Patent Publication Number: US-2023141602-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. 2021-181217, 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 
     To achieve the above objective, 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 starts charging the second battery bank after the first battery bank is fully charged, 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 charge of the battery bank unit and a state of charge (SOC) of at least one of the first battery bank and/or the second battery bank at the start of the charge of the battery bank unit. 
     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 including a first battery bank and a second battery bank that is connected in parallel to the first battery bank and starts to be charged after the first battery bank is fully charged, the method including: acquiring a temperature of the battery bank unit at a start of the charge of the battery bank unit; acquiring a state of charge (SOC) of at least one of the first battery bank and/or the second battery bank at the start of the charge of the battery bank unit; and calculating the remaining time based on the temperature and the SOC. 
     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 including a first battery bank and a second battery bank that is connected in parallel to the first battery bank and starts to be charged after the first battery bank is fully charged, the program is configured to cause the computer to perform operations including: acquiring a temperature of the battery bank unit at a start of charge of the battery bank unit, acquiring a state of charge (SOC) of at least one of the first battery bank and/or the second battery bank at the start of the charge of the battery bank unit, and calculating remaining time to complete the charge of the battery bank unit based on the temperature and the SOC. 
     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 first 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    is a flowchart illustrating a procedure performed by the control apparatus to calculate remaining time; and 
         FIG.  7    illustrates a second table. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a battery bank unit according to an embodiment 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 an embodiment 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 first table T 1  illustrated in  FIG.  3   . 
     First table T 1  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 first table T 1 , the temperature, total charge time, and charge stop time are associated with each other. In first table T 1 , the temperature is divided into six temperature zones in total, and between 0° C. and 40° C., there are four 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 total charge time and charge stop time 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 S 1 . 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 t0) 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 t0), 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 t1; 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 t1). 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 t2; 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 t2). 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 t3; 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 t3). 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 t4; 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 t4). 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 t5; 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 t5). 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 flowchart in  FIG.  6   . 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, which is between time t0 and time t5 in  FIG.  5    in particular. 
     In S 20 , control apparatus  50  acquires, from first table T 1 , the total charge time and charge stop time that are associated with the temperature at the start of the charge of 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 total charge time and charge stop time are used for calculating the remaining time in Expression 1 to be described later. 
     The total charge time is the time required for the SOC of battery bank unit  1  at the start of charge to be 100% from a first charge rate (e.g., 0%), in particular. The total charge time is determined in advance for each temperature zone by actual measurement through experiments and stored in first table T 1 . The first charge rate is any value used in, for example, an experiment to determine the total charge time stored in first table T 1  in advance. In the experiment to determine the total charge time, the collective charge processing and first and second bank charge processing are performed in the above-described manner using battery bank unit  1  with the SOC of the first charge rate, and the time required for the SOC of battery bank unit  1  to be 100% from the first charge rate is measured as the total charge time. 
     The charge stop time corresponds to the time from when the charge of first battery bank  20  is stopped to when the charge of second battery bank  30  is started (i.e., from time t2 to time t3 in  FIG.  5   ) in particular. The charge stop time is determined in advance for each temperature zone by actual measurement through experiments and stored in first table T 1 . In the above experiment to determine the total charge time, the time period in which the charge of the battery banks is stopped during the charge of battery bank unit  1  is measured as the charge stop time. 
     In a case where the temperature of battery bank unit  1  is 25° C. at the start of charge control, for example, control apparatus  50  acquires the total charge time “A3” and charge stop time “B3” that are associated with the temperature “20° C. or higher and lower than 30° C.” from first table T 1  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= m×Ts +( Tt−m×Ts )×(100− So )/(100−α)   (Expression 1)
 
     In Expression 1, “m” is the number of battery banks. In the present embodiment, m=2. Ts and Tt are respectively the charge stop time and total charge time acquired from first table T 1 . So (%) is the SOC of battery bank unit  1  at the start of charge control (time t0). α (%) is the first 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 first table T 1 . 
     Note that So may be either one of the SOCs of first and second battery banks  20  and  30  at the start of charge control. In this case, control apparatus  50  specifies, as 100%, the SOCs of first and second battery banks  20  and  30  at the end of the charge of battery bank unit  1 . 
     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 time period in which the charge is stopped in the second bank charge processing (between time t4 and time t5 in  FIG.  5   ) is considered to be approximately equal to the time period in which the charge is stopped in the first bank charge processing (between time t2 and time t3 in  FIG.  5   ). Thus, in a case where battery bank unit  1  includes m battery banks in the experiment to determine the total charge time to be stored in first table T 1 , the total of the time in which the charge of any of the battery banks is stopped corresponds to a value obtained by multiplying m and the charge stop time (Ts) together. That is, “m×Ts” in Expression 1 corresponds to the total of the time periods in which the charge of any of the battery banks is stopped while the charge of battery bank unit  1 . 
     Note that, as described above, in the time period in which the charge is stopped during the charge control of battery bank unit  1 , a battery bank that stops charging does not discharge to load apparatus  3 . For example, first battery bank  20  does not discharge between time t2 and time t3 in  FIG.  5   . Accordingly, the SOC of the battery bank that stops charging cannot be calculated based on the detection value of current sensor  60 . Thus, control apparatus  50  cannot calculate the time period in which the charge is stopped during the charge control of battery bank unit  1  using the SOC. 
     “(Tt−m×Ts)” in Expression 1 corresponds to the time obtained by subtracting the total of the time periods in which the charge of any of the battery banks is stopped (m×Ts) from the total charge time (Tt) in the experiment to determine the total charge time to be stored in first table T 1  using battery bank unit  1  with m battery banks. That is, “(Tt−m×Ts)” in Expression  1  corresponds to the time in which the charge of any of the battery banks is in progress in the time required for the SOC of battery bank unit  1  with m battery banks to be  100 % from a. 
     Additionally, “(100−So)/(100−α)” 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 α. 
     Thus, “(Tt−m×Ts)×(100−So)/(100−α)”, which is obtained by multiplying “(Tt — m×Ts)” and “(100−So)/(100−α)” together, in Expression 1 corresponds to the time in which the charge of any of the battery banks is in progress in the time required for the SOC of battery bank unit  1  with m battery banks to be 100% from So. 
     That is, in Expression 1, “(Tt−m×Ts)×(100−So)/(100−α)”, which is the time in which the charge of any of the battery banks is in progress in the time required for the SOC of battery bank unit  1  with m battery banks to be 100% from So, and “m×Ts”, which is the total of the time periods in which the charge of any of the battery banks is stopped, are added together. Thus, Expression 1 is an expression for calculating, as the remaining time at the start of charge, the time from the start to the end of the charge of battery bank unit  1  in a case where the SOC of battery bank unit  1  with m battery banks is So at the start of the charge in the charge control. 
     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  during the charge control. Control apparatus  50  then 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. 
     Subsequently, in S 22 , control apparatus  50  determines whether the first bank charge processing has ended. When the collective charge processing or the first bank charge processing is in progress (NO in S 22 ), control apparatus  50  continues updating and indicating the remaining time. While performing S 22 , control apparatus  50  measures the time actually spent for the collective charge processing and the first bank charge processing respectively (hereinafter, referred to as actual collective charge time and actual first bank charge time). 
     When the first bank charge processing has ended (YES in S 22 ), control apparatus  50  determines, in S 23 , whether the remaining time needs to be corrected at the end of the first bank charge processing, i.e., at time t3 in  FIG.  5   . Control apparatus  50  determines that the remaining time needs to be corrected at time t3 when the time difference between the remaining time at the start of charge calculated in S 21  and revised remaining time at the start to be described below is equal to or greater than a predetermined time difference. 
     The revised remaining time at the start is the time from the start to the end of charge of battery bank unit  1  calculated based on the time actually spent to complete the procedure until the first bank charge processing in the charge control. In a case where configurations of a plurality of battery banks are similar to each other, the plurality of battery banks require almost the same time to be charged. Thus, the actual first bank charge time is almost equal to the time actually required for the second bank charge processing. The revised remaining time at the start in the present embodiment is the time obtained by adding the actual collective charge time and the actual first bank charge time multiplied by two together. Note that, in a case of m battery banks, the revised remaining time at the start is the time obtained by adding the actual collective charge time and the actual first bank charge time multiplied by m together. 
     When the time difference is smaller than the predetermined time difference (NO in S 23 ), control apparatus  50  does not correct the remaining time at time t3. When the time difference is equal to or greater than the predetermined time difference (YES in S 23 ), in contrast, control apparatus  50  corrects the remaining time at time t3 in S 24 . To be more specific, control apparatus  50  replaces the remaining time at time t3 (at the end of the first bank charge processing) with the actual first bank charge time. When the remaining time at time t3 is corrected, control apparatus  50  updates the latest remaining time as follows. That is, control apparatus  50  subtracts the time elapsed from time t3 from the actual first bank charge time at predetermined time intervals after time t3 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 time actually required for the second bank charge processing is almost equal to the actual first bank charge time. Thus, control apparatus  50  can accurately correct the remaining time by replacing the remaining time with the actual first bank charge time at the end of the first bank charge processing, and indicate the corrected remaining time. Note that control apparatus  50  may correct the remaining time in S 24  without determining whether the remaining time needs to be corrected in S 23 . 
     Next, in S 25 , control apparatus  50  determines whether the second bank charge processing has ended. When the second bank charge processing is in progress (NO in S 25 ), control apparatus  50  continues updating and indicating the remaining time. 
     When the second bank charge processing has ended (YES in S 25 ), control apparatus  50  updates first table T 1  in S 26 . Control apparatus  50  updates, based on actual total charge time, the total charge time associated with the temperature of battery bank unit  1  at the start of the charge in first table T 1 . The actual total charge time is the time actually spent from the start to the end of the charge of battery bank unit  1  in the charge control. 
     To be more specific, control apparatus  50  subtracts the remaining time at the start of charge calculated in S 21  from the total charge time acquired in S 20 , and adds the actual total charge time to the subtracted time to calculate updated total charge time. That is, the updated total charge time is the time obtained by adding the charge time actually spent for the SOC to be 100% from So, which is the SOC at the start of the charge, (actual total charge time) to the charge time from the first charge rate (α) to the SOC at the start of the charge (So), which is calculated by subtracting the remaining time at the start of charge calculated in S 21  from the total charge time acquired in S 20 . In other words, the updated total charge time is the time obtained by revising the total charge time stored in first table T 1  using the difference between the measured value (actual total charge time) and the calculated value (remaining time at the start of charge calculated in S 21 ) of the time from the start to the end of the charge of battery bank unit  1  in the charge control. 
     Control apparatus  50  updates the total charge time associated with the temperature of battery bank unit  1  at the start of the charge in first table T 1  with the calculated updated total charge time. For example, in the case where the temperature of battery bank unit  1  is 25° C. at the start of the charge control, control apparatus  50  updates the total charge time “A3” associated with the temperature “20° C. or higher and lower than 30° C.” in first table T 1  with the calculated updated total charge time. 
     Further, control apparatus  50  updates the charge stop time associated with the temperature of battery bank unit  1  at the start of the charge of battery bank unit  1  in first table T 1  with actual charge stop time. The actual charge stop time is the time actually spent from when the charge of first battery bank  20  is stopped to when the charge of second battery bank  30  is started in the charge control. That is, the charge stop time stored in first table T 1  is updated with the time in which the charge is actually stopped in the first 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 charge stop time “B3” associated with the temperature “20° C. or higher and lower than 30° C.” in first table T 1  with the actual charge stop time. 
     The actual total charge time and actual charge stop 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 first table T 1  with the updated total charge time and actual charge stop time, it is possible to make the values stored in first table T 1  match the surroundings of battery bank unit  1 , etc. Thus, control apparatus  50  updates first table T 1  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. 
     Upon updating first table T 1  in S 26 , control apparatus  50  ends the control for calculating the remaining time. Control apparatus  50  also ends the indication of the remaining time. Note that control apparatus  50  ends the control for calculating the remaining time without updating first table T 1  when at least one of first and second battery banks  20  and  30  discharges during the charge control of battery bank unit  1  due to the power outage of external power source  2 . 
     The present disclosure is not limited to the embodiment described above. Various modifications to the embodiment 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, storage  51  may store a plurality of tables. In the following, a description will be given of a case where storage  51  further stores second table T 2  in  FIG.  7   . In second table T 2 , the temperature, total charge time, and charge stop time are associated with each other as in first table T 1 . Values stored as the total charge time and charge stop time in second table T 2  are different from those in first table T 1 . 
     When calculating the remaining time at the start of charge during the charge control, control apparatus  50  selects a table from which information is acquired from first and second tables T 1  and T 2  based on the SOC of battery bank unit  1  at the start of the charge. To be more specific, control apparatus  50  selects first table T 1  when the SOC of battery bank unit  1  at the start of the charge is lower than a predetermined second charge rate (e.g., 90%). Meanwhile, control apparatus  50  selects second table T 2  when the SOC of battery bank unit  1  at the start of the charge is equal to or higher than the second charge rate. The second charge rate is any value determined based on characteristics of battery bank unit  1  during the charge, which will be described later. 
     For example, control apparatus  50  selects first table T 1  when the SOC of battery bank unit  1  is lower than the second charge rate (e.g., 90%), for example 20%, at the start of the charge control after battery bank unit  1  is connected to external power source  2 . 
     In a case where battery bank unit  1  is connected to external power source  2  and load apparatus  3 , the SOC of battery bank unit  1  decreases due to self-discharge or the like even without the power outage of external power source  2 . In a case where the charge control is determined to start when the SOC of battery bank unit  1  decreases to 90%, control apparatus  50  selects second table T 2  based on the SOC at the start of the charge control being the second charge rate (90%) or higher. 
     The SOC of a battery bank affects characteristics such as how the voltage of the battery bank increases and how the temperature of the battery bank increases during the charge. Thus, different SOCs of battery bank unit  1  at the start of the charge cause different characteristics during the charge such as an amount of increase in the bank voltage value of battery bank unit  1  per unit time and an amount of increase in the temperature of battery bank unit  1  per unit time; accordingly, the charge rate increase differently. Thus, control apparatus  50  can accurately calculate the remaining time by selecting an appropriate table based on the SOC of battery bank unit  1  at the start of the charge. 
     The configuration of first and second battery banks  20  and  30  may be different from each other. In this case, in first and second tables T 1  and T 2 , the charge stop time in the first bank charge processing, another charge stop time in the second bank charge processing, temperature, and total charge time may be associated with each other. The another charge stop time corresponds to the time in which the charge is stopped in the second bank charge processing in the experiment to determine the total charge time to be stored in first table T 1 . 
     In the case where the configuration of first and second battery banks  20  and  30  are different from each other, the charge stop time and the another charge stop time cannot be considered to be the same. In this case, when there are two battery banks, “m×Ts” in Expression 1 is replaced by a term in which the charge stop time and the another charge stop time stored in the tables are added together. 
     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 end of the first bank charge processing, that is, S 23  and S 24  in  FIG.  6    may not be performed. 
     Further, control apparatus  50  may perform the first bank charge processing and the second bank charge processing without performing the collective charge processing in the charge control. In this case, S 1  and S 2  in  FIG.  4    are not performed and the actual collective charge time is zero in the calculation of the revised remaining time at the start. 
     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 1  First table 
     T 2  Second table