes and an energy storage inductor L. The upper arm MOSFET is Q _u and the lower arm MOSFET is Q _d . The source of Q _u is connected to the Q _d The drain is connected to one end of the energy storage inductor L; the drain of Q _u is used as the output terminal a, the gate of Q _u is used as the output terminal b, the gate of Q _d is used as the output terminal c, and the source of Q _d is used as the output terminal d , the other end of L serves as the output terminal e; the output terminals b and c are connected to the control circuit, so that the turning on and off of the MOSFET is controlled by the control circuit; the balancing subcircuit connected to the upper single cell, terminal a is connected to the corresponding single cell The positive electrode of the battery is connected, the e end is connected to the negative electrode of the corresponding single cell, and the d end is connected to GND; in the balancing subcircuit connected to the lower single cell, the e end is connected to the positive electrode of the corresponding single cell, and the d end is connected to the negative electrode of the corresponding single cell. The a terminal is connected to VCC; the a terminal of the overall balancing subcircuit is connected to VCC, the d terminal is connected to GND, and the e terminal is connected to the common point k of the upper battery and the lower battery.

The balancing subcircuit works as follows.

During the charge and discharge process, if the cell B _i located above the k point needs to be discharged equalized, by controlling the upper arm switch Q _ui of the Si _i to be turned on, the B _i discharges to store energy for Li; _{Q ui is} turned on for a certain period of time Then it is turned off. At this time, the current passes through the freewheeling diode of the lower arm switch Q _di of the balancing subcircuit corresponding to Bi _, Li and _Bi+1 , Bi ₊₂ ...B _n , and _Li releases energy _. to B _i+1 , B _i+2 ...B _n , realizing the transfer of energy from B _i to B _i+1 , B _i+2 ...B _n . If the cell B _j located below the k point needs to be discharged and balanced, within a PWM cycle, the lower arm MOSFET Q _dj of the balancing sub-circuit corresponding to B _j is turned on, and the current passes through Q _dj and the balancing sub-circuit corresponding to B _j The circuit energy storage inductor L _j and B _j , B _j discharges to store energy for L _j ; Q _dj is turned off after a certain period of time. At this time, the current flows through the freewheeling of the bridge arm MOSFET Q _uj in the balancing subcircuit corresponding to B _j . Diode, L _j and B ₁ , B ₂ ...... B _i-1 , L _j releases energy to B ₁ , B ₂ ...... B _i-1 , realizing the energy transfer from B _j to B ₁ , B ₂ ...... B _{i -1} transfer.

If the cell B _i located above the k point needs to be charged and balanced, the lower arm switch Q _di of the control _Si is turned on, and B _i+1 , B _i+2 ...B _n discharges to store energy for _Li ; Q _di is turned off after being turned on for a certain period of time. At this time, the current passes through the freewheeling diode of the bridge arm switch _{Q ui} on the balancing subcircuit corresponding to Bi, Li _, and Bi ₊₁ , Bi ₊₂ ...B _n , _Li releases energy to _Bi , realizing the transfer of energy from Bi ₊₁ , Bi ₊₂ ...B _n to _Bi . If the cell B _i located below the k point needs to be charged and balanced, within a PWM cycle, the upper bridge arm MOSFET Q _uj of the balancing subcircuit corresponding to B _j is turned on, and the current flows through Q _uj and the balancing sub-circuit corresponding to B _j . The circuit energy storage inductor L _j and B ₁ , B ₂ ...... B _j-1 , B ₁ , B ₂ ...... B _j-1 discharge to store energy for L _j ; Q _uj is turned off after a certain period of time. The current passes through the freewheeling diode, L _j and B _j of the lower bridge arm MOSFET Q _dj of the balanced subcircuit corresponding to B _j . L _j releases energy to B _j , realizing the energy transfer from B ₁ , B ₂ ...B _j-1 to Transfer of B _j .

The balance index is established based on SOC and terminal voltage, and is balanced in stages within a balancing cycle. Finally, the consistency of SOC and terminal voltage of each single cell in the battery pack meets the design requirements.

The specific equilibrium control strategy includes the following contents:

S1. Set the balancing index: The detection circuit determines whether the inconsistency of each battery's SOC and terminal voltage meets the working conditions of the balancing circuit; if the balancing conditions are met, the balancing circuit starts to work; if the balancing conditions are not met, the balancing circuit does not work.

S2. The balancing process includes several balancing periods. Each balancing period takes T/2 time for voltage balancing and T/2 time for SOC balancing.

S3. At the end of each balancing cycle, the detection circuit re-detects and determines whether the SOC and terminal voltage of each battery meet the balancing conditions;

S4. Repeat step S2 until the inconsistency of the single battery does not meet the working conditions of the balancing circuit, the balancing circuit stops working, and the balancing process ends.

Further, the control circuit controls the balancing subcircuit by outputting a control signal. The frequency of the control signal is based on the inductance value of the energy storage inductor of the balancing subcircuit, the switching loss of the MOSFET, and the battery terminal voltage of the single cell. , depends on the single capacity of the single battery.

Furthermore, the duty cycle of the control signal output by the control circuit can reset the energy storage inductor in each signal period, that is, the current of the energy storage inductor first rises from zero and finally drops to zero.

Further, in step S2, during the operation of the balancing circuit, by reducing the open circuit voltage of the single cell corresponding to the maximum SOC value, and increasing the terminal voltage of the single cell corresponding to the minimum terminal voltage, so that decreases and gradually meets the battery pack consistency index. When each single cell in the battery pack/> When they tend to be consistent, the dynamic performance of single cells can be consistent.

The lithium-ion battery balancing method provided by the present invention can be applied to various energy dissipative balancing circuits and energy non-dissipating balancing circuits.

Also suitable for capacitor type balancing circuits, converter type balancing circuits and transformer type balancing circuits.

Since the present invention adopts the above-mentioned balancing technology in the battery management system of the series battery pack, it can ensure that each battery will not be overcharged or overdischarged during the charging and discharging process. At the same time, using the battery terminal voltage and SOC as inconsistency indicators, it can essentially to improve the consistency of single cells in the battery pack; through staged balancing, dual-target balancing of terminal voltage and SOC can be achieved simultaneously without increasing program calculations and control complexity. This control strategy method is reliable and has a small amount of online calculations. It can significantly improve battery safety and reliability, improve battery energy utilization, extend battery life, and reduce the cost of battery energy storage systems in hybrid vehicles, electric vehicles, and power stations.

Description of the drawings

Figure 1 is a schematic diagram of the equalization circuit in the present invention.

Figure 2 is a schematic diagram of the equalization subcircuit in the present invention.

Figure 3 is a schematic diagram of the balancing strategy in the present invention.

Figure 4 is a schematic diagram of the working process of the balancing circuit during the charging process of a four-cell series battery pack.

Figure 5 is a schematic diagram of the working process of the balancing circuit during the discharge process of the four-cell series battery pack.

Detailed ways

The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

Figure 1 is the schematic diagram of the equalization circuit. Among them, the series battery pack is divided into upper and lower parts. The upper half of the single cells is the upper battery, and the lower half of the single cells is the lower battery. When the total number of single cells n is an even number, the number of single cells in the upper and lower parts is Both are (n/2). When the total number of single cells n is an odd number, the number of upper single cells is [(n+1)/2] and the number of lower single cells is [(n-1)/2]; The single cells are named B ₁ , B ₂ , B ₃ ,...B _n from top to bottom. The positive electrode of B ₁ is connected to VCC, and the negative electrode of B _n is connected to GND. Each single cell is connected to a balancing subcircuit.

Figure 2 is the schematic diagram of the equalization subcircuit. Each balancing subcircuit consists of two MOSFETs with freewheeling diodes and an energy storage inductor L. The upper arm MOSFET is Q _u and the lower arm MOSFET is Q _d . The source of Q _u and the drain and storage of Q _d One end of the energy inductor L is connected; the drain of Q _u is used as the output terminal a, the gate of Q _u is used as the output terminal b, the gate of Q _d is used as the output terminal c, the source of Q _d is used as the output terminal d, and the other terminal of L One end serves as the output terminal e; the output terminals b and c are connected to the control circuit, so that the turning on and off of the MOSFET is controlled by the control circuit; the balancing subcircuit connected to the upper single cell, terminal a is connected to the positive electrode of the corresponding single cell, The e terminal is connected to the negative electrode of the corresponding single cell, the d terminal is connected to GND; the balancing subcircuit is connected to the lower single cell, the e terminal is connected to the corresponding single cell positive electrode, the d terminal is connected to the corresponding single battery negative electrode, and the a terminal is connected to VCC .

Figure 3 is a schematic diagram of the dual-objective staged equilibrium strategy. Dual goals refer to using SOC and terminal voltage as balance indicators at the same time. Balance to ensure the consistency of the essential working status of each single cell in the battery pack. Stage-by-stage means that in each balancing cycle, half a cycle is used to achieve terminal voltage balance. This process is achieved by charging and balancing the single battery with the lowest terminal voltage; half a cycle is used to achieve SOC balance, that is, open circuit voltage. Balance, this process is achieved by discharging and equalizing the single cell with the highest open circuit voltage. The detection circuit determines whether the inconsistency of the SOC and terminal voltage of each battery meets the working conditions of the balancing circuit; if the balancing conditions are met, the balancing circuit starts to work; if the balancing conditions are not met, the balancing circuit does not work.

At the end of each balancing cycle, the detection circuit re-detects and determines whether the SOC and terminal voltage of each battery meet the working conditions of the balancing circuit. At the end of a balancing cycle, if the SOC and terminal voltage of each single cell meet the working conditions of the balancing circuit, the balancing circuit continues to work. If the working conditions of the balancing circuit are not met, the balancing circuit stops working and the balancing process ends.

Figure 4 and Figure 5 are schematic diagrams of the working principle of the balancing circuit during the charge and discharge process. Taking the number of single cells n = 4 as an example,

Figure 4 is the process of discharging equalization for B _1. During a PWM cycle, the upper bridge arm MOSFET Q _u1 of the equalizing subcircuit corresponding to B ₁ is turned on, and the current i _u1 passes through Q _u1 and the equalizing subcircuit corresponding to B ₁ . The energy storage inductor L ₁ and B ₁ , B ₁ discharge to store energy for L ₁ ; Q _u1 is turned off after a certain period of time, and the current passes through the freewheeling diode of the lower bridge arm MOSFET Q _d1 of the balancing subcircuit corresponding to B ₁ , L ₁ and B ₂ , B ₃ , B ₄ , L ₁ releases energy to B ₂ , B _{3 ,} and B ₄ , realizing the energy transfer from B ₁ to B ₂ , B ₃ , and B ₄ .

Figure 5 is the process of charging and balancing B _3. In a PWM cycle, the upper bridge arm MOSFET Q _u3 of the balancing sub-circuit corresponding to B ₃ is turned on, and the current passes through Q _u3 and the balancing sub-circuit corresponding to B ₃ to store energy. Inductor L ₃ and B ₁ , B ₂ , B ₁ , B ₂ discharge to store energy for L ₃ ; Q _u3 turns off after a certain period of time. At this time, the current passes through the lower arm MOSFET Q _d3 of the balancing subcircuit corresponding to B ₃ . Freewheeling diode, _L3 and _B3 , _L3 releases energy to _B3 , realizing the transfer of energy from _B1 , _B2 to _B3 .

Claims (5)

1. A balancing method for a lithium-ion battery pack, characterized by: balancing the lithium-ion battery pack based on a dual-target direct balancing circuit of the battery pack, The battery pack dual-target direct balancing circuit includes a series battery pack composed of multiple single cells connected in series, a balancing sub-circuit and a control circuit. The number of the single cells is not less than 3, and each single cell They are all connected to a balancing sub-circuit, and each balancing sub-circuit is connected to a control circuit; the series battery pack is divided into an upper part and a lower part, and the number of single cells in the upper part is the same as the number of single cells in the lower part, or One more; the balancing sub-circuit connected to the upper part of the single cell is connected to the GND terminal of the series battery pack, and the balancing sub-circuit connected to the lower part of the single battery is connected to the VCC terminal of the series battery pack; The balancing subcircuit is composed of two MOSFETs with freewheeling diodes and an energy storage inductor connected. The control circuit is connected to the gates of the two MOSFETs; the positive and negative ends of the single battery are respectively connected to the drain and energy storage of one of the MOSFETs. The second end of the inductor is connected; the source of the other MOSFET is connected to the GND end or VCC end of the series battery pack; the single battery is a secondary battery; The equalization method includes the following steps: S1. Connect the balancing circuit to the detection circuit and set the balancing index: the detection circuit determines whether the inconsistency of each battery's SOC and terminal voltage meets the working conditions of the balancing circuit; if the balancing conditions are met, the balancing circuit starts to work; if the balancing conditions are not met , the balancing circuit does not work, Set the average open circuit voltage of each single cell in the battery pack to U _{oc_are} , and the average terminal voltage of each single cell to U _{L_aw} , let: D _i =U _{oc_j} -U _{L_i} (1) U _{oc_i} =f(soc _i ) (2) D _max =U _{oc_max} -U _{L_min} | (3) D _ave =U _{oc_aw} -U _{L_aw} (4) The working judgment conditions of the balancing circuit are: D _max -D _ave > v _ref , v _ref is the reference voltage value of the balancing circuit; S2. The balancing process includes several balancing cycles T. The battery terminal voltage is balanced in the first half cycle T/2 of the balancing cycle T, and the battery SOC is balanced in the second half cycle T/2 of the balancing cycle T. to balance, During the charge and discharge process, if D _max -D _ave ≤ v _ref , the balancing circuit does not work. If D _max -D _ave > v _ref , the balancing circuit starts to work. The first half cycle of each balancing cycle affects the single cell corresponding to U _{oc_max} Discharge equalization is performed to reduce U _{oc_max} . In the second half of each equalization cycle, the single battery corresponding to U _{I_min} is charged and equalized, causing U _{L_min} to increase, causing D _max to decrease, and finally making D _max -D _aw ≤ v _ref is established; S3. At the end of each balancing cycle, the detection circuit re-detects and determines whether the inconsistency of the battery terminal voltage and battery SOC of each single cell meets the balancing conditions; S4, and so on, until the inconsistency of the single battery does not meet the working conditions of the balancing circuit, and the balancing circuit stops working. 2. The balancing method of the lithium-ion battery pack according to claim 1, characterized in that: the control circuit controls the balancing sub-circuit by outputting a control signal, and the frequency of the control signal is based on the storage capacity of the balancing sub-circuit. It depends on the inductance value of the inductor, the switching loss of the MOSFET, the battery terminal voltage of the single cell, and the single cell capacity of the single cell. 3. The balancing method of the lithium-ion battery pack according to claim 2, characterized in that: the duty cycle of the control signal output by the control circuit can reset the energy storage inductor in each signal period. 4. The balancing method of the lithium-ion battery pack according to claim 1, characterized in that: the balancing method is applied to energy dissipative balancing circuits and energy non-dissipating balancing circuits. 5. The balancing method of lithium-ion battery pack according to claim 1, characterized in that: the balancing method is applied to capacitor type balancing circuit, converter type balancing circuit and transformer type balancing circuit.