Abstract:
To provide a method of controlling charge and discharge of a secondary battery for automatic guided vehicle that can decide the timing of refresh charge and discharge accurately and minimize the frequency of refresh charge and discharge. The method of controlling charge and discharge of a secondary battery for automatic guided vehicle comprises a first discharge step of making the secondary battery drive an automatic guided vehicle and discharge electricity with a predetermined amount; a voltage measurement step of measuring a discharge end voltage of the secondary battery at the completion of the first discharge step; and a charge step of performing a first charge step of charging the secondary battery incompletely at a first charging current value when the discharge end voltage is higher than a preset minimum voltage, and performing a second charge step of discharging the secondary battery fully and then charging the secondary battery fully at a second charging current value smaller than the first charging current value when the discharge end voltage equals the preset minimum voltage or less.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of controlling charge and discharge of a secondary battery for automatic guided vehicle. 
     An automatic guided vehicle is used for automation and laborsaving of the delivery of materials in factories and warehouses. The automatic guided vehicle mounts a secondary battery (battery) as a power source for a traction electric motor and a driving source for other loads thereon and is controlled so as to run automatically on various driving paths along a guide line such as a guide tape adhered to the floor of factories, warehouses, etc. When the battery needs to be charged or there is no request to deliver materials from an operational control station, the automatic guided vehicle moves to a charge station as a destination and is connected to a battery charger at the charge station to charge the secondary battery. 
     Conventionally, a lead storage battery has been used as the secondary battery mounted on such automatic guided vehicle. However, the lead storage battery takes a long time to be charged and requires equipment for its charge and reloading as well as personnel for maintenance. Accordingly, in these days, an alkali storage battery such as nickel metal hydride battery, which is capable of being quickly charged at the charge station in a short time and requires no maintenance, has become widespread in place of lead storage battery. 
     Generally, an automatic guided vehicle system placed in factories comprises a minimum number of automatic guided vehicles and battery chargers essential to perform desired operations so as to reduce its total cost and prevent unnecessary space from being occupied. After the secondary battery mounted on the automatic guided vehicle supplies electric power to the loads for a few hours (the automatic guided vehicle runs automatically on the driving path), the automatic guided vehicle moves to the battery charger automatically according to an instruction of a central control unit. The battery charger charges the secondary battery quickly in a few minutes. If the nickel metal hydride battery is charged fully by quick charge, a dangerous increase in its internal pressure causes and therefore the battery is not charged fully at quick charge. As a result, incomplete charge and discharge of the secondary battery is repeated. When the nickel metal hydride battery or nickel cadmium battery is incompletely charged and discharged repeatedly without full charge, memory effect of increasing a voltage drop during discharge generates. In the case where the secondary battery with the memory effect is used for equipment in which discharge end voltage is set at a fixed value, the apparent capacity decreases with drop in voltage. For the secondary battery consisting of a plurality of single cells, voltages of respective single cells differ in proportion to accumulated discharged amount, and drop in apparent capacity and terminal voltage of the secondary battery occurs. 
     The above-mentioned drop in apparent capacity and terminal voltage can be resolved by discharging the secondary battery fully and then performing full charge or overcharge of the secondary battery, that is, refresh charge and discharge. A battery charge control device for automatic guided vehicle disclosed in a patent document 1 (Patent Publication No. 2560878) stores the number of battery charge times and an accumulated value of charged capacity and calculates an integrated discharged amount. When the integrated discharged amount reaches a preset amount, the device requests the user to perform equalizing charge. 
     A method of charging and discharging a secondary battery for automatic guided vehicle of a patent document 2 (Unexamined Patent Publication No. 2002-191103) integrates the discharged amount of the secondary battery at the side of the automatic guided vehicle, and when the integrated discharged amount reaches a preset amount, requests a battery control system to perform maintenance charge and discharge (refresh charge and discharge). 
     The conventional methods only measure the number of incomplete charge (or discharge) times or total discharged amount and determine the timing of refresh charge and discharge. Accordingly, even if memory effect or nonuniformity of voltages of respective single cells does not occur in fact, the methods may request refresh charge and discharge, thereby to cause congestion at the charge station and then interfere with the operation in factories equipped with the automatic guided vehicle. Moreover, there is a problem that memory effect or nonuniformity of voltages of respectiive single cells may not be able to be detected, resulting in sudden stop of the automatic guided vehicle. 
     To solve the above-mentioned conventional problems, the present invention provides a method of controlling charge and discharge of the secondary battery for automatic guided vehicle that can decide the timing of refresh charge and discharge of the secondary battery for automatic guided vehicle accurately and minimize the frequency of refresh charge and discharge. 
     BRIEF SUMMARY OF THE INVENTION 
     To solve the above-mentioned problems, the present invention has the following configuration. A method of controlling charge and discharge of the secondary battery for automatic guided vehicle from one aspect of the present invention has a first discharge step of making the secondary battery drive an automatic guided vehicle and discharge electricity with a predetermined amount; a voltage measurement step of measuring a discharge end voltage of the above-mentioned secondary battery at the completion of the above-mentioned first discharge step; and a charge step of performing a first charge step of charging the above-mentioned secondary battery incompletely at a first charging current value when the above-mentioned discharge end voltage is higher than a preset minimum voltage, and performing a second charge step of discharging the above-mentioned secondary battery fully and then charging the above-mentioned secondary battery fully at a second charging current value smaller than the above-mentioned first charging current value when the above-mentioned discharge end voltage equals the above-mentioned preset minimum voltage or less. 
     The present invention has the effect of realizing the method of controlling charge and discharge of the secondary battery for automatic guided vehicle that can decide the timing of refresh charge and discharge of the secondary battery for automatic guided vehicle accurately and minimize the frequency of refresh charge and discharge. 
     The secondary battery is a single cell or a battery pack consisting of a plurality of single cells. Preferably, the single cell is a nickel metal hydride battery or a nickel cadmium storage battery. 
     Preferably, the first charge step is quick charge of the secondary battery with a high current (the first charging current value). The preset minimum voltage is close to a minimum voltage at which the automatic guided vehicle using the secondary battery as a power source is operational. In the period during which the discharge end voltage when the secondary battery for automatic guided vehicle supplies electricity (discharges electricity) to loads is higher than the preset minimum voltage, the secondary battery can be used by repeating quick charge (the first charge step) and discharge (the first discharge step). 
     With the repetition of the first charge step and the first discharge step, the discharge end voltage lowers due to memory effect and nonuniformity of capacities of respective single cells. When the charge start voltage is smaller than the preset minimum voltage, the secondary voltage is discharged fully and then charged fully at the second charging current value smaller than the first charging current value (the second charge step). This can recover the terminal voltage lowered by memory effect and nonuniformity of capacities of respective single cells. 
     In the above-mentioned method of controlling charge and discharge of the secondary battery for automatic guided vehicle from another aspect of the present invention, in the above-mentioned first charge step, voltage and temperature of the above-mentioned secondary battery are monitored continuously, a preset charge end voltage is decided based on the above-mentioned temperature and charge is finished when the above-mentioned voltage equals the above-mentioned preset charge end voltage or more. 
     When the temperature of the secondary battery increases, its internal resistance becomes smaller. The present invention can improve control accuracy of charge by lowering the preset charge end voltage for discontinueing quick charge as the temperature increases (the dischargeable capacity at the completion of charge corresponds to a target value). It can be prevented that the life of the secondary battery is shortened due to overcharge and that a failure in the operation of the automatic guided vehicle is caused by insufficient charge. 
     In the above-mentioned method of controlling charge and discharge of the secondary battery for automatic guided vehicle from another aspect of the present invention, the above-mentioned preset charge end voltage is decided based on the above-mentioned first charging current value and the above-mentioned temperature. 
     The present invention can improve control accuracy of charge by lowering the preset charge end voltage for discontinueing quick charge as the temperature increases and the charging current value decreases (the dischargeable capacity at the completion of charge corresponds to a target value). It can be prevented that the life of the secondary battery is shortened due to overcharge and that a failure in the operation of the automatic guided vehicle is caused by insufficient charge. 
     In the above-mentioned method of controlling charge and discharge of the secondary battery for automatic guided vehicle from another aspect of the present invention, the above-mentioned second charge step is finished after a predetermined period from start of charge. 
     Preferably, the second charge step is a fixed amount charge. 
     In the above-mentioned method of controlling charge and discharge of the secondary battery for automatic guided vehicle from another aspect of the present invention, in the above-mentioned second charge step, temperature of the above-mentioned secondary battery is monitored continuously and charge is finished when a rate of increase per unit of time in the above-mentioned temperature becomes a predetermined value or more. 
     The temperature of the secondary battery rises steeply just before the completion of full charge. In the second charge step, when the rate of increase in temperature becomes a predetermined value or more, charge is finished. 
     In the above-mentioned method of controlling charge and discharge of the secondary battery for automatic guided vehicle from another aspect of the present invention, the above-mentioned secondary battery is a nickel metal hydride battery. 
     The present invention is useful as, in particular, a method of controlling charge and discharge of the nickel metal hydride battery mounted on the automatic guided vehicle. 
     The novel features of the invention are set forth with particularity in the appended claims. The invention as to both structure and content, and other objects and features thereof will best be understood from the detailed description when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration view of a system of controlling charge and discharge of automatic guided vehicle to which a method of controlling charge and discharge of a secondary battery for automatic guided vehicle in accordance with a first embodiment and a second embodiment of the present invention is applied. 
         FIG. 2  is a flow chart of the method of controlling charge and discharge of a secondary battery for automatic guided vehicle in accordance with the first embodiment of the present invention. 
         FIG. 3  is a graph showing the change in a discharge end voltage Vend to the number of charge and discharge cycles. 
         FIG. 4  is a graph showing function expression of a preset charge end voltage Vc to the battery temperature Tb of the secondary battery  11  in the case of charge at the charging current value I 1  of 160 A, 180 A and 200 A. 
         FIG. 5  is a flow chart of a second charge control method in accordance with the second embodiment of the present invention. 
     
    
    
     Part or All of the drawings are drawn schematically for diagrammatic representation and it should be considered that they do not necessarily reflect relative size and position of components shown therein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments that specifically describe best modes for conducting the present invention will be described referring to figures below. 
     First Embodiment 
     With reference to  FIGS. 1 to 4 , a method of controlling charge and discharge of a secondary battery for automatic guided vehicle in accordance with a first embodiment of the present invention will be described. 
       FIG. 1  is a configuration view of a system of controlling charge and discharge of automatic guided vehicle to which the method of controlling charge and discharge of the secondary battery for automatic guided vehicle in accordance with the first embodiment of the present invention is applied. Reference numeral  10  denotes the automatic guided vehicle. Reference numeral  20  denotes a charge/discharge unit installed at a charge station (not shown). 
     There is provided a secondary battery  11  and a battery control part  12  for controlling the secondary battery  11  on the automatic guided vehicle  10 . Numeral  13  denotes a battery temperature detection part and  14  denotes a voltage detection part. 
     The secondary battery  11  supplies electric power to a traction electric motor and other loads (not shown). The secondary battery  11  is formed of a plurality of single cells as nickel metal hydride batteries that are connected in series and in parallel with each other. 
     The battery temperature detection part  13  measures anyone of internal temperature, surface temperature and ambient temperature of the secondary battery  11 , and transmits it to the battery control part  12 . In the first embodiment, the battery temperature detection part  13  measures a surface temperature Tb of the secondary battery  11 . Hereinafter, the surface temperature Tb of the secondary battery  11  is referred to as a “battery temperature Tb”. The voltage detection part  14  measures voltage of the secondary battery  11  and transmits it to the battery control part  12 . 
     In response to a departure request or delivery request from an operational control station (not shown), the battery control part  12  makes the secondary battery  11  start supplying electric power to the traction electric motor and other loads. When the automatic guided vehicle moves to the charge station and is connected to the charge/discharge unit  20  via a charge/discharge line  31  and a control signal line  32 , the battery control part  12  discontinues supply of electric power by the secondary battery  11  to the traction electric motor and other loads. Further, the battery control part  12  transmits data on the battery temperature Tb and a battery voltage V to the charge/discharge unit  20  through the control signal line  32 . 
     The charge/discharge unit  20  has a charge/discharge terminal  21 , a control terminal  22 , a source terminal  23 , a rectifier filter  24 , a charge/discharge part  25 , a current detection part  27  and a charge/discharge control part  28 . The charge/discharge terminal  21  is connected to the charge/discharge line  31 . The control terminal  22  is connected to the control signal line  32 . The source terminal  23  is connected to an AC power source. The rectifier filter  24  rectifies an AC source voltage from the source terminal  23 , transforms the rectified voltage to the DC source voltage and supplies the transformed voltage to the charge/discharge part  25 . The current detection part  27  detects a charge/discharge current sent to the secondary battery  11 . 
     The charge/discharge control part  28  controls the charge/discharge part  25  based on the battery temperature Tb and voltage received from the automatic guided vehicle  10  and the current received from the current detection part  27 . The charge/discharge part  25  charges or discharges the secondary battery  11  based on the control signal sent from the charge/discharge control part  28 . 
     The automatic guided vehicle  10  is driven by discharge of the secondary battery  11 , performs its operation, moves to the charge station every predetermined timing according to a preset program (for example, every one hour) and is charged quickly by the charge/discharge unit  20  according to a first charge control method (described later). The quick charge is set to be performed when the state of charge of the secondary battery  11  (the ratio (%) of the current dischargeable capacity to the dischargeable capacity at full charge) is the order of 65%. The quick charge finishes when the state of charge of the secondary battery  11  reaches 75%. In the quick charge according to the first charge control method, as the secondary battery  11  is not charged fully, increase in internal pressure of the secondary battery  11  can be prevented. The secondary battery  11  is used in the state of charge of 65 to 75%. 
       FIG. 3  is a graph showing the change in a discharge end voltage Vend (when the automatic guided vehicle  10  is connected to the charge/discharge unit) to the number of charge and discharge cycles of quick charge (incomplete charge) and slight discharge. At each cycle, the secondary battery  11  is charged with electricity with a certain amount to go into a state of charge of 75% from a state of charge of 65%, and discharges electricity with a certain amount to go into a state of charge of 65% from a state of charge of 75%. As the charge and discharge cycle is repeated, the discharge end voltage Vend lowers and when the discharge end voltage Vend is below a preset minimum voltage Vr (12.0 in the first embodiment), the secondary battery  11  cannot supply sufficient power to the loads. Memory effect is one of the factors of lowering the discharge end voltage Vend. Since the discharged amount from the start to the completion of discharge is substantially constant each time (the battery discharges electricity from a state of charge of 75% to a state of charge of 65% each time), discharge voltage drop due to memory effect is accumulated and the discharge end voltage Vend lowers gradually. Nonuniformity of voltages of respective single cells constituting the secondary battery  11  also lowers the discharge end voltage Vend. In the first embodiment, when the discharge end voltage Vend becomes the preset minimum voltage Vr or less, the secondary battery  11  is discharged fully and charged fully at a low current to resolve voltage drop. 
       FIG. 2  is a flow chart of the method of controlling charge and discharge of the secondary battery in accordance with the first embodiment of the present invention. Preferably, the automatic guided vehicle  10  is connected to the charge/discharge unit  20  each time it discharges electricity with a substantially constant amount. The battery control part  12  monitors voltage of the secondary battery  11  through the voltage detection part  14  at all times (step  201 ). At a step  202 , the discharge end voltage Vend is acquired. At a step  203 , it is determined whether or not the discharge end voltage Vend equals the preset minimum voltage Vr or more. When the discharge end voltage Vend equals the preset minimum voltage Vr or more, the operation proceeds to a step  210 . 
     The charge/discharge control part  28  switches the charge/discharge part  25  to a charger, controls a charging current value I 1  received from the current detection part  27  to be 200 A, and start quick charge (step  210 ). At a step  211 , the charge/discharge control part  28  acquires the voltage V of the secondary battery  11  measured by the voltage detection part  14  from the battery control part  12  through the control signal line  32 . At a step  212 , the charge/discharge control part  28  acquires the battery temperature Tb from the battery control part  12  through the control signal line  32 . 
       FIG. 4  is a graph showing function expression of a preset charge end voltage Vc to the battery temperature Tb of the secondary battery  11  in the case of charge at the charging current value I 1  of 160 A, 180 A and 200 A. The function expression is set so that the state of charge at the completion of charge is 75%. The preset charge end voltage Vc is a linear function of the battery temperature Tb by applying the charging current value I 1  as a parameter. As the battery temperature Tb becomes higher and the charging current value I 1  becomes lower, the preset charge end voltage Vc becomes lower. The charge/discharge control part  28  stores the function expression in  FIG. 4  for each charging current value I 1 . 
     Control accuracy of charge of the secondary battery  11  can be improved (dischargeable capacity at the completion of charge corresponds to a target value) by lowering the preset charge end voltage Vc for discontinuing quick charge (by deciding the preset charge end voltage Vc according the function expression in  FIG. 4 ) as the battery temperature Tb becomes higher and the charging current value I 1  becomes smaller. It can be prevented that the life of the secondary battery  11  is shortened due to overcharge and that a failure in the operation of the automatic guided vehicle is caused by insufficient charge. 
     Returning to  FIG. 2 , at a step  213 , the charge/discharge control part  28  acquires the preset charge end voltage Vc from the charging current value I 1  and the battery temperature Tb (according to the function expression in  FIG. 4 ). It is determined whether or not the voltage V of the secondary battery  11  equals the preset charge end voltage Vc or more (step  214 ). When the voltage V of the secondary battery  11  equals the preset charge end voltage Vc or less, the operation returns to a step  211  and continues quick charge. When the voltage V of the secondary battery  11  is higher than the preset charge end voltage Vc, quick charge is finished (step  215 ). The automatic guided vehicle  10  start running and leaves the charge station. The operation returns to the step  201 . At the steps  210  to  215 , charge is conducted according to the first charge control method. As mentioned above, since the secondary battery  11  is not charged fully in quick charge according to the first charge control method, increase in internal pressure of the secondary battery  11  can be prevented. Further, since the secondary battery  11  is charged with a high current in a first charge control method, charge can be completed in a short time (many secondary batteries  11  for automatic guided vehicle can be maintained by a small number of charge/discharge units). 
     In the case where it is determined that the discharge end voltage Vend is smaller than the preset minimum voltage Vr at the step  203 , the operation proceeds to a step  220 . The charge/discharge control part  28  switches the charge/discharge part  25  to a discharger and discharges the battery  11  until the voltage acquired from the battery control part  12  reaches the discharge end voltage (full discharge) (step  220 ). The charge/discharge control part  28  switches the charge/discharge part  25  to a charger, controls a charging current value I 2  received from the current detection part  27  to be 10 A, and starts charging (step  221 ). The charging current value I 2  is smaller than the charging current value I 1  according to the first charge control method. Charge is continued for a predetermined period (step  222 ). The predetermined period at the step  222  is set to be a period necessary for full charge of the secondary battery  11 . Charge is finished at a step  223 . The operation returns to the step  201 . At the steps  220  to  223 , charge is conducted according to a second charge control method. 
     Since the secondary battery  11  is discharged fully at the step  220  and charged fully according to the second charge control method, it is possible to equalize memory effect on the secondary battery  11  and the dispersion of voltages of respective single cells. Since the secondary battery  11  is charged fully with a relatively low current in the second charge control method, damage of the secondary battery  11  due to rapid increase in its internal pressure can be prevented. 
     Second Embodiment 
     Referring to  FIG. 1  and  FIG. 5 , a method of controlling charge and discharge of a secondary battery for automatic guided vehicle in accordance with a second embodiment of the present invention will be described. The method of controlling charge and discharge of the secondary battery for automatic guided vehicle in accordance with the second embodiment applies to the same system of controlling charge and discharge of the automatic guided vehicle  10  as in the first embodiment ( FIG. 1 ).  FIG. 1  has been already described above. 
     The method of controlling charge and discharge of a secondary battery for automatic guided vehicle in accordance with a second embodiment is different from the method of the first embodiment only in the second charge control method. Specifically, the steps  221  to  223  in  FIG. 2  (the second charge control method) are replaced with steps  220 ,  221  and  501  to  503  in  FIG. 5 . Only the different steps in the second embodiment will be described below. 
       FIG. 5  is a flow chart of the method of controlling charge and discharge of a secondary battery for automatic guided vehicle in accordance with the second embodiment. After full discharge (step  220 ), the charge/discharge control part  28  switches the charge/discharge part  25  to a charger, controls a charging current value I 2  received from the current detection part  27  to be 10 A, and starts charging (step  221 ). The charging current value I 2  is smaller than the charging current value I 1  according to the first charge control method. At a step  501 , the charge/discharge control part  28  acquires the battery temperature Tb from the battery control part  12  through the control signal line  32 . At a step  502 , the charge/discharge control part  28  calculates a time variation rate dTb/dt (t is time) and determines whether or not the rate equals a threshold value or more. When the rate is smaller than the threshold value, the operation returns to a step  501  and charge is continued. When the rate equals the threshold value or more, the operation proceeds to a step  503  and charge is finished. 
     The temperature Tb of the secondary battery  11  increases with the progress of charge and the rate of increase per unit of time rises steeply just before the completion of full charge. Therefore, full charge can be detected by the rate of increase of the battery temperature Tb. 
     According to the present invention, it is possible to obtain an advantageous effect of realizing a method of controlling charge and discharge of the secondary battery for automatic guided vehicle that accurately judges the timing of refresh charge and discharge of the secondary battery for automatic guided vehicle and minimizes the frequency of refresh charge and discharge. 
     While preferred embodiments of the present invention have been described in detail to a certain degree, it is to be understood that, within the scope and spirit of the claims made herein, the invention may be practiced otherwise than as specifically described herein, the invention may be modified in arrangement and detail without departing from such scope and spirit.