Patent Publication Number: US-2023144462-A1

Title: Battery bank unit, remaining charge time calculation method, and remaining charge time calculation program

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is entitled to or claims the benefit of Japanese Patent Application No. 2021-181218, filed on Nov. 5, 2021, the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a battery bank unit, a remaining charge time calculation method, and a remaining charge time calculation program. 
     BACKGROUND ART 
     Patent Literature 1 discloses a battery bank unit that discharges electricity to a load apparatus connected to an external power source when the external power source is unable to supply power due to a power outage. The battery bank unit includes a plurality of battery banks. The plurality of battery banks are each composed of a plurality of secondary batteries, and are connected in parallel to each other. The plurality of battery banks are charged by electric power from the external power source under ordinary circumstances. 
     The battery bank unit is configured so that the plurality of battery banks are switched in turn to be charged and a battery bank that is not being charged can discharge electricity to the load apparatus. This allows the battery bank unit to discharge electricity to the load apparatus even while the battery bank unit is charging. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1 
         Japanese Patent Application Laid-Open No. 2016-10250 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     For the load apparatus management, for example, there is a need to know how long it takes to complete the charge of a battery bank unit while the battery bank unit is charging. 
     The objective of the present disclosure is to provide a battery bank unit capable of accurately calculating time required to complete charging. 
     Solution to Problem 
     A battery bank unit according to the present disclosure includes: a first battery bank and a second battery bank that are connected in parallel to each other; and a control apparatus that performs collective charge for collectively charging the first battery bank and the second battery bank, performs first bank charge for charging only the first battery bank after the collective charge, and performs second bank charge for charging only the second battery bank after the first bank charge, wherein, the control apparatus calculates remaining time to complete charge of the battery bank unit based on a temperature of the battery bank unit at a start of the collective charge. 
     A remaining charge time calculation method according to the present disclosure is a method for a computer to calculate remaining time to complete charge of a battery bank unit, the charge including collective charge for collectively charging a first battery bank and a second battery bank connected in parallel to the first battery bank, first bank charge for charging only the first battery bank after the collective charge, and second bank charge for charging only the second battery bank after the first bank charge, the method including: acquiring a temperature of the battery bank unit at a start of the collective charge; and calculating the remaining time based on the temperature of the battery bank unit. 
     A remaining charge time calculation program according to the present disclosure is a program stored in a non-transitory storage medium, wherein, when the program is executed by a computer that controls a battery bank unit for which collective charge for collectively charging a first battery bank and a second battery bank connected in parallel to the first battery bank is performed, first bank charge for charging only the first battery bank is performed after the collective charge, and second bank charge for charging only the second battery bank is performed after the first bank charge, the program is configured to cause the computer to perform operations including: acquiring a temperature of the battery bank unit at a start of the collective charge; and calculating remaining time to complete charge of the battery bank unit based on the temperature of the battery bank unit. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide a battery bank unit capable of accurately calculating time required to complete charging. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram illustrating a battery bank unit in Embodiment 1 of the present disclosure; 
         FIG.  2    is a block diagram of the battery bank unit; 
         FIG.  3    illustrates a table; 
         FIG.  4    is a flowchart illustrating a procedure performed by a control apparatus to charge the battery bank unit; 
         FIG.  5    is a timing chart for the procedure in the flowchart in  FIG.  4   ; 
         FIG.  6 A  is a flowchart illustrating a procedure performed by the control apparatus to calculate remaining time; 
         FIG.  6 B  is a flowchart following the procedure in  FIG.  6 A ; 
         FIG.  7 A  is a flowchart illustrating a procedure performed by the control apparatus to calculate remaining time according to Embodiment 2 of the present disclosure; and 
         FIG.  7 B  is a flowchart following the procedure in  FIG.  7 A . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Embodiment 1&gt; 
     Hereinafter, a battery bank unit according to Embodiment 1 of the present disclosure will be described with reference to the accompanying drawings.  FIG.  1    is a schematic diagram illustrating battery bank unit  1  in Embodiment 1 of the present disclosure. Battery bank unit  1  supplies power to load apparatus  3  connected to external power source  2  when external power source  2  loses power. Battery bank unit  1  is charged by power from external power source  2 . 
     External power source  2  is, for example, an apparatus that converts commercial AC power into DC power and outputs the DC power. Load apparatus  3  is an apparatus (e.g., server apparatus) that operates with DC power. 
     As illustrated in  FIG.  1   , battery bank unit  1  includes input/output terminal  10 , first and second battery banks  20  and  30 , charge/discharge circuitry  40 , and control apparatus  50 . 
     Input/output terminal  10  is connected to power line  4  that supplies power to load apparatus  3  from external power source  2 . 
     First and second battery banks  20  and  30  are composed of a plurality of secondary batteries (e.g., nickel-hydrogen secondary batteries) connected in series, for example. Note that the secondary battery is not necessarily a nickel-hydrogen secondary battery, and may be another secondary battery such as a lithium-ion secondary battery. The configurations of first and second battery banks  20  and  30  are similar to each other. First and second battery banks  20  and  30  are connected in parallel to each other. 
     Charge/discharge circuitry  40  functions as circuitry that performs charge and discharge of first and second battery banks  20  and  30  via input/output terminal  10 . Charge/discharge circuitry  40  includes boost DC/DC converter  41 , switch  42 , first charge switch  43 , first discharge switch  44 , second charge switch  45 , and second discharge switch  46 . 
     Boost DC/DC converter  41  is a power conversion apparatus that boosts power supplied from external power source  2  and outputs the boosted power. 
     Switch  42  switches a value of voltage applied to first and second battery banks  20  and  30 . In switch  42 , first terminal  42   a  is connected to an output terminal of boost DC/DC converter  41 , and second terminal  42   b  is connected to input/output terminal  10 . Additionally, third terminal  42   c  is connected to first and second battery banks  20  and  30  via first and second charge switches  43  and  45 . 
     When switch  42  is in the on state, first terminal  42   a  and third terminal  42   c  are connected to each other, and the power outputted from boost DC/DC converter  41  is supplied to first and second battery banks  20  and  30  via first and second charge switches  43  and  45 . In contrast, when switch  42  is in the off state, second terminal  42   b  and third terminal  42   c  are connected to each other, and the power outputted from external power source  2  is supplied to first and second battery banks  20  and  30  via first and second charge switches  43  and  45 . 
     First charge switch  43  allows first battery bank  20  to be charged when in the on state, and does not allow first battery bank  20  to be charged when in the off state. In first charge switch  43 , first terminal  43   a  is connected to third terminal  42   c  of switch  42  and second terminal  43   b  is connected to the positive electrode of first battery bank  20 . Note that the negative electrode of first battery bank  20  is connected to the ground. 
     First discharge switch  44  allows first battery bank  20  to discharge when in the on state, and does not allow first battery bank  20  to discharge when in the off state. In first discharge switch  44 , first terminal  44   a  is connected to the positive electrode of first battery bank  20  and second terminal  44   b  is connected to input/output terminal  10 . 
     Second charge switch  45  allows second battery bank  30  to be charged when in the on state, and does not allow second battery bank  30  to be charged when in the off state. In second charge switch  45 , first terminal  45   a  is connected to third terminal  42   c  of switch  42  and second terminal  45   b  is connected to the positive electrode of second battery bank  30 . Note that the negative electrode of second battery bank  30  is connected to the ground. 
     Second discharge switch  46  allows second battery bank  30  to discharge when in the on state, and does not allow second battery bank  30  to discharge when in the off state. In second discharge switch  46 , first terminal  46 a is connected to the positive electrode of second battery bank  30  and second terminal  46 b is connected to input/output terminal  10 . 
       FIG.  2    is a block diagram of battery bank unit  1 . As illustrated in  FIG.  2   , battery bank unit  1  further includes current sensor  60 , first voltage sensor  61 , first temperature sensor  62 , second voltage sensor  63 , and second temperature sensor  64 . 
     Current sensor  60  detects a value of current flowing in or out of power line  4  via input/output terminal  10 . To be more specific, current sensor  60  detects a value of current between input/output terminal  10  and connecting point  40 a of charge/discharge circuitry  40 . First voltage sensor  61  detects a voltage value of first battery bank  20 . First temperature sensor  62  detects the temperature of first battery bank  20 . 
     Second voltage sensor  63  detects a voltage value of second battery bank  30 . Second temperature sensor  64  detects the temperature of second battery bank  30 . Current sensor  60 , first voltage sensor  61 , first temperature sensor  62 , second voltage sensor  63 , and second temperature sensor  64  each transmit the detected value to control apparatus  50 . 
     Battery bank unit  1  further includes a third voltage sensor (not illustrated) that detects a power source voltage value that is a voltage value of external power source  2 . Control apparatus  50  detects a power outage of external power source  2  based on the power source voltage value detected by the third voltage sensor. 
     Control apparatus  50  controls the charge/discharge of battery bank unit  1  by controlling the states of switches  42  to  46 . Control apparatus  50  includes storage  51 . Storage  51  stores table T illustrated in  FIG.  3   . 
     Table T is a table that is referred to when control apparatus  50  calculates remaining time that is the time required to complete the charge of battery bank unit  1  to be described later. In table T, the temperature, collective charge time, first bank charge time, second bank charge time, and voltage drop amount are associated with each other. In table T, the temperature is divided into eight temperature zones in total, and between 0° C. and 60° C., there are six zones each including a range of 10° C. Needless to say, the temperature range in each temperature zone and the number of temperature zones are not limited to those illustrated in  FIG.  3   . The collective charge time, first bank charge time, second bank charge time, and voltage drop amount will be described later in detail. 
     Control apparatus  50  also calculates the state of charge (SOC) of battery bank unit  1  by a known method based on the current value detected by current sensor  60 . The SOC of battery bank unit  1  is a charge rate (%) corresponding to the sum of the charge amounts of first and second battery banks  20  and  30 . 
     Next, charge control for battery bank unit  1  performed by control apparatus  50  will be described with reference to the flowchart in  FIG.  4    and the timing chart in  FIG.  5   . 
     In a state where the charge control is not started, switch  42  and first and second charge switches  43  and  45  are all in the off state and first and second discharge switches  44  and  46  are both in the on state: accordingly, the discharge of first and second battery banks  20  and  30  are allowed. As described above, the configurations of first and second battery banks  20  and  30  are similar to each other, and they are connected in parallel. Thus, the voltage values and charge amounts of first and second battery banks  20  and  30  are approximately equal to each other. That is, the SOC of battery bank unit  1  is approximately equal to each of the SOCs of first and second battery banks  20  and  30 . 
     Control apparatus  50  starts the charge control when detecting connection to external power source  2  or detecting the end of the power outage of external power source  2  based on the detection value of the third voltage sensor. 
     Control apparatus  50  starts collective charge processing in  51 . The collective charge processing is processing of charging first and second battery banks  20  and  30  collectively. To be more specific, as illustrated in  FIG.  5   , control apparatus  50  switches switch  42  and first and second charge switches  43  and  45  to the on state (time t 0 ) from the state where switch  42  and first and second charge switches  43  and  45  are all in the off state and first and second discharge switches  44  and  46  are both in the on state. 
     First and second discharge switches  44  and  46  remain the on state. This allows battery bank unit  1  to discharge to load apparatus  3  even when external power source  2  loses power during the collective charge processing. 
     When the collective charge processing is started (time t 0 ), power is supplied from boost DC/DC converter  41  to first and second battery banks  20  and  30 , and the voltage values of first and second battery banks  20  and  30  increase. 
     In  FIG.  5   , the solid-line voltage value indicates the voltage value of first battery bank  20 , and the chain-line voltage value indicates the voltage value of second battery bank  30 . The voltage values of first and second battery banks  20  and  30  are approximately equal before the start of the collective charge processing and during the collective charge processing. Thus, the lines indicating the voltage values of first and second battery banks  20  and  30  are overlapped with each other, resulting in the solid line. 
     Next, in S 2 , control apparatus  50  determines whether the bank voltage value, which is the voltage value of battery bank unit  1 , is equal to or greater than the power source voltage value. The bank voltage value is specifically a mean value of the voltage value of first battery bank  20  and the voltage value of second battery bank  30 . Note that the bank voltage value may be either one of the voltage values of first and second battery banks  20  and  30 . When the bank voltage value is lower than the power source voltage value (NO in S 2 ), the collective charge processing is continued. 
     Meanwhile, when the voltage values of first and second battery banks  20  and  30  increase and the bank voltage value becomes equal to or higher than the power source voltage value (time t 1 ; YES in S 2 ), control apparatus  50  ends the collective charge processing and starts first bank charge processing in S 3 . 
     The first bank charge processing is processing of charging only first battery bank  20 . In the first bank charge processing, first battery bank  20  is fully charged at a voltage value higher than the power source voltage value. Second battery bank  30  is not charged in the first bank charge processing. 
     To be more specific, control apparatus  50  switches second charge switch  45  to the off state and first discharge switch  44  to the off state (time t 1 ). As a result, the power of boost DC/DC converter  41  is supplied only to first battery bank  20 , and the voltage value of first battery bank  20  further increases from the power source voltage value. In the first bank charge processing, first discharge switch  44  is in the off state and first battery bank  20  does not discharge. This makes it possible to prevent application of a voltage value higher than the power source voltage value to load apparatus  3 , thereby preventing failure of load apparatus  3 , for example. 
     Meanwhile, the charge of second battery bank  30  is stopped, and the voltage value of second battery bank  30  gradually decreases due to self-discharge. Second discharge switch  46  is in the on state in first bank charge processing. Thus, second battery bank  30  can discharge to load apparatus  3  even when external power source  2  loses power during the first bank charge processing. 
     Subsequently, control apparatus  50  determines whether first battery bank  20  is fully charged in S 4 . To be more specific, control apparatus  50  determines whether the detection value of first temperature sensor  62  has reached a predetermined first temperature. The first temperature is a temperature at which first battery bank  20  is fully charged. When the detection value of first temperature sensor  62  is lower than the first temperature (NO in S 4 ), control apparatus  50  continues to charge first battery bank  20  only. 
     In contrast, when first battery bank  20  is fully charged and the detection value of first temperature sensor  62  reaches the first temperature (time t 2 ; YES in S 4 ), control apparatus  50  stops charging first battery bank  20  in S 5 . 
     To be more specific, control apparatus  50  switches first charge switch  43  to the off state (time t 2 ). As a result, the charge of first battery bank  20  is stopped, and the voltage value of first battery bank  20  gradually decreases due to self-discharge. At this time, the temperature of first battery bank  20  is higher than the temperature of second battery bank  30 . Thus, the drop amount of the voltage value of first battery bank  20  per unit time is larger than the drop amount of the voltage value of second battery bank  30  per unit time. 
     Next, in S 6 , control apparatus  50  determines whether the voltage value of first battery bank  20  is equal to or lower than the power source voltage value. When the voltage value of first battery bank  20  is higher than the power source voltage value (NO in S 6 ), control apparatus  50  continues the state where first and second battery banks  20  and  30  are not charged. 
     When the voltage value of first battery bank  20  is equal to or lower than the power source voltage value (time t 3 ; YES in S 6 ), in contrast, control apparatus  50  ends the first bank charge processing and starts second bank charge processing in S 7 . 
     The second bank charge processing is processing of charging only second battery bank  30 . In the second bank charge processing, second battery bank  30  is fully charged at a voltage value higher than the power source voltage value. First battery bank  20  is not charged in the second bank charge processing. 
     To be more specific, control apparatus  50  switches second charge switch  45  to the on state, first discharge switch  44  to the on state, and second discharge switch  46  to the off state (time t 3 ). As a result, power is supplied from boost DC/DC converter  41  to second battery bank  30  only, and the voltage value of second battery bank  30  increases and exceeds the power source voltage value. In the second bank charge processing, second discharge switch  46  is in the off state and second battery bank  30  does not discharge. This makes it possible to prevent application of a voltage value higher than the power source voltage value to load apparatus  3 , thereby preventing failure of load apparatus  3 , for example. 
     Meanwhile, the charge of first battery bank  20  remains stopped, and the voltage value of first battery bank  20  gradually decreases due to self-discharge. First discharge switch  44  is in the on state in second bank charge processing. Thus, first battery bank  20  can discharge to load apparatus  3  even when external power source  2  loses power during the second bank charge processing. 
     Then, control apparatus  50  determines whether second battery bank  30  is fully charged in S 8 . To be more specific, control apparatus  50  determines whether the detection value of second temperature sensor  64  has reached a predetermined second temperature. The second temperature is the temperature at which second battery bank  30  is fully charged. When the detection value of second temperature sensor  64  is lower than the second temperature (NO in S 8 ), control apparatus  50  continues to charge second battery bank  30  only. Note that the second temperature may be the same as the first temperature, which is the temperature at which first battery bank  20  is fully charged. 
     In contrast, when second battery bank  30  is fully charged and the detection value of second temperature sensor  64  reaches the second temperature (time t 4 ; YES in S 8 ), control apparatus  50  stops charging second battery bank  30  in S 9 . 
     To be more specific, control apparatus  50  switches second charge switch  45  to the off state (time t 4 ). Accordingly, the charge of second battery bank  30  is stopped, and the voltage of second battery bank  30  gradually decreases due to self-discharge. At this time, the temperature of second battery bank  30  is higher than the temperature of first battery bank  20 . Thus, the drop amount of the voltage value of second battery bank  30  per unit time is larger than the drop amount of the voltage value of first battery bank  20  per unit time. 
     Next, in S 10 , control apparatus  50  determines whether the voltage value of second battery bank  30  is equal to or lower than the power source voltage value. When the voltage value of second battery bank  30  is higher than the power source voltage value (NO in S 10 ), control apparatus  50  continues the state where first and second battery banks  20  and  30  are not charged. 
     When the voltage value of second battery bank  30  is equal to or lower than the power source voltage value (time t 5 ; YES in S 10 ), in contrast, control apparatus  50  ends the second bank charge processing in S 11 . To be more specific, control apparatus  50  switches switch  42  to the off state and second discharge switch  46  to the on state (time t 5 ). This is the end of the charge of battery bank unit  1 . Control apparatus  50  specifies the SOC of battery bank unit  1  at the end of the charge of battery bank unit  1  as 100%. 
     Note that battery bank unit  1  may include three or more battery banks. In a case of including m battery banks, the m battery banks are collectively charged in the collective charge processing. When the collective charge processing is finished, m battery banks are charged one by one in turn as is the case with the above first and second bank charge processing. 
     Next, control for calculating remaining time performed by control apparatus  50  will be described with reference to the flowcharts in  FIGS.  6 A and  6 B . The remaining time is time required to complete the charge of battery bank unit  1 . Control apparatus  50  calculates the remaining time while performing the charge control described above. 
     In S 20 , control apparatus  50  acquires, from table T, collective charge time, first bank charge time, and second bank charge time that are associated with the temperature at the start of the collective charge processing for battery bank unit  1 . The temperature of battery bank unit  1  is, for example, the mean temperature of first and second battery banks  20  and  30 . Note that the temperature of battery bank unit  1  may be either one of the temperatures of first and second battery banks  20  and  30 . 
     The collective charge time corresponds to the time required from the start (time tO in  FIG.  5   ) to the end (time tl in  FIG.  5   ) of collective charge processing in a case where the SOC of battery bank unit  1  at the start of the collective charge processing is a first predetermined charge rate (e.g., 0%). The first predetermined charge rate is any value determined in, for example, an experiment performed to determine the collective charge time stored in table T in advance. 
     The first bank charge time corresponds to the time required from the start (time t 1 ) to the end (time t 3 ) of first bank charge processing. The second bank charge time corresponds to the time required from the start (time t 3 ) to the end (time t 5 ) of second bank charge processing. The collective charge time, first bank charge time, and second bank charge time are determined for each temperature zone by actual measurement performed in advance through experiments, for example, and are stored in table T. 
     In a case where the temperature of battery bank unit  1  is 25° C. at the start of the charge of battery bank unit  1 , for example, control apparatus  50  acquires the collective charge time “A 3 ”, first bank charge time “B 3 ”, and second bank charge time “C 3 ” that are associated with the temperature “20° C. or higher and lower than 30° C.” from table T in  FIG.  3   . 
     Subsequently, control apparatus  50  calculates the remaining time in S 21 . To be more specific, control apparatus  50  calculates the remaining time at the start of charge, which is the remaining time at the time of starting the charge, using Expression 1. 
       [1] 
       Remaining time at the start of charge= Tm 0+ Tm 1+ Tm 2   (Expression 1)
 
     In Expression 1, Tm 0 , Tm 1 , and Tm 2  respectively represent the collective charge time, first bank charge time, and second bank charge time acquired from table T. 
     Control apparatus  50  also indicates the calculated remaining time at the start of charge to load apparatus  3 . Load apparatus  3  displays the remaining time at the start of charge on a display section such as a display. This allows an administrator of load apparatus  3  to recognize the time from the start to the completion of the charge of battery bank unit  1 . 
     Further, control apparatus  50  measures the time elapsed from the start of the charge of battery bank unit  1 . Control apparatus  50  subtracts the elapsed time from the remaining time at the start of charge calculated in S 21  at predetermined time intervals to calculate the remaining time at that time, updates the remaining time to the latest, and indicates the updated remaining time to load apparatus  3 . Load apparatus  3  displays the updated remaining time on the display section. 
     Next, in S 22 , control apparatus  50  determines whether the collective charge processing has ended. When the collective charge processing is in progress (NO in S 22 ), control apparatus  50  continues updating and indicating the remaining time. 
     When the collective charge processing has ended (YES in S 22 ), control apparatus  50  corrects the remaining time in S 23 . To be more specific, control apparatus  50  corrects the remaining time based on the actual collective charge time, which is the time actually spent for the collective charge processing. Control apparatus  50  subtracts the actual collective charge time from the collective charge time acquired from table T, and adds the calculated value to the remaining time at the end of the collective charge processing. 
     That is, control apparatus  50  corrects the remaining time by using the difference between the experimental value (collective charge time) and the measured value (actual collective charge time) of the time required for the collective charge processing. This allows control apparatus  50  to accurately correct the remaining time at the end of collective charge. Note that control apparatus  50  need not correct the remaining time in S 23 . 
     Then, control apparatus  50  updates the collective charge time in S 24 . To be more specific, control apparatus  50  updates the collective charge time associated with the temperature of battery bank unit  1  at the start of the charge with the actual collective charge time in table T. For example, in the case where the temperature of battery bank unit  1  is 25° C. at the start of the charge of battery bank unit  1 , control apparatus  50  updates the collective charge time “A 3 ” associated with the temperature “20° C. or higher and lower than 30° C.” in table T with the actual collective charge time. 
     Next, in S 25 , control apparatus  50  determines whether the charge of first battery bank  20  is stopped during the first bank charge processing. When first battery bank  20  is not fully charged and the charge of first battery bank  20  is not stopped (NO in S 25 ), control apparatus  50  continues updating and indicating the remaining time. 
     Meanwhile, when first battery bank  20  is fully charged and the charge of first battery bank  20  is stopped (YES in S 25 ), control apparatus  50  determines, in S 26 , whether the temperature of first battery bank  20  is equal to or lower than a first determination temperature. The first determination temperature is a temperature of first battery bank  20  when the drop amount of the voltage value of first battery bank  20  per unit time is almost constant. When the charge of first battery bank  20  is stopped (time t 2 ), the temperature of first battery bank  20  is relatively high and the drop amount of the voltage value of first battery bank  20  per unit time is relatively large. After that, as the voltage value of first battery bank  20  decreases due to self-discharge, the drop amount of the voltage value of first battery bank  20  per unit time becomes smaller and almost constant at and after point P (time t 10 ) in  FIG.  5   . That is, the first determination temperature is specified to be the temperature of first battery bank  20  at point P. The first determination temperature is specified by actual measurement performed in advance through experiments or the like. 
     Note that control apparatus  50  may calculate the first determination temperature. To be more specific, control apparatus  50  calculates the first determination temperature by adding the first predetermined temperature to the temperature of first battery bank  20  at the start of the collective charge processing. The first predetermined temperature is determined in advance by experimentally measuring the relation between the temperature of first battery bank  20  at the start of the collective charge processing and the temperature of first battery bank  20  at which the drop amount of the voltage value of first battery bank  20  per unit time is almost constant. Note that the first determination temperature may be a temperature obtained by adding the first predetermined temperature to the temperature of first battery bank  20  at the start of the first bank charge processing. 
     When the temperature of first battery bank  20  is higher than the first determination temperature (NO in S 26 ), control apparatus  50  continues updating and indicating the remaining time. When the temperature of first battery bank  20  is decreased and becomes equal to or lower than the first determination temperature (YES in S 26 ), control apparatus  50  acquires the voltage drop amount from table T in S 27 . Specifically, control apparatus  50  acquires, from table T, the voltage drop amount associated with the temperature of first battery bank  20  at that time. 
     For example, when the temperature of first battery bank  20  is 45° C. at that time, control apparatus  50  acquires the voltage drop amount “V 5 ” associated with the temperature “40° C. or higher and lower than 50° C.” from table T in  FIG.  3   . The voltage drop amount is a drop amount of the voltage value of first battery bank  20  per unit time after first battery bank  20  is fully charged and stops charging. The voltage drop amount is determined for each temperature zone by actual measurement performed in advance through experiments, for example, and is stored in table T. 
     Subsequently, control apparatus  50  corrects the remaining time in S 28 . First, control apparatus  50  calculates, based on the voltage drop amount acquired from table T, a first charge stop time, which is the time required from when the temperature of first battery bank  20  becomes equal to or lower than the first determination temperature to when the second bank charge processing is started (time t 3 ). To be more specific, control apparatus  50  calculates the first charge stop time by subtracting the power source voltage value from the voltage value of first battery bank  20  at the time when the temperature of first battery bank  20  becomes equal to or lower than the first determination temperature, and dividing the subtracted value by the voltage drop amount acquired from table T. 
     Control apparatus  50  corrects the remaining time based on the calculated first charge stop time. Specifically, control apparatus  50  corrects the remaining time by replacing the remaining time at the time when the temperature of first battery bank  20  becomes equal to or lower than the first determination temperature with the sum of the calculated first charge stop time and the second bank charge time acquired in S 20 . After the temperature of first battery bank  20  becomes equal to or lower than the first determination temperature, control apparatus  50  subtracts the time elapsed from when the temperature of first battery bank  20  becomes equal to or lower than the first determination temperature from the corrected remaining time at predetermined time intervals to calculate the remaining time at that time and update the remaining time to the latest. Control apparatus  50  then indicates the updated remaining time to load apparatus  3 . This allows control apparatus  50  to accurately correct the remaining time while the charge of first battery bank  20  is stopped during the first bank charge processing. 
     Next, in S 29 , control apparatus  50  determines whether the second bank charge processing has started. When the second bank charge processing has not started (NO in S 29 ), control apparatus  50  continues updating and indicating the remaining time. 
     When second bank charge processing has started (YES in S 29 ), in contrast, control apparatus  50  corrects the remaining time in S 30 . Specifically, control apparatus  50  replaces the remaining time at time t 3  (at the start of the second bank charge processing) with the actual first bank charge time that is the time actually spent for the first bank charge processing. When the remaining time at time t 3  is corrected, control apparatus  50  updates the latest remaining time as follows. That is, control apparatus  50  subtracts the time elapsed from time t 3  from the actual first bank charge time at predetermined time intervals after time t 3  to calculate the remaining time at that time and update the remaining time to the latest. Control apparatus  50  then indicates the updated remaining time to load apparatus  3 . 
     The configurations of first and second battery banks  20  and  30  are similar to each other as described above, and the surroundings (e.g., temperature and humidity) of first and second battery banks  20  and  30  are almost the same. Accordingly, the actual duration of the second bank charge processing is approximately equal to the actual first bank charge time. That is, when the first bank charge processing is finished, control apparatus  50  can accurately correct the remaining time by replacing the remaining time with the actual first bank charge time, and indicate the corrected remaining time. Note that control apparatus  50  need not correct the remaining time in S 30 . 
     Next, in S 31 , control apparatus  50  determines whether the charge of second battery bank  30  is stopped during the second bank charge processing. When second battery bank  30  is not fully charged and the charge of second battery bank  30  is not stopped (NO in S 31 ), control apparatus  50  continues updating and indicating the remaining time. 
     Meanwhile, when second battery bank  30  is fully charged and the charge of second battery bank  30  is stopped (YES in S 31 ), control apparatus  50  determines, in S 32 , whether the temperature of second battery bank  30  is equal to or lower than a second determination temperature. The second determination temperature is a temperature of second battery bank  30  when the drop amount of the voltage value of second battery bank  30  per unit time is almost constant. When the charge of second battery bank  30  is stopped (time t 4 ), the temperature of second battery bank  30  is relatively high and the drop amount of the voltage value of second battery bank  30  per unit time is relatively large. After that, as the voltage value of second battery bank  30  decreases due to self-discharge, the drop amount of the voltage value of second battery bank  30  per unit time becomes smaller and almost constant at and after point Q (time t 11 ) in  FIG.  5   . That is, the second determination temperature is specified to be the temperature of second battery bank  30  at point Q. The second determination temperature is specified in advance by actual measurement through experiments or the like. Note that the second determination temperature may be the same as the first determination temperature in a case where the configurations of first and second battery banks  20  and  30  are similar to each other. 
     Note that control apparatus  50  may calculate the second determination temperature. To be more specific, control apparatus  50  calculates the second determination temperature by adding a second predetermined temperature to the temperature of second battery bank  30  at the start of the collective charge processing. The second predetermined temperature is determined in advance by experimentally measuring the relation between the temperature of second battery bank  30  at the start of the collective charge processing and the temperature of second battery bank  30  at which the drop amount of the voltage value of second battery bank  30  per unit time is almost constant. Note that the second determination temperature may be a temperature obtained by adding the second predetermined temperature to the temperature of second battery bank  30  at the start of the second bank charge processing. In addition, the second predetermined temperature may be the same as the first predetermined temperature in a case where the configurations of first and second battery banks  20  and  30  are similar to each other. 
     When the temperature of second battery bank  30  is higher than the second determination temperature (NO in S 32 ), control apparatus  50  continues updating and indicating the remaining time. When the temperature of second battery bank  30  is decreased and becomes equal to or lower than the second determination temperature (YES in S 32 ), control apparatus  50  acquires the voltage drop amount from table T in S 33 . Specifically, control apparatus  50  acquires, from table T, the voltage drop amount associated with the temperature of second battery bank  30  at that time. 
     For example, when the temperature of second battery bank  30  is 45° C. at that time, control apparatus  50  acquires the voltage drop amount “V 5 ” associated with the temperature “40° C. or higher and lower than 50° C.” from table T. 
     Subsequently, control apparatus  50  corrects the remaining time in S 34 . First, control apparatus  50  calculates, based on the voltage drop amount acquired from table T, a second charge stop time, which is the time required from when the temperature of second battery bank  30  becomes equal to or lower than the second determination temperature to when the second bank charge processing is finished (time t 5 ). To be more specific, control apparatus  50  calculates the second charge stop time by subtracting the power source voltage value from the voltage value of second battery bank  30  at the time when the temperature of second battery bank  30  becomes equal to or lower than the second determination temperature, and dividing the subtracted value by the voltage drop amount acquired from table T. 
     Control apparatus  50  corrects the remaining time based on the calculated second charge stop time. Specifically, control apparatus  50  corrects the remaining time by replacing the remaining time at the time when the temperature of the second battery bank  20  becomes equal to or lower than the second determination temperature with the calculated second charge stop time. After the temperature of second battery bank  30  becomes equal to or lower than the second determination temperature, control apparatus  50  subtracts the time elapsed from when the temperature of second battery bank  30  becomes equal to or lower than the second determination temperature from the corrected remaining time at predetermined time intervals to calculate the remaining time at that time and update the remaining time to the latest. Control apparatus  50  then indicates the updated remaining time to load apparatus  3 . This allows control apparatus  50  to accurately correct the remaining time while the charge of second battery bank  30  is stopped during the second bank charge processing. 
     Next, in S 35 , control apparatus  50  determines whether the second bank charge processing has ended. When the second bank charge processing has not ended (NO in S 35 ), control apparatus  50  continues updating and indicating the remaining time. 
     Meanwhile, when the second bank charge processing has ended (YES in S 35 ), control apparatus  50  updates the first and second bank charge times in S 36 . To be more specific, control apparatus  50  updates the first bank charge time associated with the temperature of battery bank unit  1  at the start of the charge with the actual first bank charge time in table T. For example, in the case where the temperature of battery bank unit  1  is 25° C. at the start of the charge of battery bank unit  1 , control apparatus  50  updates the first bank charge time “B 3 ” associated with the temperature “20° C. or higher and lower than 30° C.” in table T with the actual first bank charge time. 
     Additionally, in table T, control apparatus  50  updates the second bank charge time associated with the temperature of battery bank unit  1  at the start of the charge with the actual second bank charge time that is the time actually spent for the second bank charge processing. For example, in the case where the temperature of battery bank unit  1  is 25° C. at the start of the charge of battery bank unit  1 , control apparatus  50  updates the second bank charge time “C 3 ” associated with the temperature “20° C. or higher and lower than 30° C.” in table T with the actual second bank charge time. After updating the second bank charge time, control apparatus  50  ends the control for calculating the remaining time. 
     The actual collective charge time, actual first bank charge time, and actual second bank charge time vary depending on the surroundings of battery bank unit  1 , power source voltage value, temperatures of first and second battery banks  20  and  30 , aging of battery bank unit  1 , and degree of deterioration of first and second battery banks  20  and  30  (hereinafter, referred to as the surroundings of battery bank unit  1 , etc.). By updating table T with the actual collective charge time, actual first bank charge time, and actual second bank charge time, it is possible to make the values stored in table T match the surroundings of battery bank unit  1 , etc. Thus, control apparatus  50  updates table T so as to adapt to a change in the surroundings of battery bank unit  1 , etc. every time first and second battery banks  20  and  30  are charged, thereby accurately calculating the remaining time in performing the charge control. 
     Embodiment 2 
     Next, Embodiment 2 of the present disclosure will be described mainly about aspects different from the above Embodiment 1. Table T in Embodiment 2 has no voltage drop amount. In addition, the control for calculating the remaining time performed by control apparatus  50  in Embodiment 2 is different from that in the above Embodiment 1. In the following, the control for calculating the remaining time in Embodiment 2 will be described with reference to  FIGS.  6 A,  7 A, and  7 B . 
     When starting the control for calculating the remaining time, control apparatus  50  performs S 20  to S 25  in  FIG.  6 A  as in the above Embodiment 1. When the charge of first battery bank  20  is stopped (YES in S 25 ), control apparatus  50  calculates, in S 126  of  FIG.  7 A , a first actual drop amount that is the actual drop amount of the voltage value of first battery bank  20  per unit time at that time. 
     Then, in S 127 , control apparatus  50  determines whether the first actual drop amount is equal to or less than a first predetermined amount. The first predetermined amount is a value determined in advance by actual measurement through experiments or the like, and is determined to be the value of the first actual drop amount at point Pin  FIG.  5   . That is, the first predetermined amount is the value of the first actual drop amount when the first actual drop amount becomes smaller and almost constant after the charge of first battery bank  20  is stopped. 
     When the temperature of first battery bank  20  is relatively high and the first actual drop amount is greater than the first predetermined amount (NO in S 127 ), control apparatus  50  continues calculating the first actual drop amount (S 126 ) and updating and indicating the remaining time. In contrast, when the voltage value of first battery bank  20  and the temperature of first battery bank  20  decrease and the first actual drop amount becomes equal to or less than the first predetermined amount (YES in S 127 ), control apparatus  50  corrects the remaining time in S 128 . 
     To be more specific, control apparatus  50  calculates another first charge stop time, which is the time required from when the first actual drop amount becomes equal to or less than the first predetermined amount to when the second bank charge processing is started, and corrects the remaining time based on the calculated another first charge stop time. 
     Control apparatus  50  calculates the another first charge stop time by subtracting the power source voltage value from the voltage value of first battery bank  20  at the time when the first actual drop amount becomes equal to or less than the first predetermined amount, and dividing the subtracted value by the first actual drop amount at the time when the first actual drop amount becomes equal to or less than the first predetermined amount. 
     Further, control apparatus  50  corrects the remaining time by replacing the remaining time at the time when the first actual drop amount becomes equal to or less than the first predetermined amount with the sum of the calculated another first charge stop time and the second bank charge time acquired in S 20 . After the first actual drop amount becomes equal to or less than the first predetermined amount, control apparatus  50  subtracts the time elapsed from when the first actual drop amount becomes equal to or less than the first predetermined amount from the corrected remaining time at predetermined time intervals to calculate the remaining time at that time and update the remaining time to the latest. Control apparatus  50  then indicates the updated remaining time to load apparatus  3 . 
     Subsequently, control apparatus  50  performs S 129  to S 131  as in S 29  to S 31  in Embodiment 1. 
     When the charge of second battery bank  30  is stopped (YES in S 131 ), in S 132 , control apparatus  50  calculates a second actual drop amount that is the actual drop amount of the voltage value of second battery bank  30  per unit time at that time. 
     Then, in S 133 , control apparatus  50  determines whether the second actual drop amount is equal to or less than a second predetermined amount. The second predetermined amount is a value determined in advance by actual measurement through experiments or the like, and is determined to be the value of the second actual drop amount at point Q in  FIG.  5   . That is, the second predetermined amount is the value of the second actual drop amount when the second actual drop amount becomes smaller and almost constant after the charge of second battery bank  30  is stopped. Note that the second predetermined amount may be the same as the first predetermined amount in a case where the configurations of first and second battery banks  20  and  30  are similar to each other. 
     When the temperature of second battery bank  30  is relatively high and the second actual drop amount is greater than the second predetermined amount (NO in S 133 ), control apparatus  50  continues calculating the second actual drop amount (S 132 ) and updating and indicating the remaining time. In contrast, when the voltage value of second battery bank  30  and the temperature of second battery bank  30  decrease and the second actual drop amount becomes equal to or less than the second predetermined amount (YES in S 133 ), control apparatus  50  corrects the remaining time in S 134 . 
     To be more specific, control apparatus  50  calculates another second charge stop time, which is the time required from when the second actual drop amount becomes equal to or less than the second predetermined amount to when the second bank charge processing is ended, and corrects the remaining time based on the calculated another second charge stop time. 
     Control apparatus  50  calculates the another second charge stop time by subtracting the power source voltage value from the voltage value of second battery bank  30  at the time when the second actual drop amount becomes equal to or less than the second predetermined amount, and dividing the subtracted value by the second actual drop amount at the time when the second actual drop amount becomes equal to or less than the second predetermined amount. 
     Further, control apparatus  50  corrects the remaining time by replacing the remaining time at the time when the second actual drop amount becomes equal to or less than the second predetermined amount with the calculated another second charge stop time. After the second actual drop amount becomes equal to or less than the second predetermined amount, control apparatus  50  subtracts the time elapsed from when the second actual drop amount becomes equal to or less than the second predetermined amount from the corrected remaining time at predetermined time intervals to calculate the remaining time at that time and update the remaining time to the latest. Control apparatus  50  then indicates the updated remaining time to load apparatus  3 . 
     Next, control apparatus  50  performs S 135  and S 136  as in S 35  and S 36  in Embodiment  1 , and ends the control for calculating the remaining time. 
     The present disclosure is not limited to the embodiments described above. Various modifications to the embodiments and embodiments with components in different embodiments combined are also included within the scope of the present disclosure, as long as they do not depart from the spirit of the present disclosure. 
     For example, Expression 1 may be altered into the following Expression 2. 
       [2] 
       Remaining time at the start of charge= Tm 0×(100−So)/(100−α)+ Tm 1+ Tm 2   (Expression 2)
 
     In Expression 2, So (%) is the SOC of battery bank unit  1  at the start of charge control (time t 0 ). α (%) is the first predetermined charge rate and is the SOC of a battery bank unit at the start of an experiment to determine the total charge time to be stored in table T. 
     “Tm 0 ” is multiplied by “(100−So)/(100α)” in Expression 2, which is a difference from Expression 1. As described above, Tm 0  is the collective charge time, and corresponds to the time required from the start (time t 0 ) to the end (time t 1 ) of collective charge processing in a case where the SOC of battery bank unit  1  at the start of charge is the first predetermined charge rate (a). 
     “(100−So)/(100α)” is a ratio of the amount of charge for the SOC of battery bank unit  1  to be 100% from So to the amount of charge for the SOC of battery bank unit  1  to be 100% from a. 
     Thus, “Tm 0 ×(100−So)/(100−α)” in Expression 2 corresponds to the time required from the start (time t 0 ) to the end (time tl) of collective charge processing in a case where the SOC of battery bank unit  1  at the start of collective charge is So. 
     That is, Expression 2 makes it possible to calculate the remaining time at the start of charge excluding the time required for charging the charge amount corresponding to the SOC of battery bank unit  1  at the start of collective charge. Control apparatus  50  calculates the remaining time to complete the charge of battery bank unit  1  based on the temperature of battery bank unit  1  and the SOC of battery bank unit  1  at the start of collective charge processing. Thus, control apparatus  50  can accurately calculate the remaining time at the start of charge. 
     Control apparatus  50  may start the charge control in a case where the SOC of battery bank unit  1  decreases due to self-discharge of first and second battery banks  20  and  30  and becomes equal to or lower than a second predetermined charge rate (e.g., 90%). The second predetermined charge rate is determined so that the SOC of battery bank unit  1  is not relatively low due to self-discharge. Note that the second predetermined charge rate may be determined to be equal to the first predetermined charge rate. In this case, Tm 0  (collective charge time) in Expressions 1 and 2 is determined based on the second predetermined charge rate, and is relatively highly accurate in a case where the SOC of battery bank unit  1  at the start of charge control is equal to the second predetermined charge rate. Thus, control apparatus  50  can calculate the remaining time more accurately in the case where the SOC of battery bank unit  1  at the start of charge control is equal to the second predetermined charge rate. 
     Further, control apparatus  50  may correct the remaining time in S 30  based on the corrected charge time obtained by adding, to the actual first bank charge time, the charge time corresponding to the drop amount of the voltage value dropped due to self-discharge of second battery bank  30  during the first bank charge processing. Specifically, control apparatus  50  calculates the corrected charge time by multiplying the actual first bank charge time by a predetermined factor (e.g., 1.05). The predetermined factor is determined in advance by experimentally measuring the drop amount of the voltage value dropped due to self-discharge of second battery bank  30  during the first bank charge processing. Control apparatus  50  corrects the remaining time at time t 3  (at the start of the second bank charge processing) by replacing it with the corrected charge time. 
     The configuration of first and second battery banks  20  and  30  may be different from each other. In this case, in table T, the temperature, collective charge time, first bank charge time, second bank charge time, voltage drop amount, and second voltage drop amount, which is the drop amount of the voltage value of second battery bank  30  per unit time after second battery bank  30  is fully charged, may be associated with each other. In this case, control apparatus  50  acquires the second voltage drop amount in S 33 . 
     In the case where the configuration of first and second battery banks  20  and  30  are different from each other, the remaining time may not be corrected at the start of the second bank charge processing, that is, S 30  in  FIG.  6 B  and S 130  in  FIG.  7 A  may not be performed. 
     Battery bank unit  1  may be configured so that first and second battery banks  20  and  30  are removable. In addition, battery bank unit  1  need not include first and second battery banks  20  and  30  as the components of battery bank unit  1  itself. That is, battery bank unit  1  may be configured by including input/output terminal  10 , charge/discharge circuitry  40 , control apparatus  50 , and sensors  60  to  64 . In this case, battery bank unit  1  can function as a back-up apparatus for external power source  2  by retrofitting separately arranged first and second battery banks  20  and  30 . 
     Further, storage  51  may be configured separately from control apparatus  50  and communicable with control apparatus  50 . In this case, storage  51  may be communicably connected to control apparatus  50  via a network such as the Internet. With such a configuration, the information of the table can be shared by a plurality of battery bank units  1 ; furthermore, the table can be updated by the plurality of battery bank units  1  so as to store more accurate information. Also, control apparatus  50  may be configured separately from battery bank unit  1 . In this case, control apparatus  50  can remotely control battery bank unit  1  and calculate the remaining time via a network such as the Internet. Further, storage  51  may be a non-transitory storage medium that stores a remaining charge time calculation program for calculating the remaining time, and control apparatus  50  may calculate the remaining time as described above by reading and executing the remaining charge time calculation program. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is particularly useful as a battery bank unit. 
     REFERENCE SIGNS LIST 
     
         
           1  Battery bank unit 
           20  First battery bank 
           30  Second battery bank 
           50  Control apparatus 
         T Table