A method and device for representing a residual capacity of a battery

Abstract: A method includes the steps of: measuring a charging amount in a charging mode where a charger having a function for detecting a fully charged state is connected to the battery; adding, if the charging amount in the charging mode is more than a trickle charging amount, the measured charging amount to a present charging amount to obtain the present charging amount and thereby indicating a ratio of the present charging amount to a fully charged state; and judging, if the charging amount in the charging mode is less than a trickle charging amount, the present charging amount as the fully charged amount and representing this amount to be fully charged state. Further, a device for indicating a residual capacity of a battery includes: a battery (1), a charger (2), a current-to-voltage converting circuit (3), an analog-to-digital converter (4), an operational unit (5) such as a MPU, and a display unit (6) and executes the operation in accordance with the above method.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and device for representing the amount of residual charge remaining in a battery, and more particularly to a method and device for representing the residual capacity of a battery when a nickel-cadmium battery or the like is connected to a battery charger equipped with a detector for detecting a fully charged condition and is charged thereby. 2. Description of the Related Art When a secondary cell such as a nickel-cadmium battery or the like, the output voltage of which is substantially constant, is charged and reaches a fully charged state, a trickle charging current from the battery charger often continues indefinitely, so that it is desirable to determine whether or not a charging operation has been completed. For this reason, it is necessary to connect the secondary cell or battery not only to a trickle charger that provides a so-called microcurrent from the beginning of a charge cycle in the form of a charging current, but also to a charger that has a function for detecting a fully charged state so that a change in the charging current may be made immediately before or after a constant charge has been completed. In general, in the case of the prior art where the residual capacity of a battery is indicated, there is a well known method and apparatus for measuring and indicating a charging current and discharging current. The charging amount is indicated by a value I c × T c by which a charging current I c is multiplied by a time Tc and, for example, if a charging current Ic of 50 mA is maintained for ten hours, the charging amount would be I c × T c = 500 mAh. Then a value of 500 mAh is reached, if the charger detects a full charge, the value of 500 mAh is considered to be the full charging amount. The discharging amount is designated by the product of I D × T D by which a discharging current I D is multiplied by a discharging time T D . If I D is 50 mA and the discharging time is 10 hours, the full discharging amount is 500 mAh, which is designated a dischargeable amount. For a time until a discharge-terminating voltage is reached so that the voltage between both terminals of a battery becomes IV per cell, 10 mA per hour is discharged and the discharged amount after ten hours is, for example, I D × T D = 100 mA, the residual capacity of a battery at this time, or the remaining discharge amount, is indicated as 500 - 100 = 400 mAh. In the prior art method and device for indicating the residual capacity of a battery, it is assumed that all of the charged amount can be discharged. This causes a problem that the charging amount and the discharging amount are not linearly related on a 1 to 1 ratio based on the charging efficiency or the surrounding environment and an error in the indication of the remaining capacity may occur in the last stage of the discharge processes. From EP-A-0 225 106 a method for representing a correct residual capacity of a battery is known, comprising the steps of measuring a charging amount in a charging mode where a charger is connected to the battery; adding, if the battery voltage is below a maximum voltage, said measured charging amount to a present charging amount to obtain the present charging amount and thereby indicating a ratio of the present charging amount to a fully charge state; and judging, if the battery voltage is above said maximum voltage, the present charging amount as the fully charged amount and representing this amount to be a fully charged state. This document moreover discloses the steps of determining, whether a charger is connected to the battery; measuring the voltage and the temperature of the battery; measuring a change of charge in the discharge mode; and subtracting this change of charge from the stored charge value. From Patent Abstracts of Japan, volume 16, No. 37 (E-1160) corresponding to JP-A-3 245 741 a battery charger is known that provides for shifting the charging current from a high level in full charging mode to a low level in trickle charging mode upon detection of a full charge through a full charge detector. SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and device for continuously indicating an accurate residual capacity of a battery. According to the invention this object is solved as defined in claims 1, 3 and 5, respectively. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flowchart showing a charging mode in the method for representing the residual capacity of a battery in accordance with the present invention; Fig. 2 is a flowchart showing a first discharge mode in the method in accordance with the present invention; Fig. 3 is a flowchart showing a second discharge mode in the method in accordance with the present invention; Fig. 4 is an explanatory view showing charging amount and discharging amount in the present invention; Fig. 5 is a block diagram showing a first example of the device for representing a residual capacity of a battery in accordance with the present invention; Fig. 6 is a block diagram showing a second example of the device in accordance with the present invention; Fig. 7 is a flowchart showing an embodiment having a charging mode in the present invention; Figs. 8A and 8B form a flowchart showing an embodiment having a first discharge mode in the present invention; Figs. 9A and 9B form a flowchart showing an embodiment having a second discharge mode in the present invention; and Fig. 10 is a selection table of the discharge efficiency of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a view showing the steps of the method for indicating the residual capacity of a battery in accordance with the present invention. First, a charging mode (S001), in which a charger having a function for detecting a fully charged state is connected to a battery, is initiated as shown in Fig. 1. Then, the charging current value (or charging amount) of a current flowing from the charger in the charging mode (S001) to the battery, is measured (S002). If the charging amount is larger than a trickle charging amount (S003) (or an estimated value for completion of charging), a new present charging amount is obtained (S004) by adding the present charging amount (which is 0 at the starting time) and the measured charging amount (S002). Since the fully charged amount is judged as a charging amount which has been determined in a preceding charging mode, present charging amount is divided by the predetermined fully charged amount, to be represented as a ratio of the division (S005). If the charging amount is less than a trickle charging amount, since it is considered that the charging operation has already been completed, the present charging amount represents a fully charged state (S006). This fully charged amount is equivalent to the maximum charging amount, as will be shown in an upper part of Fig. 4 where a battery is compared to a bucket. Figure 2 is a view showing the steps of a discharging mode, after the charging mode operation is completed, as shown in Fig. 1. A first discharging mode (S011) is a mode where the charger is removed from the battery after the battery has been charged in the charging mode as shown in Fig. 1. In this mode of step S011, a discharge efficiency α is found by dividing a latest dischargeable amount by a fully charged amount. The discharge efficiency α is a value denoting that to that extent the charged battery will discharge depending on surrounding conditions. The full charge amount used to find the discharge efficiency α is a charging amount after trickle charging as shown in Fig. 1 and Fig. 4(a), and the latest dischargeable amount is equivalent to the dischargeable amount from which is subtracted a remaining discharge amount (an amount that could not be discharged in the preceding discharge mode when the voltage of the battery has reached a discharge-terminating voltage). Regarding the discharge efficiency α thus obtained, the present remaining discharge amount (refer to Fig. 4(b)) can be obtained by multiplying the present charging amount (a value determined by the charging mode) by the above-noted discharge efficiency α. Meanwhile, the discharge current value or the discharging amount is measured and an operation in which the above measured discharge amount is subtracted from the present remaining discharge amount is effected to obtain a new present remaining discharge amount (S015). Further, the above operation when the above estimated present remaining discharge amount (S015) is divided by the latest dischargeable amount (S012) is effected to enable the residual amount of the battery to be denoted as a percentage. (S016). In Fig. 2, in the discharge mode, when the discharge mode changes back to the charging mode, before the value of the voltage of the battery between both terminals reaches a discharge-terminating voltage, the present remaining discharge amount is divided by the above discharge efficiency α to derive the present charging amount, so the converted value can be used in the charging mode. Fig. 3 is a view showing a series of steps in a second discharge mode. In the discharge mode in Fig 2, it is assumed that if the battery is used in the same environment, the discharge efficiency α is constant, whereas in the case of the discharge mode shown in Fig. 3, it should be noted that the ratio of the discharge amount to the charging amount varies depending on the surrounding temperature or the discharge current. Therefore, not only the discharge current value (discharge amount), but also the temperature is measured and the discharge efficiency is determined from the measured discharge amount and temperature. The discharge efficiency in Fig. 3 is not obtained by calculation as in Fig. 2, but the discharge efficiency, which is a function of the discharge amount and the temperature, is stored and the discharge efficiency corresponding to the respective variable of the function is read out. Thus, the discharge efficiency as determined above is multiplied by a sampling period to form a variable sampling period. For example, the discharge efficiency may worsen if the surrounding temperature is low and the discharge amount is large. Therefore, the sampling period is shortened as necessary and the above measured discharge amount is subtracted from the present remaining discharge amount by the shortened sampling period to obtain a new present remaining discharge amount. Then, the obtained present remaining discharge amount is divided by the fully charged amount depending on the required charging time to represent the remaining battery discharge amount (%). Also, in the discharge mode of Fig. 3, when the voltage value of the battery between both terminals reduces to the discharge-terminating voltage, if the present remaining discharge amount is zero, the present charging amount in the charging mode can be regarded as zero. Figure 5 is a view showing a device for performing the method for representing the residual capacity of a battery in accordance with the present invention as shown in Figs. 1 and 2. In Fig. 5, reference numeral 1 denotes a battery, 2 a charger having a function for detecting a fully charged state, 3 a current-to-voltage conversion circuit which detects a charging current value (charging amount) flowing from the charger 2 to the battery 1 during charging or a discharging current value (discharge amount) from the battery 1 when the charger 2 is removed and converts the value into a corresponding voltage value, 4 an analog-to-digital converter for outputting a current value represented by a voltage value from the current-voltage conversion circuit 3 and voltage value between both terminals of the battery 1, in the form of a digital signal, 5 an operation means or a microprocessor unit (MPU) means for controlling the operation in the charging mode shown in Fig. 1 and the discharge mode shown in Fig. 2, upon receiving a digital current value and digital voltage value from the A/D converter 4, and 6 a display unit for displaying the result of an operation carried out by the operation means 5. The operation means 5 is formed such that the mounting or unmounting of the charger 2 can be detected and in order to enter the charging mode shown in Fig. 1, the charged current value is obtained from the voltage value output by the A/D converter 4 in the case of the charging mode where the charger 2 is connected therewith. If the required charging current value is more than a trickle charging amount, the present charging amount is defined by a sum of the present charging amount and the charging current value, and a ratio of the full charging amount to the present charging amount is output to the display unit 6. On the other hand, if the required charging current value is less than the trickle charging amount, the above present charging amount is regarded as the full charging amount to be updated and a fully charged state is indicated by the display unit 6. In order to enable the discharge mode shown in Fig. 2, the present charging amount is multiplied by a discharge efficiency which is the ratio of the latest dischargeable amount to the fully charged amount, to calculate the present remaining discharge amount. At the same time, the discharge current value is calculated by the current-to-voltage conversion circuit 3 and A/D converter 4, and the calculated discharge current value is subtracted from the calculated present remaining discharge amount, and then the ratio of the present discharge amount to the latest dischargeable amount is indicated by the display unit 6. Then, when the voltage value of the battery 1 which was detected through the A/D converter 4 reaches the discharge-terminating voltage, the latest dischargeable amount is obtained from the present remaining discharge amount. When the operation means 5 returns to the charging mode before the voltage value reaches the discharge-terminating voltage in the discharge mode, the present remaining discharge amount is divided by the discharge efficiency to convert the quotient into the present charging amount. Figure 6 is a view showing a device for initiating the charging mode shown in Fig. 1 and the discharge mode shown in Fig. 3. The difference between the present device and the device shown in Fig. 5 is that a temperature detection means 7 is provided with the output signal thereof being supplied to the A/D converter 4, and the temperature data is transferred to the operation means 5. In order to operate in the discharge mode shown in Fig. 6, the operation means (or MPU) 5 receives a discharge current value in the case of the discharge mode where the charger 2 is not connected to the battery 1, from the current-to-voltage conversion circuit 3 and the A/D converter 4. At the same time, the operation means 5 receives the temperature data through the A/D converter 4, and a present remaining discharge amount is calculated by subtracting the discharge current value from the present remaining discharge value, and e ratio of the present charging amount to the fully charged amount in the charging mode is indicated by the display unit 6. Figure 7 is a flowchart showing the operation of an embodiment having a charging mode shown in Fig. 1 that includes an operational algorithm which is executed by a microprocessor (MPU) 5 in the operation means shown in Fig. 5. The operation of the display device showing the residual capacity of a battery shown in Fig. 5 and the method for representing the residual capacity of the battery shown in Fig. 1 will be described. A microprocessor 5 determines whether or not a charger 2 is provided (Step S1). When the charger 2 is not provided or connected, the process transfers to the discharge mode (Subroutine T0), whereas when the charger 2 is provided or connected, the process proceeds to a step of measurement of a charging current value (Charging amount) through the current-to-voltage conversion circuit 3 and the A/D converter 4 (Step S2). Then, the charging operation proceeds and the process discriminates whether a trickle charging has begun or not, that is, whether or not charging has been completed, from the charging current value measured in the step S2 (Step S3). As described at first, since the present charger is equipped with a sensor for detecting a fully charged state, if a state of trickle charging is entered, the charging current will always change. In the prior art trickle charger, a microcurrent always flows from the beginning of a charging process, and it is difficult to discriminate whether or not trickle charging has begun. However, with a charger having the function for detecting a fully charged state, it is possible to discriminate easily whether or not trickle charging has begun. If trickle charging has not started, the normal charging operation continues. Therefore, the addition of the present charging amount (which was regarded as being zero when the charging started), to the measured charging amount obtained in the step S2 enables calculation of the present charging amount (Step S4). The present charging amount obtained in step S4 is then divided by the fully charged amount (which means a charging amount of a constant value that is given in advance, and thereafter values are obtained by the fully charged remaining amount in the preceding discharge mode) to indicate the residual battery capacity in %. (Steps S5 and S6). In the above-described Steps S1 to S6, the charger 2 is mounted and the process is continued until the trickle charging begins. When the trickle charging begins, the present charging amount at that time point obtained by step S4 is judged as the fully charged amount (Step S7), to represent a fully charged state (Step S8), and whether or not the charger 2 is provided or connected is determined (Step S9). Then, at a step where the charger 2 is removed, a discharge mode (Subroutine T0) is initiated. Fig. 8 shows an embodiment having a first discharge mode shown in Fig. 2 and is a flowchart showing an operational algorithm which is executed by a microprocessor 5 shown in Fig. 5. The flowchart illustrates a method for indicating the residual capacity of a battery shown in Fig. 2 and the operation of the display device for displaying the residual capacity of the battery shown in Fig. 5. First, the microprocessor 5 divides a latest dischargeable amount by a full charging amount to obtain a discharge efficiency α (Step T1). The latest dischargeable amount defining the discharge efficiency α, is obtained by step T8 described hereinafter. The fully charged amount shows the updated fully charged amount shown in step S7 of Fig. 7. If the discharge efficiency α obtained above is multiplied by the present charging amount (Step T2), an equivalent present remaining dischargeable capacity will be obtained. In this case, the present charging amount is the present charging amount obtained in step S4 of Fig 7, and if the operation in the charging mode in Fig. 7 is performed up to the point of trickle charging, the present charging amount will be the full charging amount. Thereafter, the microprocessor 5 measures a discharge current value through a current-to-voltage converting circuit 3 and an analog-to-digital converter 4 (Step T3). The discharge amount measured in step T3 is subtracted from the present remaining discharge amount obtained in the above step T2 to be able to calculate the present remaining discharge amount (Step T4) and if the present remaining discharge amount is divided by the latest dischargeable amount, the present remaining discharge amount of the battery can be represented by a percentage (%) (Step T5). Then, whether or not the discharge mode in which the charger 2 is removed continues is discriminated (Step T6), and if the charger 2 is still removed, the voltage between both terminals of the battery 1 is fetched through the analog-to-digital converter 4 and whether or not the voltage is a discharge-terminating voltage, that is, whether or not discharge has been completed, is discriminated (Step T7). If the voltage has not reached a value of a discharge-terminating voltage, the process returns to the step T3 and the same operation is repeated. When it becomes clear that the voltage has reached the discharge-terminating voltage in step T7, the present remaining discharge amount (Step T4) is subtracted from the latest dischargeable amount and the discharge amount that is necessary between a period from a state of the present latest dischargeable amount, viz., the fully charged amount (Step S7) until discharge has been completed, can be obtained by a remaining discharge amount when discharge has not yet been completed (Step T8). Then, the process further checks whether or not the charger 2 is provided (Step T9), and transfers to a charging mode (Subroutine S0). On the other hand, when it becomes clear in step T6 that the process has transferred to a charging mode in which the charger 2 is provided, the present remaining discharge amount (Step T4) is divided by the discharge efficiency α (Step T1) to obtain the present charging amount, and it is left in a converted state so that when the process changes to a charging mode (Subroutine S0), it can be used as the present charging amount. Figure 9 shows an embodiment having the second discharge mode denoted in Fig. 3, and is a flowchart showing an operational algorithm that is executed by the microprocessor 5 in the device shown in Fig. 6. As shown in the flowchart, a method for indicating the residual capacity of a battery shown in Fig. 3 and the operation of an embodiment of the display device for displaying the residual amount of the battery shown in Fig. 6 is carried out. First, the microprocessor 5 reads a temperature signal from a thermistor 7 of a temperature detection means as temperature data converted by the analog-to-digital converter 4 to effect a temperature measurement (Step T11), and measures the discharge current value as illustrated in Fig. 8 (Step T12). The discharge efficiency is determined by the thus measured temperature and a discharge current valve (Step T13). The value of the discharge efficiency is given as a ratio when a period of an integrating operation in the case of charging shown in Fig. 7 (Step S4) is 1, whereas in the case of discharge a discharge efficiency E is multiplied by a sampling period τ to produce an abridged sampling period τE (Step T14). An example of a selection table for obtaining the discharge efficiency E is shown in Fig. 10. In the figure, at a position where the discharge efficiency is 1.0, discharge is effected for the same period as in the charging mode, but in the case of a ratio other than 1, the period is shorter than the period in the case of charging. For example, in the case where the surrounding temperature is 15°C and the discharge current value is 200 mA, the discharge efficiency E is equal to 0.9. The sampling period τE is set in this way, and the measured discharge amount is subtracted from the present remaining discharge amount or the present charge amount for the sampling period, and the present discharge amount is obtained (Step T15). In other words, since a sampling period changes according to temperature and the discharge current value, the present charge amount is calculated more frequently and therefore, a more accurate present charge amount can be obtained. If the so obtained present charge amount is divided by the full charging amount which is updated by the step S7 in Fig. 7, to obtain the present residual capacity of the battery denoted by a percentage and to represent it by the display unit 6 (Step T16 and T17). Further, the operation means discriminates whether or not the charger 2 is provided (Step T18), and when the charger is provided, the process transfers to the charging mode (Subroutine S0), whereas when the charger is not provided, since the discharge mode is continuing, the operation means further monitors the voltage between both terminals of the battery 1. If the voltage between both terminals is not a discharge terminating voltage, the process returns to the above step T11 and the same steps are repeated. If the voltage between both terminals reaches a discharge-terminating voltage, the present charge amount (Charging amount) is made zero (Step T20), and if the charger 2 is provided, the process transfers to the charging mode (Step T21, S0). Therefore, the present device has a self-learning function in accordance with the latest present charge amount and the pattern of use can represent a correct present charge amount under certain conditions.

Claims: A method for representing a correct residual capacity of a battery, comprising the steps of: measuring (S002) charge amounts in a charging mode, where a charger having a function for detecting a fully charged state and which provides a trickle charging current after detection of the fully charged state, is connected to the battery, and if the charging current in the charging mode is more than a trickle charging current (S003), adding up said measured charge amounts (S004) to obtain a present charge amount; indicating (S005) a ratio of the present charge amount to a full charge amount; if the charging current in the charging mode is not more than said trickle charging current (S003) judging (S006) and indicating the present charge amount as the full charge amount representing a fully charged state; measuring (S014, T3) a discharge amount in a discharge mode in which said charger is not connected to said battery; multiplying (S013, T2) the present charge amount by a discharge efficiency (α) represented by a ratio of a latest dischargeable amount to the full charge amount to calculate the present remaining discharge amount; subtracting (S015, T4) said measured discharge amount from said calculated present remaining discharge amount to calculate an updated present remaining discharge amount and indicating (S016, T5) the ratio of said present remaining discharge amount to said latest dischargeable amount; and measuring a voltage value of said battery and obtaining (T8), when said voltage value of said battery (1) reaches a discharge terminating voltage, an updated latest dischargeable amount by subtracting said present remaining discharge amount that could not be discharged in the preceding discharge mode from the previous latest dischargeable amount. The method according to claim 1, further comprising the step of: dividing (T10) said present remaining discharge amount by said discharge efficiency (α) when changing from said discharge mode to said charging mode (T6) before said voltage reaches the discharge-terminating voltage, to obtain said present charging amount. A method for representing a correct residual capacity of a battery, comprising the steps of: measuring (S002) charge amounts in a charging mode, where a charger having a function for detecting a fully charged state and which provides a trickle charging current after detection of the fully charged state, is connected to the battery, and if the charging current in the charging mode is more than a trickle charging current (S003), adding up said measured charge amounts (S004) to obtain a present charge amount; indicating (S005) a ratio of the present charge amount to a full charge amount; if the charging current in the charging mode is not more than said trickle charging current (S003) judging and indicating (S006) the present charge amount as the full charge amount representing a fully charged state; measuring (S022, T12) a discharge current in a discharge mode in which said charger is not connected to said battery; setting a discharge efficiency (E) depending on temperature and the discharge current value; subtracting (T15) said discharge current value from said present charge amount at a rate depending on said discharge efficiency, the corresponding variable sampling period being obtained by multiplying a predetermined sampling period (τ) by the discharge efficiency (E), to calculate an updated present charge amount; and representing (T16) the ratio of the present charge amount to the full charge amount determined in the charging mode. The method according to claim 3, wherein when the voltage value of said battery reduces to the discharge terminating voltage, the present remaining discharge amount is set to zero (T20). A device for indicating a residual capacity of a battery (1) comprising: said battery (1); a current-to-voltage converting circuit (3) for converting a charging current value in the case where said charger (2) is connected to said battery (1) to a voltage value and for converting a discharge current to a voltage value in the case where said charger is not connected to said battery (1); an analog-to-digital converter (4) for converting said voltage value from said converting circuit (3) to a digital value and for converting a voltage value of the battery (1) to a digital value; a display unit (6); a charger (2) having a function of detecting a fully charged state and which provides a trickle charging current after detection of the fully charged state; characterized by operational means (5) for detecting whether or not said charger (2) is connected to said battery (1) and whether said charger (2) is in the charging mode when said charger (2) is connected to said battery (1), and adapted for carrying out the method according to any one of the preceding claims. The device according to claim 5 adapted for carrying out the method according to claim 3, wherein a temperature measuring means (7) is provided to be connected to said analog-to-digital converter (4), and said operation means (5) is adapted to receive a discharge current value in a discharge mode where said charger (2) is not connected to said battery (1) from said current-to-voltage converting circuit (3) and said analog-to-digital converter (4) and adapted to receive said temperature through said analog-to-digital converter (4) and adapted to indicate on said display unit (6) the ratio of the present charging amount to the full charging amount determined in the charging mode.

Inventors: GOTOH SEIJI, ORITA KENJI, TAKAYAMA TOMIO, GOTOH, SEIJI, ORITA, KENJI, TAKAYAMA, TOMIO, Gotoh, Seiji, c/o FUJITSU KYUSHU DIGITAL, Orita, Kenji, c/o FUJITSU KYUSHU DIGITAL, Takayama, Tomio, c/o FUJITSU KYUSHU DIGITAL
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G01R31
IPC codes: H02J7, H01M10, G01R31