10 caused by a decrease in the calculation accuracy of the control SOC. In this way, the display SOC calculation process is useful for a fail-safe function for preventing power shortage of the vehicle 10.

The index is preferably calculated according to the elapsed time since the control SOC was previously corrected using the map 400. In this case, the index value is calculated by ECU 160 as a percentage [%] of the multiplication value of the length of the elapsed time and the largest value of the offset error of the current sensor 107 with respect to the full charge capacity of the battery 105. This value is calculated as indicating the largest value of the error resulting from the calculation of the control SOC using the current sensor 107. The largest value of the offset error is determined in advance depending on the current sensor 107, and is stored in the memories of ECU 160. The index value exceeding the threshold value corresponds to the elapsed time exceeding a predetermined threshold time as the threshold value.

As the elapsed time is longer, the effect of the integrated value of the detected error on the calculation result of the control SOC is larger, so that the calculation accuracy of the control SOC is more likely to be lowered. Therefore, this elapsed time can appropriately reflect the degree of decrease in the calculation accuracy of the control SOC. With the above configuration, the index value is determined according to the elapsed time (for example, the longer the elapsed time, the larger the index value). Accordingly, ECU 160 can appropriately determine a decrease in the calculation accuracy of the control SOC according to the index.

FIG. 3 is a diagram illustrating an exemplary transition of the control SOC and the display SOC. Referring to FIG. 3, a line 605 represents a transition of the control SOC. A line 610 represents a transition of the display SOC.

In the time t0, it is assumed that the control SOC is X0 (>Xb) and is corrected using the map 400. X0 is, for example, 100%. During the period from the time to to the time t1, since the elapsed time from the time to does not exceed the threshold time THT, the index value does not exceed the threshold value.

In the time t1, the elapsed time from the time to exceeds the threshold time THT. As a result, since the index value exceeds the threshold value, ECU 160 starts the display SOC calculation process. After the time t1, ECU 160 calculates the display SOC while continuously calculating the control SOC.

ECU 160 preferably calculates the display SOC such that the magnitude of the difference dif between the control SOC and the display SOC increases as the elapsed time from the time to increases. In this embodiment, the magnitude of the difference dif at the time tb (db) is larger than the magnitude of the difference dif at the time ta (da).

For example, ECU 160 calculates the display SOC such that the magnitude of the difference dif is determined by multiplying the offset error of the current sensor 107 by the length of the elapsed time. ECU 160 may calculate the display SOC such that the magnitude of the difference dif is determined by multiplying the offset error by the length of the elapsed time from the time t1 (the elapsed time from the time t0 minus the threshold time THT). Accordingly, it is possible to avoid a situation in which SOC displayed on the display device suddenly decreases when the index value exceeds the threshold value. The magnitude of the difference dif may be determined in accordance with the control SOC or may be determined in accordance with the integrated value of the detected value of the current sensor 107.

When the display SOC is calculated as described above, as indicated by the line 605,610, the display SOC becomes lower with respect to the control SOC as the elapsed time from the time to becomes longer (as the calculation accuracy of the control SOC decreases). Thus, even when the calculation accuracy of the control SOC is greatly reduced after a long period of time has elapsed since the time to, the display SOC is surely displayed lower than the actual SOC of the battery 105. As a result, the user of the vehicle 10 can be motivated more effectively to perform external charging earlier than usual. Therefore, it is possible to reliably prevent the electric power shortage of the vehicle 10 caused by the decrease in the calculation accuracy of the control SOC.

FIG. 4 is a flow chart illustrating an exemplary process executed by ECU 160 in the first embodiment. This flow chart is started when the correction of the control SOC is completed.

Referring to FIG. 4, ECU 160 calculates the index according to the elapsed time from the time when the control SOC is corrected last time (e.g., time t0) (S105).

ECU 160 determines whether the index value exceeds a threshold (S110). If the index value has not yet exceeded the threshold (NO in S110), the process returns to S105. When the index value exceeds the threshold value (YES in S110), ECU 160 determines that the calculation accuracy of the control SOC has decreased (S112), and starts the display SOC calculation process (S115). Thereafter, the process ends.

As described above, according to the first embodiment, SOC calculated by ECU 160 according to the detection value of the current sensor 107 includes the control SOC and the display SOC. When the index value exceeds the threshold value, ECU 160 starts a display SOC calculation process of calculating the display SOC lower than the control SOC. Accordingly, when the calculation accuracy of the control SOC is deteriorated, the user is motivated to perform the external charge earlier than normal. Therefore, it is possible to prevent unexpected power shortage of the vehicle 10 caused by a decrease in the calculation accuracy of the control SOC.

First Modification Example of First Embodiment

If the index value exceeds the threshold (e.g., at time t1 of FIG. 3), ECU 160 may perform a screen display process for displaying a screen on the display device that prompts the user to fully charge the battery 105.

With such a configuration, the user is prompted to perform external charging so as to fully charge the battery 105. When the battery 105 is fully charged, the actual SOC is 100%, and the calculated control SOC is basically equal to or larger than Xb (FIG. 2), so that the control SOC is corrected by ECU 160. Thus, even if the calculation accuracy of the control SOC is deteriorated, the process returns to the original state. As a consequence, it is possible to more effectively prevent unexpected power shortage of the vehicle 10 caused by a decrease in the calculation accuracy of the control SOC.

FIGS. 5A and 5B are diagrams illustrating exemplary screens displayed on a display device. In the example of FIG. 5A, the display device is an instrument panel 140. The screen display process corresponds to ECU 160 controlling the instrument panel 140 to display the screen 500. The screen 500 includes a message