Charging control method and system for an electric vehicle

Abstract

Charge control procedures for an electric vehicle, comprehensive: Monitoring, by means of a control system, the state of an auxiliary battery, which includes a state of charge (SOC), a temperature and a charging current of the auxiliary battery; Determine, by controlling, a charging mode of the electric vehicle, based on the monitored state of the auxiliary battery; Setting, by means of the control, a charging voltage of the auxiliary battery, based on the monitored SOC and temperature of the auxiliary battery; Charging, via the control unit, of the auxiliary battery with the set charging voltage; and Switching, by controlling, the charging mode of the electric vehicle, based on the monitored state of the auxiliary battery, wherein the charging mode includes a charge-oriented mode for charging the auxiliary battery and a low-voltage charging mode for maintaining a voltage of the auxiliary battery, which includes determining the charge mode: If the monitored SOC of the auxiliary battery is less than or equal to a preset first SOC criterion, determine, by controlling, the charge-oriented mode of the electric vehicle as the charge mode, and which includes changing the charging mode: If the monitored SOC of the auxiliary battery is less than or equal to the first SOC criterion, and greater than a second SOC criterion that is preset to a value less than that of the first SOC criterion, a preset second minimum holding time will be applied, and if the average charging current of the auxiliary battery during charging is greater than a charging current criterion, the control system will switch the charging mode of the electric vehicle to a low-voltage charging mode.

Description

BACKGROUND 1. Field of the invention

[0001] The present invention relates to a charging control method and system for an electric vehicle which sets different charging control methods based on the state of charge (SOC) of the auxiliary battery of the electric vehicle, thereby improving the charging efficiency of the electric vehicle. 2. State of the art

[0002] Recently, due to the inadequacy of environmentally friendly products and energy sources attributable to pollution, the conservation of energy resources has been emphasized. Accordingly, even the automotive industry has developed electric vehicles that are environmentally friendly and highly energy-efficient to improve competitiveness. Lately, thanks to improvements in battery performance and reduced battery costs, electric vehicles have become a viable option for general-purpose vehicles. Electric vehicles are characterized by the fact that they emit no carbon dioxide while driving, exhibit excellent environmental performance, and offer enhanced vehicle appeal, such as improved acceleration responsiveness and a quiet, smooth driving experience.

[0003] Typically, an electric vehicle requires an operating current for its electronic components and a drive current to power the motor that moves the vehicle. Both the operating and drive currents are supplied by a battery. Generally, a main battery, consisting of multiple battery cells connected in series, is used to provide the drive current. However, because the electronic components of an electric vehicle operate at a lower voltage than the operating and drive currents, they are configured to draw power either from a voltage-down converter of current supplied by the main battery or from an auxiliary battery that provides suitable current for electronic components. In particular, the electronic components also include an electronic starter unit.

[0004] Therefore, a technology to prevent the discharge of an auxiliary battery has been continuously developed for electric vehicles to prevent situations where starting or using electronic units becomes impossible. The state of the art includes a control method that allows an auxiliary battery to be charged as needed, even when an electric vehicle is in an ignition-on state, thereby improving the reliability and marketability of electric vehicles.

[0005] However, such conventional technology only provides a charging control method when the auxiliary battery voltage is low and does not provide a charging control strategy for the auxiliary battery when the auxiliary battery voltage is sufficiently high or maintained at a suitable voltage level. Consequently, the auxiliary battery may continue to be charged even when its voltage is sufficiently high, thus potentially worsening the electric vehicle's fuel efficiency.

[0006] DE 10 2011 007 713 A1 describes a vehicle power management system (VPMS) that controls the charging voltage for a battery in a vehicle, wherein a VPMS control unit evaluates the state of charge (SOC), temperature, and charging current of the battery to determine a charging mode. A fast-charging mode is used when the state of charge is below a first threshold, wherein the VPMS control unit selects a target fast-charging voltage, compensates for the battery temperature, and transmits the compensated fast-charging voltage to the charging source.

[0007] DE 10 2014 208 431 A1 relates to a method and a device for controlling the charging of a low-voltage battery. The method includes assigning a preset voltage to the low-voltage battery based on a vehicle's operating mode and the state of charge of a high-voltage battery by means of a controller. Furthermore, the controller compares the preset voltage with the current charging voltage of the low-voltage battery and increases the current charging voltage with an initial slope until it equals the preset voltage if the preset voltage is higher than the current charging voltage.

[0008] The foregoing is intended only to aid a better understanding of the background of the present invention and is not intended to imply that the present invention falls within the scope of the prior art already known to those skilled in the field. SUMMARY

[0009] Accordingly, the present invention provides a charging control method and system for an electric vehicle which improves the charging efficiency of an auxiliary battery and the fuel efficiency of the electric vehicle by establishing different charging strategies for the auxiliary battery based on whether the state of charge (SOC) of the auxiliary battery is high or low.

[0010] The invention is defined by the independent claims.

[0011] A charging control procedure for an electric vehicle may, for example, include monitoring, by a controller, the state of an auxiliary battery, which includes a state of charge (SOC), temperature, and charging current of the auxiliary battery; determining, by the controller, a charging mode of the electric vehicle based on the monitored state of the auxiliary battery; setting, by the controller, a charging voltage of the auxiliary battery when determining the charging mode, based on the monitored SOC and temperature of the auxiliary battery; and charging, by the controller, the auxiliary battery with the set charging voltage.

[0012] The charging mode can include a charge-oriented mode for charging the auxiliary battery and a low-voltage charging mode for maintaining the auxiliary battery's voltage. Determining the charging mode can involve, if the monitored state of charge (SOC) of the auxiliary battery is greater than a predetermined initial SOC criterion for a preset minimum holding time, selecting the vehicle's low-voltage charging mode as the charging mode. The initial minimum holding time can be modified based on the monitored SOC value of the auxiliary battery. Additionally, determining the charging mode can involve, if the monitored SOC of the auxiliary battery is less than or equal to a preset initial SOC criterion, selecting the electric vehicle's charge-oriented mode as the charging mode.

[0013] The charge control procedure can further include, after charging the auxiliary battery, changing the charging mode of the electric vehicle based on the monitored state of charge of the auxiliary battery. Additionally, if the holding time of the charge-oriented mode exceeds a maximum time, the charging mode of the electric vehicle can be changed to a low-voltage charging mode. The maximum time can be modified based on a value of the monitored state of charge (SOC) of the auxiliary battery.

[0014] Furthermore, changing the charging mode can involve switching the electric vehicle's charging mode to low-voltage charging mode if the monitored state of charge (SOC) of the auxiliary battery is less than or equal to the first SOC criterion for a preset second minimum holding time and greater than a second SOC criterion set to a value less than the first SOC criterion, and if the average charging current of the auxiliary battery during charging is greater than a charging current criterion. The second minimum holding time can be changed based on the value of the monitored SOC of the auxiliary battery. Setting the charging voltage can involve adjusting the charging voltage using map data, where the temperature and the SOC of the auxiliary battery, based on the specific charging mode, are used as inputs and the charging voltage is an output. BRIEF DESCRIPTION OF THE DRAWING

[0015] The above and other problems, features and advantages of the present invention will become more apparent from the following detailed description, when viewed together with the accompanying drawing, wherein: Fig. 1 is a flowchart showing a charge control method for an electric vehicle according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0016] It is understood that the term "vehicle" or "vehicle-" or any other similar term used herein includes motor vehicles in general, such as passenger cars including sports utility vehicles (SUVs), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles, and other alternative fuel vehicles (e.g., fuels derived from resources other than petroleum). As referenced herein, a hybrid vehicle is a vehicle that has two or more power sources, for example, vehicles that are powered by both gasoline and electricity.

[0017] Although an exemplary embodiment is described using a plurality of units to perform the exemplary process, it is understood that the exemplary process can also be performed by one or more modules. It is also understood that the term controller / control unit refers to a hardware device containing memory and a processor. The memory is configured to store the modules, and the processor is specifically configured to execute the modules to perform one or more processes described below.

[0018] Furthermore, the control logic of the present invention can also be implemented as non-transitory computer-readable media on a computer-readable medium containing executable program instructions that are executed by a processor, a controller / control unit, or the like. Examples of the computer-readable medium include, but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes, floppy disks, flash drives, smart cards, and optical data storage devices. The computer-readable recording medium can also be distributed in network-connected computer systems, so that the computer-readable media are stored and executed in a distributed manner, e.g., by a telematics server or a controller area network (CAN).

[0019] The terminology used herein serves only to describe certain embodiments and is not intended to limit the invention. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It is further understood that the terms "comprise" and / or "comprehensive," when used in this specification, specify the presence of the aforementioned features, integers, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. As used herein, the term "and / or" includes any and all combinations of one or more of the associated, listed elements.

[0020] Unless specifically stated or clear from the context, the term "approximately," as used herein, means within a range of normal prior art tolerance, for example, within two standard deviations of the mean. "Approximately" may be understood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated value. Unless otherwise clear from the context, all numerical values ​​provided herein are modified by the term "approximately."

[0021] Exemplary embodiments of the present invention are described in detail below with reference to the attached drawing.

[0022] As in Fig. As shown in Figure 1, a charging control method for an electric vehicle according to the present invention can include monitoring (S100) by a controller the state of an auxiliary battery, including the state of charge (SOC), temperature and charging current of the auxiliary battery; determining (S200) by the controller the charging mode of the electric vehicle based on the monitored state of the auxiliary battery; setting (S300) by the controller the charging voltage of the auxiliary battery in the determined charging mode based on the SOC and temperature of the auxiliary battery; and charging (S400) by the controller the auxiliary battery with the set voltage.

[0023] Monitoring can include tracking the auxiliary battery's condition, including its state of charge (SOC), temperature, and charging current. Specifically, the SOC and charging current can be used as criteria to determine the auxiliary battery's charging mode, as described later, and the SOC and temperature can be used to set the charging voltage. The monitored information can represent the auxiliary battery's condition, encompassing its SOC, temperature, and charging current. Additionally, information related to the auxiliary battery, such as its voltage and wear, can also be included in the battery's condition report.

[0024] After monitoring the auxiliary battery's condition, the vehicle's charging mode can be determined based on this monitoring. The method for determining the vehicle's charging mode can be implemented in various ways. It may be possible to monitor the auxiliary battery's condition in real time and then determine the charging mode. Alternatively, the time period can be divided into uniform time intervals, and the charging mode determined separately for each interval. Furthermore, different charging mode types can be implemented, such as a fast charging mode or a slow charging mode.As an example of different forms, the present invention can include the configuration of dividing the charging modes of the auxiliary battery into a charge-oriented mode for charging the auxiliary battery and a low-voltage charging mode for maintaining the voltage of the auxiliary battery.

[0025] The charge-oriented mode can be described as a mode for charging the auxiliary battery. This mode is intended for use when the auxiliary battery requires charging and is at risk of becoming completely discharged. Therefore, the charging voltage can be set to the highest possible value within the extended charging voltage range. Meanwhile, the low-voltage charging mode can be used when the auxiliary battery voltage is already sufficiently high, requiring the charging voltage to simply maintain the battery's charge level. Therefore, the charging voltage can be set to a value lower than the charging voltage used in the charge-oriented mode.

[0026] In particular, one point to be addressed is the concept of charging voltage. The term "charging voltage" as described in the present application refers to the charging voltage required to charge the auxiliary battery. In a typical electric vehicle, the auxiliary battery is charged with a low voltage obtained by converting a high voltage from a high-voltage battery to a low voltage via a low-voltage converter, that is, a low-current direct current (DC) / DC converter (LDC). Therefore, the voltage of the auxiliary battery can be the same as the output voltage of the LDC. This concept is applied to the present invention in the same way, and the charging voltage described above or subsequently has the same meaning as the output voltage of the LDC.

[0027] As described above, assuming that the identifiable modes are a charge-oriented mode and a low-voltage charge mode, the criteria for classifying the charge mode based on the monitoring state of the auxiliary battery can become problematic. Therefore, the present invention presents criteria for classifying charge modes based on the state of the auxiliary battery. In particular, classification methods using the state of charge (SOC) and the average charge current of the auxiliary battery are described separately.

[0028] As a first classification criterion, the method presented in the present invention is configured such that if the monitored state of charge (SOC) of the auxiliary battery exceeds a first SOC criterion for a preset first minimum holding time, the controller can be configured to determine a low-voltage charging mode as the vehicle's charging mode. As the SOC increases, the battery approaches a fully charged state. Therefore, the first SOC criterion must have a sufficiently high SOC value so that the low-voltage charging mode described above can be determined as the charging mode. The first SOC criterion can have various values ​​based on a designer's selection. However, if the first SOC criterion is set to an excessively low value, the auxiliary battery may be at risk of being discharged, and thus the first SOC criterion can preferably be set to approximately 90%.

[0029] However, even if the monitored state of charge (SOC) of the auxiliary battery is greater than the first SOC criterion, the low-voltage charging mode may not be determined as the charging mode. In the present invention, the criterion for determining the low-voltage charging mode as the charging mode stipulates that the SOC must exceed the first SOC criterion for a minimum holding time. This prevents errors that may occur due to noise during the monitoring process or voltage spikes in the auxiliary battery and compensates for the phenomenon of the SOC decreasing over time. In other words, the first minimum holding time can be freely set based on designer choice. However, to minimize errors caused by noise or voltage spikes, the first minimum holding time can be set to a large value, for example, approximately 3600 s.However, if the initial minimum holding time is set to an excessively large value, the time required to determine the charging mode increases, potentially impairing the vehicle's charging efficiency and thus affecting its fuel efficiency. Therefore, the initial minimum holding time should be adjusted to a suitable value, taking into account both its advantages and disadvantages.

[0030] Furthermore, although the initial minimum hold time can be freely set, the set value can be modified based on the monitored state of charge (SOC) of the auxiliary battery. This is because, if the initial SOC criterion is assumed to be, for example, 90%, the system's efficiency may be negatively affected when the auxiliary battery's SOC is 91% or 99%, and is controlled in the same way. Since the case where the SOC is 99% differs more from the initial SOC criterion than the case where the SOC is 91%, there is no need to set the initial minimum hold time to a significant period. Therefore, the initial minimum hold time can be modified based on the value of the monitored auxiliary battery's SOC, and the initial minimum hold time can be set to a shorter duration as the difference from the initial SOC criterion increases.

[0031] In contrast, if the monitored state of charge (SOC) of the auxiliary battery is less than or equal to the preset first SOC criterion, the controller can be configured to determine the charge-oriented mode as the vehicle's charging mode. If the monitored SOC is less than or equal to the first SOC criterion, this indicates the battery's state of charge, at which the auxiliary battery needs to be charged to some extent, as described above. Therefore, in this case, the low-voltage charging mode, rather than the charge-oriented mode, may be determined.

[0032] Regardless of whether the charge-oriented mode or the low-voltage charge mode is selected as the charging mode, the controller can be configured to adjust the auxiliary battery's charging voltage based on the monitored state of charge (SOC) and temperature of the auxiliary battery after the charging mode has been selected. Therefore, the present invention can secondarily improve charging efficiency by adjusting the charging voltage differently, taking into account both the SOC and temperature of the auxiliary battery, even within the selected charging mode. Primarily, charging efficiency is improved by classifying the auxiliary battery charging mode based on the auxiliary battery's SOC.

[0033] In particular, the state of charge (SOC) and temperature can be considered as variables required to set the charging voltage, as the SOC and temperature of the auxiliary battery are indicators that allow for easier detection of its state of charge. The SOC of the auxiliary battery indicates its state of charge, which can be considered the most direct factor in setting the charging voltage. Therefore, the lower the SOC of the auxiliary battery, the higher the probability that the battery will discharge, and thus the charging voltage can be higher.

[0034] However, the temperature of the auxiliary battery exhibits a slightly different characteristic than that of the state of charge (SOC). Generally, the temperature of an electrical device can be determined based on the amount of current flowing through it. Furthermore, every electrical device has its own specific load resistance. The temperature can rise proportionally to a value determined by multiplying the square of the current flowing through a load resistance by the load resistance. In particular, since the load resistance is a unique value defined for each electrical device, it can be assumed that, as a result, the temperature is proportional to the square of the current. "Current" in the literal sense of the word means the flow of current through electrodes.Accordingly, it can be deduced that if the electrode flow is significant, i.e., if the auxiliary battery is charged rapidly, the temperature can rise significantly.

[0035] Therefore, considering these temperature characteristics, a high auxiliary battery temperature may indicate that the battery is already being charged at a high rate. Consequently, continuing to charge the auxiliary battery in this case can accelerate battery degradation, negatively impacting its lifespan. Conversely, a low auxiliary battery temperature indicates a lower probability of battery degradation. Therefore, in this case, the charging voltage can be set to a high value, minimizing the likelihood of the auxiliary battery being discharged.

[0036] In other words, during the voltage adjustment process (S300), the charging voltage can be set taking into account the state of charge (SOC) and temperature of the auxiliary battery. However, the charging voltage can also be set to a higher value if the SOC or temperature of the auxiliary battery decreases. Such a charging voltage can be set to different values ​​based on the type and condition of the vehicle. As an example, the present invention presents a method for setting the charging voltage by inserting characteristic map data, in which the temperature and SOC of the auxiliary battery, based on the charging mode, are used as inputs, and the charging voltage is an output.

[0037] Once the voltage has been set, the controller can be configured to charge the auxiliary battery, performing the set charging voltage as described in Fig. Figure 1 shows. As described above, the most commonly used method for charging the auxiliary battery is to convert a high voltage from a high-voltage battery into a low voltage using the LDC and to charge the auxiliary battery with the low voltage.

[0038] After the charging process (S400), if the auxiliary battery is charged in low-voltage charging mode, the low-voltage charging mode can be maintained continuously until the auxiliary battery's state of charge (SOC) decreases to less than or equal to the first SOC criterion. However, if the auxiliary battery's charging mode is charge-oriented mode, the situation may differ slightly from the above. Since charge-oriented mode is a mode in which the auxiliary battery is charged continuously, the auxiliary battery's SOC may increase over time. Therefore, the auxiliary battery may not require further charging after this time has elapsed.Therefore, in order to respond to this situation, the present invention performs the mode change process (S500) through the controller, which can be configured to select the vehicle's charging mode based on the monitored state of the auxiliary battery after the charging process has been carried out, as shown in . Fig. 1 shown, to change.

[0039] The monitoring process (S100) can be the first step of the present invention. Even after the mode determination (S200), voltage setting (S300), and charging (S400) have been performed, the controller can be configured to continuously monitor the auxiliary battery's state. Therefore, during mode determination (S200), even if the charge-oriented mode has been selected as the charging mode and charging the auxiliary battery is required, it may be possible to change the charging mode if, as a result of continuously monitoring the auxiliary battery's state, the battery's state corresponds to a state in which charging is no longer necessary.

[0040] The state of the auxiliary battery, which serves as the criterion for changing the charging mode, can be determined using various variables, such as the current and temperature of the auxiliary battery. Therefore, several methods based on variables identified as the most reasonable from among the various variables that can be considered are presented in the present invention. The first method is configured such that if the holding time of the charge-oriented mode exceeds the maximum time, the control system can be configured to switch the vehicle's charging mode to a low-voltage charging mode.

[0041] Switching the charging mode based on the holding time of the charge-oriented mode allows for easier determination of the state of charge (SOC) while ensuring accuracy. As described above, charging can be continuous in charge-oriented mode, allowing the auxiliary battery's SOC to gradually increase over time. Accordingly, once a time sufficient to exceed a predetermined SOC—that is, once the first SOC criterion has been met—charging efficiency can be improved by switching to the low-voltage charging mode, resulting in improved vehicle fuel efficiency. Even the maximum holding time can be adjusted in this case, based on the monitored SOC of the auxiliary battery, in the same way as the initial minimum holding time described above. The maximum holding time can decrease as the auxiliary battery's SOC increases.

[0042] The second procedure is configured such that if the monitored SOC of the auxiliary battery is less than or equal to the first SOC criterion and greater than a second SOC criterion, which is preset to a value less than the first SOC criterion, a preset second minimum holding time will be maintained, and if the average charging current of the auxiliary battery during charging is greater than a charging current criterion, the controller may be configured to switch the vehicle's charging mode to the low-voltage charging mode.

[0043] The second method described above, which is more specific than the first, requires more information to change the mode but can ensure greater reliability in terms of accuracy. This method takes into account the state of charge (SOC), holding time, and average charging current of the auxiliary battery. Since the average charging current is closely related to temperature, similar to voltage setting (S300), this method can also use the SOC and temperature as determining criteria.

[0044] In this method, the value of the second SOC criterion is smaller than that of the first SOC criterion. Accordingly, if the first SOC criterion is approximately 90%, as described above in the exemplary embodiment, the second SOC criterion can be set to approximately 85%. However, this value can be freely changed as the designer deems necessary. The charging current criterion can also have different values ​​as the designer deems necessary or depending on the vehicle type. However, assuming a typical battery voltage, the charging current criterion can preferably be set to approximately 20 A. Additionally, the average charging current of the auxiliary battery can represent the charging current used to charge the auxiliary battery during a second minimum holding period.

[0045] The second minimum hold time can be freely set based on a selection by the designer, similar to the first minimum hold time, and can be changed based on the monitored state of charge (SOC) of the auxiliary battery. However, the second minimum hold time can be set so that as the SOC of the auxiliary battery increases, the second minimum hold time can decrease, similar to the maximum hold time.

[0046] As described above, when the present invention is used, the following advantages can be obtained.

[0047] First, when the auxiliary battery's state of charge (SOC) is essentially high, charge control is not performed for a long period of time, thereby increasing the charging efficiency of the auxiliary battery and improving the fuel efficiency of the electric vehicle.

[0048] Secondly, charging efficiency can be improved by adjusting the charging voltage based on the SOC and temperature of the auxiliary battery.

[0049] Although the exemplary embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will recognize various modifications, additions and substitutions as possible without deviating from the scope of protection of the invention as disclosed in the attached claims.

Claims

A charging control procedure for an electric vehicle, comprising: monitoring, by a controller, the state of an auxiliary battery, including a state of charge (SOC), temperature, and charging current of the auxiliary battery; determining, by the controller, a charging mode of the electric vehicle based on the monitored state of the auxiliary battery; setting, by the controller, a charging voltage of the auxiliary battery based on the monitored SOC and temperature of the auxiliary battery; charging, by the controller, the auxiliary battery with the set charging voltage; and changing, by the controller, the charging mode of the electric vehicle based on the monitored state of the auxiliary battery, wherein the charging mode includes a charge-oriented mode for charging the auxiliary battery and a low-voltage charging mode for maintaining an auxiliary battery voltage.where determining the charging mode includes: if the monitored SOC of the auxiliary battery is less than or equal to a preset first SOC criterion, determining, by the controller, the charge-oriented mode of the electric vehicle as the charging mode; and where changing the charging mode includes: if the monitored SOC of the auxiliary battery is less than or equal to the first SOC criterion and greater than a second SOC criterion preset to a value less than that of the first SOC criterion, holding for a preset second minimum holding time; and if the average charging current of the auxiliary battery during charging is greater than a charging current criterion, changing, by the controller, the charging mode of the electric vehicle to a low-voltage charging mode. Charge control method according to claim 1, wherein the determination of the charge mode includes: when the monitored SOC of the auxiliary battery is greater than a preset first SOC criterion for a preset first minimum holding time, determining, by controlling, the low-voltage charge mode of the vehicle as the charge mode. Charge control method according to claim 2, wherein the minimum holding time is changed based on a value of the monitored SOC of the auxiliary battery. Charge control method according to claim 1, wherein changing the charge mode includes: when a holding time of the charge-oriented mode is greater than a maximum time, changing, by control, the charge mode of the electric vehicle to a low-voltage charge mode. Charge control method according to claim 4, wherein the maximum time is changed based on a value of the monitored SOC of the auxiliary battery. Charge control method according to claim 1, wherein the second minimum holding time is changed based on the value of the monitored SOC of the auxiliary battery. Charge control method according to claim 1, wherein the adjustment of the charge voltage includes the adjustment, by control, of the charge voltage using characteristic map data, in which the temperature and the SOC of the auxiliary battery, which are based on the specified charge mode, are used as inputs and the charge voltage is an output. A charging control system for an electric vehicle, comprising: a memory configured to store program instructions; and a processor configured to execute the program instructions, wherein the program instructions, upon execution, are configured to: monitor the state of an auxiliary battery, including the state of charge (SOC), temperature, and charging current of the auxiliary battery; determine a charging mode of the electric vehicle based on the monitored state of the auxiliary battery; set a charging voltage of the auxiliary battery based on the monitored SOC and temperature of the auxiliary battery; and charge the auxiliary battery at the set charging voltage, wherein the charging mode includes a charge-oriented mode for charging the auxiliary battery and a low-voltage charging mode for maintaining a voltage of the auxiliary battery.wherein the program instructions are further configured upon execution: to determine the charge-oriented mode of the electric vehicle as the charge mode if the monitored SOC of the auxiliary battery is less than or equal to a first preset SOC criterion; wherein the program instructions are further configured upon execution: to switch the charge mode of the electric vehicle based on the monitored state of the auxiliary battery after charging the auxiliary battery; and wherein the program instructions are further configured upon execution: to switch the charge mode of the electric vehicle to the low-voltage charge mode if the monitored SOC of the auxiliary battery is less than or equal to the first SOC criterion and greater than a second SOC criterion preset to a value smaller than that of the first SOC criterion, for a second preset minimum holding time.and if the average charging current of the auxiliary battery during charging is greater than a charging current criterion. Charge control system according to claim 8, wherein the program instructions are further configured during execution to determine the low-voltage charging mode of the vehicle as the charging mode when the monitored SOC of the auxiliary battery is greater than a preset first SOC criterion for a preset first minimum holding time. Charge control system according to claim 9, wherein the first minimum holding time is changed based on a value of the monitored SOC of the auxiliary battery. Charge control system according to claim 8, wherein the program instructions are further configured during execution to: switch the charging mode of the electric vehicle to the low-voltage charging mode if a holding time of the charge-oriented mode is greater than a maximum time. Charge control system according to claim 11, wherein the maximum time is changed based on a value of the monitored SOC of the auxiliary battery.

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