A vehicle dc-dc output power management method and system

Abstract

The application discloses a kind of vehicle DCDC output power management method and system, the method includes: based on the load type of low-voltage load on vehicle, the low-voltage load is classified;Obtain the use probability K of each kind of low-voltage load under load type, the consumption power Pi of low-voltage load;Maximum cumulative continuous load P1ΣC is obtained, conversion load total power P2Σ is obtained, based on the maximum cumulative continuous load P1ΣC obtained using the first method to determine the rated output power Pe1 of DCDC;Based on the conversion load total power P2Σ obtained using the second method to determine the rated output power Pe2 of DCDC;Select one of rated output power Pe1, rated output power Pe2.The application can solve the selection problem of DCDC power of pure electric or hybrid vehicle.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of power electronics, and more particularly relates to a vehicle DCDC output power management method and system. BACKGROUND

[0002] The DCDC converter converts the high voltage of the power battery pack into constant 12V, 14V or 24V low voltage, which can supply power to all vehicle electrical appliances and charge the auxiliary storage battery. It is the main power supply for the low-voltage electrical system of the automobile, and supplies power to all low-voltage electrical devices of the vehicle, and simultaneously charges the low-voltage storage battery.

[0003] Pure electric or hybrid vehicles will basically be equipped with various intelligent products, and the number of low-voltage load controllers on the vehicle will be much higher than that of traditional vehicles. Because the low-voltage load controller is different from the original traditional vehicle, the generator selection scheme of the traditional vehicle cannot be completely applicable to the pure electric or hybrid vehicle, and there is an urgent need for a DCDC power design selection scheme applicable to the pure electric or hybrid vehicle to meet the power demand of the low-voltage load controller of the pure electric or hybrid vehicle. SUMMARY

[0004] To solve the above technical problems, the present application provides a vehicle DCDC output power management method and system, which solves the problem that the output power of the DCDC should meet the premise that various use conditions and various low-voltage electrical devices can work normally, so as to achieve the balance between the supply and demand of electrical energy among the DCDC, the storage battery and the electrical load during the operation of the vehicle.

[0005] In a first aspect, the vehicle DCDC output power management method provided by the embodiments of the present application comprises:

[0006] Classifying the low-voltage loads based on the load types of the low-voltage loads on the vehicle;

[0007] Obtaining the use probability K of each type of low-voltage load under the load type;

[0008] Obtaining the consumption power Pi of each type of low-voltage load;

[0009] Obtaining the maximum cumulative continuous load P1ΣC, which is the sum of the consumption powers Pi of various types of low-voltage loads;

[0010] Obtaining the converted total load power P2Σ, which is obtained by multiplying the consumption power Pi of each type of low-voltage load by the corresponding use probability K and then summing them up;

[0011] Determining the rated output power Pe1 of the DCDC by using a first method based on the obtained maximum cumulative continuous load P1ΣC;

[0012] The rated output power Pe2 of the DCDC is determined by a second method based on the total power P2Σ of the converted load;

[0013] One of the rated output power Pe1 and the rated output power Pe2 is selected.

[0014] As a possible implementation, the acquisition of the usage probability K of each type of low-voltage load under the load type further includes:

[0015] N vehicles are selected as samples, and M vehicles are randomly selected from the N vehicles within a preset time interval;

[0016] The time period of the usage of the low-voltage load on the selected vehicle is determined.

[0017] Based on the total usage time, the usage probability Ki of each low-voltage load within a time period is determined.

[0018] The above operation is repeated multiple times to obtain multiple usage probabilities Ki of the low-voltage load, and the usage probability Ki is the ratio of the usage time of the low-voltage load to the DCDC operation time.

[0019] As a possible implementation, Y times are randomly selected to obtain Y Ki coefficients, and the usage probability K of each different low-voltage load is obtained, and the calculation formula of the usage probability K is as follows:

[0020] K = (ΣKi) / Y.

[0021] As a possible implementation, the rated output power Pe1 of the DCDC is determined by a first method based on the maximum cumulative continuous load P1ΣC further includes:

[0022] The first method includes that the rated output power Pe1 of the DCDC should be at least a larger than the maximum cumulative continuous load P1ΣC, a is a proportional value, the calculation formula of the rated output power Pe1 of the DCDC is Pe1≥P1ΣC(1+a), and the a is between 10% and 40%.

[0023] As a possible implementation, the rated output power Pe2 of the DCDC is determined by a second method based on the total power P2Σ of the converted load further includes:

[0024] The second method includes that the calculation formula of the rated output power Pe2 of the DCDC is Pe2≥P2Σ / (1-τ) = ΣKiPi / (1-τ), and the experience value τ is between 0.05 and 0.3.

[0025] As a possible implementation, the experience value a is obtained based on experience, and the numerical value of the experience value a is 25%.

[0026] As a possible implementation, the experience value τ is obtained based on an experience algorithm, and the value of τ is 0.15.

[0027] As a possible implementation, the load type based on the low-voltage load on the vehicle is used to classify the low-voltage load.

[0028] The load type is obtained based on the use frequency of the low-voltage load in different scenarios.

[0029] The load type includes long-term continuous, night continuous, cold-season continuous, high-temperature continuous, rain continuous, frequent intermittent, and short-time intermittent.

[0030] As a possible implementation, the rated power value of the DCDC in the market is selected based on the obtained rated output power Pe1 and rated output power Pe2.

[0031] In a second aspect, the vehicle DCDC output power management system provided by the embodiments of the present application,

[0032] comprises:

[0033] a preprocessing unit configured to classify the low-voltage load based on the load type of the low-voltage load on the vehicle;

[0034] a calculation unit configured to obtain a use probability K of each type of low-voltage load under the load type;

[0035] obtain a consumption power Pi of each type of low-voltage load; obtain a maximum cumulative continuous load P1ΣC, which is the sum of the consumption power Pi of each type of low-voltage load; obtain a converted total load power P2Σ, which is obtained by summing the product of the consumption power Pi of each type of low-voltage load and the corresponding use probability K; determine the rated output power Pe1 of the DCDC based on the obtained maximum cumulative continuous load P1ΣC using a first method; and determine the rated output power Pe2 of the DCDC based on the obtained converted total load power P2Σ using a second method;

[0036] an output unit configured to select one of the rated output power Pe1 and the rated output power Pe2.

[0037] The vehicle DCDC output power management method and system provided by the present application can solve the selection problem of the DCDC power of a pure electric or hybrid vehicle, so as to meet the power demand of the low-voltage load controller of the pure electric or hybrid vehicle. In particular, during the operation of the vehicle, the output power of the DCDC should meet the premise that various use conditions and low-voltage electrical devices can work normally, so as to achieve the balance between the supply and demand of electrical energy among the DCDC, the battery, and the electrical load. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 is a flow chart of the vehicle DCDC output power management method implemented by the present application.

[0039] Figure 2 is an architecture diagram of the vehicle DCDC output power management system implemented by the present application. DETAILED DESCRIPTION

[0040] In order to make the objectives, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with the accompanying drawings and embodiments. It should be understood that the present application is a quantitative evaluation system method, and the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0041] In the present application, the DC-DC converter is a kind of power conversion device that converts a certain DC voltage source into a DC voltage source required by the load. The DC-DC converter is a basic component of many other types of power converters, which is widely used in electric locomotives, electric vehicles, and switching power supplies.

[0042] Because pure electric or hybrid vehicles will basically carry various intelligent products, the number of low-voltage load controllers on the whole vehicle will be much higher than that of traditional vehicles. Because the low-voltage load controller is different from the original traditional vehicle, the generator selection scheme of the traditional vehicle cannot be completely applicable to the pure electric or hybrid vehicle. Therefore, the present application provides a vehicle DCDC output power management method and system to meet the power demand of the low-voltage load controller of the pure electric or hybrid vehicle. Especially during the running of the vehicle, the output power of the DCDC should meet the premise that various use conditions and each low-voltage electrical device can work normally, so that the balance between the DCDC, the storage battery and the electrical load can be achieved. It should be noted that the low-voltage load mentioned in the present application can be understood as a low-voltage load controller.

[0043] Reference Figure 1 , Figure 1 is a flow chart of the vehicle DCDC output power management method implemented by the present application. Specifically, it can include the following steps:

[0044] Step S100, based on the load type of the low-voltage load on the vehicle, the low-voltage load is classified.

[0045] Specifically, it includes step S110, obtaining the load type based on the use frequency of the low-voltage load in different scenes; the load type includes long-term continuous, night continuous, cold season continuous, high temperature continuous, rain continuous, frequent intermittent and short time intermittent.

[0046] As an example, the low-voltage components on the vehicle are classified into A-G categories according to the frequency of use in different scenarios, specifically long-term continuous, night continuous, cold season continuous, high-temperature continuous, rain continuous, frequent intermittent, and short-time intermittent. For example, the vehicle controller, battery controller, and motor controller are used continuously for a long time, the night driving light is used continuously at night, the PTC heater controller is used continuously in the cold season, the compressor controller is used continuously at high temperature, the wiper controller is used continuously in the rain, the steering controller is used frequently intermittently, and the brake controller is used intermittently for a short time. Here, all the vehicle controllers are not listed one by one.

[0047] Step S200, obtaining the use probability K of each type of low-voltage load under the load type.

[0048] The calculation of the use probability K uses the interval estimation method of the overall average number. For the whole vehicle, the individual use difference is large, and the use probability of the low-voltage load is not completely the same. We can calculate the possible region of the overall parameter value on the basis of selecting samples, and determine the K value in the possible region. This method is called interval estimation value method, which specifically includes the following steps:

[0049] Step S210, selecting N vehicles as samples, and randomly selecting M vehicles in a predetermined time interval, M≤N.

[0050] Step S220, determining the time period of use of the low-voltage load on the selected vehicle;

[0051] Step S230, determining the use probability Ki of each low-voltage load in a time period based on the overall use time;

[0052] Step S240, repeating the above operation N times to obtain multiple use probabilities Ki of the low-voltage load. The use probability Ki is the ratio of the use time of the low-voltage load to the DCDC operation time.

[0053] According to the method shown in steps S210-S240, Y times (N>Y) are randomly selected, so that Y Ki coefficients of a certain type of low-voltage load can be obtained, and the average value can be determined to determine the use probability K value of each different load. The calculation formula of the use probability K value is:

[0054] K=(ΣKi) / Y (Formula 1).

[0055] Use the above formula 1 to calculate the use probability K of each type of low-voltage load (in Table 1, Ka, Kb, Kc, Kd, Kf, Kh, Ki, etc.)

[0056] Step S300, obtaining the consumption power Pi of each type of low-voltage load.

[0057] The consumed power Pi is calculated after long-term data accumulation for each type of low-voltage load.

[0058] The consumed power Pa, Pb, Pc, Pd, etc. of each type of low-voltage load in the following table is obtained respectively.

[0059] Table 1: Low-voltage electrical system power consumption information table

[0060] Low voltage load type Load type Consumed power (W) Usage probability (K) Class A Long continuous Pa Ka Class B Night continuous Pb Kb Class C Cold season continuous Pc Kc Class D High temperature continuous Pd Kd Class E Rain continuous Pf Kf Class F Frequent intermittent Ph Kh Class G Short intermittent Pi Ki

[0061] Step S400: Obtain the maximum cumulative continuous load P1ΣC, which is the sum of the consumed power Pi of each type of low-voltage load.

[0062] For example, the sum of the consumed power Pi of each type of low-voltage load (A-G type) in Table 1, i.e. the formula of the maximum cumulative continuous load P1ΣC is:

[0063] P1ΣC = Pa + Pb + Pc + Pd + Pf + Ph + Pi (Formula 2)

[0064] Step S500: Obtain the converted total load power consumption P2Σ, which is the sum of the product of the consumed power Pi of each type of low-voltage load and its corresponding usage probability K.

[0065] For example, the converted total load power consumption P2Σ of each type of low-voltage load (A-G type) in Table 1, the calculation formula is:

[0066] P2Σ = Pa*Ka + Pb*Kb + Pc*Kc + Pd*Kd + Pf*Kf + Ph*Kh + Pi*Ki (Formula 3)

[0067] Step S600: Based on the obtained maximum cumulative continuous load P1ΣC, the first method is used to determine the rated output power Pe1 of the DCDC.

[0068] The first method is that the rated output power Pe1 of the DCDC should be at least a larger than the maximum cumulative continuous load P1ΣC, a is a proportional value, and the calculation formula of the rated output power Pe1 of the DCDC is:

[0069] Pe1 ≥ P1ΣC(1 + a), (Formula 4);

[0070] Wherein a is between 10% and 40%, and the empirical value a is obtained based on experience.

[0071] For example, the optimal value of the empirical value a is 25%, based on the power consumption devices, load types and usage probabilities in Table 1, the corresponding maximum cumulative continuous load P1ΣC is calculated, based on the calculation of P1ΣC, the rated output power Pe1 of the DCDC is calculated using the above formula 4:

[0072] Pe1≥ P1∑C (1 + 25%) ;

[0073] P1∑C = Pa + Pb + Pc + Pd + Pf + Ph + Pi;

[0074] Pe1≥ (Pa + Pb + Pc + Pd + Pe) x 1.25W.

[0075] Step S700, based on the obtained converted load total power P2∑, the second method is used to determine the rated output power Pe2 of the DCDC.

[0076] The second method is specifically the rated output power Pe2 of the DCDC, and the calculation formula is:

[0077] Pe2≥ P2∑ / (1-τ) = ∑KiPi / (1-τ) (Formula 5)

[0078] The empirical value τ is between 0.05 and 0.3, and the empirical value τ is obtained based on the empirical algorithm.

[0079] For example, the optimal value of τ is 0.15, based on the table 1, the load type and the use probability of each electrical device, the corresponding converted load total power P2∑ is calculated, and Pe2 is obtained based on P2∑, and the calculation formula of Pe2 is as follows:

[0080] Pe2≥ P2∑ / (1-0.15)

[0081] P2∑ = Pa*Ka + Pb*Kb + Pc*Kc + Pd*Kd + Pf*Kf + Ph*Kh + Pi*Ki

[0082] Pe2≥ Pa*Ka + Pb*Kb + Pc*Kc + Pd*Kd + Pf*Kf + Ph*Kh + Pi*Ki / (1-0.15)W.

[0083] Step S800, one of the rated output power Pe1 and the rated output power Pe2 is selected.

[0084] Specifically, the rated power value of the DCDC in the market is selected based on the obtained rated output power Pe1 and rated output power Pe2. Both of the two power values Pe1 and Pe2 can meet the requirements, if Pe1 is used, the maximum power value is used, which will cause the cost to increase. The value of Pe2 is the minimum, which is the most cost-saving. The DCDC has the maximum rated power, and the cost is higher, but the DCDC in the market has a fixed power value, such as 2.2, 3.5, 4, and 5 kW or so. If 5 kW is calculated according to Pe1, and 3.4 kW is calculated according to Pe2, 3.5 kW can be selected at this time, because there is no 3.4 kW in the market. If 3.8 kW is calculated according to Pe2, 4 kW can be selected at this time, because there is no 3.8 kW in the market. The calculated power value is the maximum and minimum power value that meets the requirements of the whole vehicle, but the final selection needs to be calculated according to the rated power of the DCDC product in the market.

[0085] In a second aspect of the present application, based on the above-mentioned vehicle DCDC output power management method, an embodiment of the present application further discloses a vehicle DCDC output power management system, comprising:

[0086] The preprocessing unit classifies the low-voltage loads based on the load types of the low-voltage loads on the vehicle.

[0087] The calculation unit obtains the usage probability K of each type of low-voltage load under the load type; obtains the consumption power Pi of each type of low-voltage load; obtains the maximum cumulative continuous load P1ΣC, which is the sum of the consumption power Pi of each type of low-voltage load; obtains the converted total load power P2Σ, which is the sum of the product of the consumption power Pi of each type of low-voltage load and its corresponding usage probability K; determines the rated output power Pe1 of the DCDC by using the first method based on the obtained maximum cumulative continuous load P1ΣC; determines the rated output power Pe2 of the DCDC by using the second method based on the obtained converted total load power P2Σ;

[0088] The output unit selects one of the rated output power Pe1 and the rated output power Pe2.

[0089] In a possible case, the acquiring the usage probability K of each low-voltage load under the load type further comprises: selecting N vehicles as samples, randomly selecting M vehicles in a preset time interval, and determining the time period of usage of the low-voltage load on the selected vehicles; determining the usage probability Ki of each low-voltage load in a time period based on the total usage time; repeating the above operation multiple times to acquire multiple usage probabilities Ki of the low-voltage load, and the usage probability Ki is the proportion of the usage time of the low-voltage load to the DCDC operation time.

[0090] For example, the above operation is repeated N times, and Y times (N>Y) are randomly selected, so that Y Ki coefficients of a certain type of low-voltage load are obtained, and the average value is calculated to determine the usage probability K value of each different load. The calculation formula of the usage probability K value is:

[0091] K=(ΣKi) / Y (Formula 1).

[0092] Wherein, Ki represents the usage probability of the low-voltage load.

[0093] In a possible case, the first method is used to determine the rated output power Pe1 of the DCDC based on the acquired maximum cumulative continuous load P1ΣC further comprises:

[0094] The first method comprises that the rated output power Pe1 of the DCDC is at least greater than the maximum cumulative continuous load P1ΣC by a, a is a proportional value, and the calculation formula of the rated output power Pe1 of the DCDC is Pe1≥P1ΣC(1+a), and the a is between 10% and 40%.

[0095] For example, the optimal value of the experience value a is 25%, the corresponding maximum cumulative continuous load P1ΣC is calculated based on the electrical devices, load types and usage probabilities in Table 1, and the rated output power Pe1 of the DCDC is calculated based on the calculation P1ΣC and the above formula 4:

[0096] Pe1>>P1ΣC(1+25%);

[0097] P1ΣC=Pa+Pb+Pc+Pd+Pf+Ph+Pi;

[0098] Pe1≥(Pa+Pb+Pc+Pd+Pe)×1.25W.

[0099] Pa, Pb, Pc, Pd, etc. represent the consumption power of the low-voltage load.

[0100] In a possible case, the second method is specifically a calculation formula of the rated output power Pe2 of the DCDC, which is Pe2≥P2Σ / (1-τ)=ΣKiPi / (1-τ), where the empirical value τ is between 0.05 and 0.3, and the empirical value τ is obtained based on experience.

[0101] For example, the optimal value of τ is 0.15, the corresponding converted total power P2Σ of the load is calculated based on each electrical device, load type and use probability in Table 1, and Pe2 is obtained based on P2Σ, and the calculation formula of Pe2 is as follows:

[0102] Pe2≥P2Σ / / (1-0.15)

[0103] P2Σ=Pa*Ka+Pb*Kb+Pc*Kc+Pd*Kd+Pf*Kf+Ph*Kh+Pi*Ki

[0104] Pe2≥Pa*Ka+Pb*Kb+Pc*Kc+Pd*Kd+Pf*Kf+Ph*Kh+Pi*Ki] / (1-0.15)W.

[0105] In a possible case, the rated output power Pe1 and the rated output power Pe2 are selected based on the obtained rated output power Pe1 and the rated output power Pe2.

[0106] Both the rated output power Pe1 and the rated output power Pe2 can meet the requirements, if Pe1 is used, the maximum power value is used, which causes an increase in cost. The value of Pe2 is the minimum and the most cost-saving. The DCDC has the maximum rated power and the higher cost, but the DCDC on the market has a fixed power value, such as 2.2, 3.5, 4 and 5 kW or so. If 5 kW is calculated according to Pe1, and 3.4 kW is calculated according to Pe2, 3.5 kW can be selected at this time, because there is no 3.4 kW on the market. If 3.8 kW is calculated according to Pe2, 4 kW can be selected at this time, because there is no 3.8 kW on the market. The calculated power value is the maximum and minimum power value that meets the requirements of the whole vehicle, but the final selection needs to be calculated according to the corresponding rated power of the DCDC product on the market.

[0107] The system embodiment corresponds to the method embodiment, has the same technical effects as the method embodiment, and specific descriptions are referred to the method embodiment. The system embodiment is based on the method embodiment, and specific descriptions can be referred to the method embodiment, which will not be described herein. It can be understood by those skilled in the art that the drawings are only a schematic diagram of an embodiment, and the modules or flows in the drawings are not necessarily required to implement the present application.

[0108] The vehicle DCDC output power management method and system of the present application can solve the selection problem of DCDC power of pure electric or hybrid vehicle, so as to meet the power demand of low-voltage load controller of pure electric or hybrid vehicle, especially in the process of vehicle operation, the output power of DCDC should meet the premise that various use conditions and various low-voltage electrical devices can work normally, so that the balance between DCDC, battery and electrical load can be achieved.

[0109] Those skilled in the art can understand that the modules in the system in the embodiments can be distributed in the system of the embodiments according to the embodiment description, or can be changed and located in one or more systems different from the embodiments. The modules of the above embodiments can be combined into one module, or can be further split into multiple sub-modules.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can still be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A vehicle DCDC output power management method, characterized by, Classifying the low-voltage loads based on the load types of the low-voltage loads on the vehicle; obtaining a use probability K of each type of low-voltage load under the load type; obtaining a consumed power Pi of each type of low-voltage load; obtaining a maximum cumulative continuous load P1ΣC, which is a sum of the consumed powers Pi of the low-voltage loads of each type; obtaining a converted total power P2Σ of the loads, which is a sum of the products of the consumed powers Pi of each type of low-voltage load and the corresponding use probability K; determining a rated output power Pe1 of the DCDC based on the obtained maximum cumulative continuous load P1ΣC by using a first method; determining a rated output power Pe2 of the DCDC based on the obtained converted total power P2Σ of the loads by using a second method; Selecting one of the rated output power Pe1 and the rated output power Pe2; The first method includes that the rated output power Pe1 of the DCDC is at least greater than the maximum cumulative continuous load P1ΣC by a, which is a proportional value, and a calculation formula of the rated output power Pe1 of the DCDC is Pe1≥P1ΣC(1+a), and a is between 10% and 40%; The second method includes that a calculation formula of the rated output power Pe2 of the DCDC is Pe2≥P2Σ / (1-τ)=ΣKiPi / (1-τ), and an empirical value τ is between 0.05 and 0.

3.

2. The vehicle DCDC output power management method of claim 1, wherein, The obtaining of the use probability K of each type of low-voltage load under the load type further includes: selecting N vehicles as samples, and randomly selecting M vehicles in a preset time interval; determining a time period in which the low-voltage loads on the selected vehicles are used; determining a use probability Ki of each low-voltage load in a time period based on the total use time; repeating the above operation multiple times to obtain multiple use probabilities Ki of the low-voltage loads, and the use probability Ki is a proportion of the use time of the low-voltage load to the operation time of the DCDC.

3. The vehicle DCDC output power management method of claim 2, wherein, Y times are randomly selected to obtain Y Ki coefficients, and a use probability K of each different low-voltage load is obtained, and a calculation formula of the use probability K is as follows: K=(ΣKi) / Y.

4. The vehicle DCDC output power management method of claim 1, wherein, The a is obtained based on experience, and a value of the a is 25%.

5. The vehicle DCDC output power management method of claim 1, wherein, The τ is obtained based on an empirical algorithm, and a value of the τ is 0.

15.

6. The vehicle DCDC output power management method of claim 1, wherein, The classifying the low-voltage loads based on the load types of the low-voltage loads on the vehicle includes: obtaining the load types based on use frequencies of the low-voltage loads in different scenarios; and the load types include long-term continuous, night continuous, cold-season continuous, high-temperature continuous, rain continuous, frequent intermittent, and short-time intermittent.

7. The vehicle DCDC output power management method of claim 1, wherein, Selecting a rated power value of the DCDC in the market based on the obtained rated output power Pe1 and the rated output power Pe2.

8. A vehicle DCDC output power management system, characterized by, The method includes: A preprocessing unit classifies the low-voltage loads based on the load types of the low-voltage loads on the vehicle; The computing unit obtains a use probability K of each type of low-voltage load under a load type; obtains a consumption power Pi of each type of low-voltage load; obtains a maximum cumulative continuous load P1ΣC, which is a sum of the consumption powers Pi of the various types of low-voltage load; obtains a converted total load power P2Σ, which is a sum of the products of the consumption power Pi of each type of low-voltage load and the corresponding use probability K; determines a rated output power Pe1 of the DCDC based on the obtained maximum cumulative continuous load P1ΣC using a first method; and determines a rated output power Pe2 of the DCDC based on the obtained converted total load power P2Σ using a second method. The output unit selects one of the rated output power Pe1 and the rated output power Pe2. The first method includes that the rated output power Pe1 of the DCDC should be at least greater than the maximum cumulative continuous load P1ΣC by a, a is a proportional value, and a calculation formula of the rated output power Pe1 of the DCDC is Pe1≥P1ΣC(1+a), a is between 10% and 40%. The second method includes that a calculation formula of the rated output power Pe2 of the DCDC is Pe2≥P2Σ / (1-τ)=ΣKiPi / (1-τ), and an empirical value τ is between 0.05 and 0.3.

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