CN107369858A - A kind of battery pack Bi-objective equalizing circuit control strategy stage by stage - Google Patents

Abstract

The invention discloses a dual-objective stage-by-stage equalization circuit control strategy for a battery pack. According to the lithium battery equivalent circuit model, the consistency of the battery working state is determined by the battery terminal voltage and the open circuit voltage, and the open circuit and SOC show a one-to-one linear relationship within a certain range. Dual goals refer to using the cell terminal voltage and cell SOC as equalization indicators at the same time to achieve voltage balance and SOC balance; staged means to balance the voltage first and then balance the SOC within a balance cycle to achieve battery terminal voltage balance , SOC balance, and finally achieve the balance of battery terminal voltage and open circuit voltage. The invention can essentially eliminate the inconsistency of the single cells in the battery pack. The control strategy is suitable for battery balancing management systems of hybrid electric vehicles, pure electric vehicles or energy storage devices in energy storage power stations.

Description

A dual-objective phase-by-stage equalization circuit control strategy for battery packs

technical field

The invention relates to a battery group equalization technology, which is suitable for a battery equalization management system of a hybrid electric vehicle, a pure electric vehicle or an energy storage device in an energy storage power station.

Background technique

In recent years, with the deteriorating air quality and the scarcity of oil resources, new energy vehicles, especially pure electric vehicles, have become the development hotspots of major automobile companies in the world today. The power battery pack is a key component of an electric vehicle, and the power battery, as a key component of an electric vehicle, has a major impact on the power, economy and safety of the vehicle.

Due to the limited capacity of single cells and the low voltage of single cells, power battery packs are generally composed of multiple single cells connected in series and parallel to meet the requirements of use. In this way, in actual use, due to the inevitable inconsistency between single cells of the same type, the service life of the battery pack will be seriously affected, and overcharging and overdischarging may easily occur.

In order to improve the inconsistency of the battery pack, prolong the service life of the battery pack, greatly improve the overall performance of the battery pack, and ensure the safety and reliability of the battery pack, a large number of balancing topologies and control strategies have been proposed. For the research on the control strategy of the balance circuit, Kobzev, Tae-hoon Kim, etc. all use the battery terminal voltage as the balance index to balance the battery pack. When charging or after charging, the terminal voltage of the battery may be higher than that of other batteries. If this equalization method is adopted, the result of equalization is that the battery with low capacity will supplement the energy of the battery with high capacity, which will increase the power of each battery in the battery pack. capacity gap. Danielson, Huang W and others believe that the advantage of using SOC as the balance variable is that when the current changes suddenly under different working conditions, it will not cause the battery state of charge to fluctuate, making the change of the balance target relatively stable, which is conducive to reducing the impact of balance shocks on the battery, but This equalization method can only solve the performance degradation problem of the battery with larger capacity in the battery pack due to long-term insufficient charging, and cannot reduce or eliminate the gap in the actual capacity of each battery. Generally speaking, most of the current researches on the balance control strategy use a single terminal voltage or a single SOC as the balance index.

Contents of the invention

The present invention combines the battery equivalent circuit model, as shown in Figure 2, the consistency of the two single batteries is composed of three parts: battery open circuit voltage, "internal resistance + resistance-capacitance link", battery terminal voltage, etc., and at a certain SOC Within the range, there is a one-to-one relationship between the open circuit voltage of the battery and the SOC. As shown in Figure 3, the measured relationship between the open circuit voltage and the SOC of the Sanyo lithium-ion battery under different working conditions is in the range of 0.1-0.9, The curves are almost coincident, indicating that within this range, the open circuit voltage and SOC have a one-to-one correspondence. If only a single SOC and the terminal voltage of the monomer are used as the balance index, it cannot reflect the dynamic consistency of the battery in essence, but taking the SOC and the terminal voltage as the balance index at the same time can ensure the "internal resistance + resistance capacity link" Consistency, thereby ensuring consistent battery dynamic performance.

Based on this, the present invention proposes to formulate an equalization control strategy with SOC and terminal voltage simultaneously as equalization indicators, and to essentially improve the consistency of single cells of the power battery pack by equalizing the SOC and terminal voltage in stages.

A dual-objective phase-by-stage equalization control strategy, which refers to the establishment of equalization indicators based on SOC and terminal voltage, and equalizes them in stages within one equalization cycle, and finally achieves the consistency of SOC and terminal voltage of each single battery in the battery pack to meet the design requirements. Require.

Further, the method includes the following:

S1. Set the balance index: the detection circuit judges whether the inconsistency of the SOC and terminal voltage of each battery meets the working conditions of the balance circuit; if the balance condition is met, the balance circuit starts to work; if the balance condition is not satisfied, the balance circuit does not work.

Set the average open-circuit voltage of each single cell in the battery pack as U _{oc_ave} , and the average terminal voltage of each single cell as U _{L_ave} , so that:

D _i =U _{oc_i} -U _{L_i} (1)

U _{oc_i} = f(soc _i ) (2)

D _max = U _{oc_max} -U _{L_min} (3)

D _ave = U _{oc_ave} -U _{L_ave} (4)

The working judgment condition of the equalization circuit is: D _max -D _ave >v _ref , where v _ref is the reference voltage value of the equalization circuit.

S2. The equalization process includes several equalization periods, and each equalization period is T/2 time for voltage equalization, and T/2 time for SOC equalization.

During the charge and discharge process, if D _max -D _ave ≤ v _ref , the equalization circuit does not work, if D _max -D _ave > v _ref , the equalization circuit starts to work, and the first half of each equalization cycle is for the single battery corresponding to U _{oc_max} Perform discharge equalization to reduce U _{oc_max} , and charge and equalize the single battery corresponding to U _{L_min} in the second half of each equalization cycle, so that U _{L_min} increases, resulting in a decrease in D _max , and finally makes D _max -D _ave ≤ v _{ref is} established.

S3. At the end of each equalization period, the detection circuit re-detects and judges whether the SOC and terminal voltage of each battery meet the equalization conditions;

S4. Step S2 is repeated until the inconsistency of the single cells does not meet the working conditions of the balancing circuit, the balancing circuit stops working, and the balancing process ends.

Further, in step S2, during the working process of the balancing circuit, by reducing the open-circuit voltage of the single cell corresponding to the maximum value of SOC, increasing the terminal voltage of the single cell corresponding to the minimum terminal voltage, so that D _max decreases, gradually Satisfy the consistency index of the battery pack. When D _i = U _{oc_i} - U _{L_i} of each single battery in the battery pack tends to be consistent, the consistency of dynamic performance of the single battery can be achieved.

The battery pack in the present invention can be secondary batteries such as lead-acid batteries, lithium-ion batteries, nickel-metal hydride batteries or supercapacitors. Energy-dissipative equalization circuits such as equalizer circuits and transformer-type equalizer circuits, and energy non-dissipative equalizer circuits.

The beneficial points of the present invention are: at the same time, using the battery terminal voltage and SOC as the inconsistency index can essentially improve the consistency of the single cells in the battery pack; Under the premise, the balance of terminal voltage and SOC can be realized at the same time. This control strategy method is reliable, and the amount of online calculation is small, which can significantly improve the safety and reliability of the battery, improve the energy utilization rate of the battery, and prolong the life of the battery.

Description of drawings

In order to more clearly illustrate the principle of the present invention and the technical solution in the implementation, the technical solution involved in the present invention will be further introduced below using the drawings. The following drawings are only part of the implementation examples of the present invention. For those skilled in the art, Other technical solutions can be obtained according to the following figure without paying creative labor.

Fig. 1 schematic diagram of the present invention;

Figure 2 OCV-SOC curves of constant current intermittent discharge at different rates;

Figure 3 Lithium battery second-order RC equivalent circuit model.

detailed description

As shown in Figure 1, it is a dual-objective phase-by-stage equalization circuit control strategy. The dual-objective refers to taking SOC and terminal voltage as equalization indicators at the same time, and by realizing the balance of D _i = U _{oc_i} - U _{L_i} , to ensure that each battery pack The consistency of the working state of the single battery is essential. Phased means that in each equalization cycle, half of the 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 of the cycle is used to achieve SOC balance, that is, the open circuit voltage Equalization, this process is realized by discharging and equalizing the single battery with the highest open circuit voltage. This control strategy method is reliable, and the amount of online calculation is small, which can significantly improve the safety and reliability of the battery, improve the energy utilization rate of the battery, and prolong the life of the battery.

The novel dual-objective phased equilibrium control strategy includes the following steps:

S1. Set the balance index

The detection circuit judges 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.

The working judgment condition of the equalization circuit is: D _max -D _ave >v _ref , where v _ref is the reference voltage value of the equalization circuit. When D _max -D _ave > v _ref , the equalization circuit starts to work, when D _max -D _ave ≤ v _ref , the consistency of the battery pack meets the requirements, and the equalization circuit does not work.

S2, equalization circuit work

The equalization process includes several equalization cycles, each equalization cycle T/2 time is used for voltage equalization, and T/2 time is used for SOC equalization.

The equalization circuit starts to work. In the first half of each equalization cycle, the single battery corresponding to U _{oc_max} is discharged and balanced, so that U _{oc_max is} reduced; in the second half of each equalization cycle, the single battery corresponding to U _{L_min} is charged and balanced. Make U _{L_min} increase; the decrease of U _{oc_max} and the increase of U _{L_min} lead to the decrease of D _max = U _{oc_max} - U _{L_min} , when D _max decreases to make D _max - D _ave ≤ v _ref established, the equalization circuit stop working.

S3. An equilibrium cycle ends

At the end of each equalization period, the detection circuit re-detects and judges whether the SOC and terminal voltage of each battery meet the working conditions of the equalization circuit;

S4. The balance process ends

At the end of a balancing cycle, if the SOC and terminal voltage of each single battery meet the working conditions of the balancing circuit, the balancing circuit will continue to work. If the working conditions of the balancing circuit are not met, the balancing circuit will stop working, and the balancing process ends.

Before using this strategy in a balancing circuit, battery OCV-SOC calibration is required.

The selected experimental object is the 18650 cylindrical battery produced by Panasonic's Sanyo Company, with a rated capacity of 2600mAh, a rated voltage of 3.7V, a charge cut-off voltage of 4.2V, and a discharge cut-off voltage of 2.75V. The charge and discharge experiments of the battery in this paper are carried out under the constant temperature conditions of SOH=1, 25°C, and the OCV under the constant current intermittent discharge conditions of 0.2C, 0.3C, 0.4C, 0.5C, 0.6C, 0.75C, and 1C are respectively calibrated -SOC curve.

The calibration steps for each group are as follows:

Charge the battery with constant current (0.2C) and then constant voltage (cut-off voltage 4.25V);

Discharge the battery with constant current and constant capacity (260mAh);

After the discharge is over, let it stand for 1 hour to eliminate the polarization effect of the battery;

repeat steps , to the end of battery discharge.

Figure 3 shows the calibration experiment result curve. It can be seen from the figure that when the SOC is greater than 10%, the curves almost overlap, indicating that under the same temperature (25°C) and SOH (new battery) conditions, the OCV-SOC relationship curves corresponding to different discharge rates are similar. Any one of the curves can be used to represent the OCV-SOC curve at this temperature. In this paper, the OCV-SOC curve under the condition of 0.2C constant current intermittent discharge is selected as the reference curve, and the matlab sixth-order polynomial data is used to fit it, and it can be obtained:

U _oc ＝a ₁ ×soc ⁶ +a ₂ ×soc ⁵ +a ₃ ×soc ⁴ +a ₄ ×soc ³ +a ₅ ×soc ² +a ₆ ×soc+a ₇ (1)

Where: a ₁ =-34.72, a ₂ =120.7, a ₃ =-165.9, a ₄ =114.5, a ₅ =−40.9, a ₆ = 7.31, a ₇ =3.231.

Claims (7)

1. A kind of 1. battery pack Bi-objective Balance route strategy stage by stage, it is characterised in that：Refer in a balanced cycle T, it is right Two balanced indexs of battery pack are controlled；Described two balanced indexs refer to the battery terminal voltage of each cell, battery SOC；The battery terminal voltage be in balanced cycle T before half of cycle T/2 carry out balanced, the battery SOC is in equilibrium Second half of the cycle T/2 in cycle T carries out balanced.
2. 2. battery pack Bi-objective as claimed in claim 1 Balance route strategy stage by stage, it is characterised in that：The equilibrium of battery pack Process includes several balanced cycle Ts.
3. 3. battery pack Bi-objective as claimed in claim 2 Balance route strategy stage by stage, it is characterised in that comprise the steps of：

   S1, whether equilibrium condition is met by the inconsistency of each battery terminal voltage of detection circuit judges, battery SOC；

   S2, equilibrium condition is such as unsatisfactory for, equalizing circuit does not work；Such as meet equilibrium condition, equalizing circuit is started working；

   S3, balancing procedure include several balanced cycle Ts, battery terminal voltage be in balanced cycle T before half of cycle T/2 enter Row is balanced, and the battery SOC is that the second half of the cycle T/2 in balanced cycle T carries out equilibrium；

   S4, each balanced end cycle, detection circuit detect and judge the inconsistency of each battery terminal voltage, battery SOC again Whether equilibrium condition is met；

   S5, and so on, until cell inconsistency is unsatisfactory for equalizing circuit condition of work, equalizing circuit is stopped.
4. 4. battery pack Bi-objective according to any one of claims 1 to 3 Balance route strategy stage by stage, it is characterised in that： The battery pack is secondary cell.
5. 5. battery pack Bi-objective according to claim 4 Balance route strategy stage by stage, it is characterised in that：The battery pack It is lead-acid battery, lithium ion battery, Ni-MH battery or ultracapacitor.
6. 6. battery pack Bi-objective according to claims 1 to 3 Balance route strategy stage by stage, it is characterised in that：It is described equal Weighing apparatus control strategy is applied to energy-dissipating equalizing circuit and energy non-dissipative type equalizing circuit.
7. 7. battery pack Bi-objective according to claim 6 Balance route strategy stage by stage, it is characterised in that：The balanced control It is balanced that system strategy is applied to conductive discharge formula equalizing circuit, capacitor type equalizing circuit, converter type equalizing circuit and transformer type Circuit.