JPH06189466A - Secondary battery system and its charging method - Google Patents

Abstract

PURPOSE:To avoid overcharge and prolong a charging/discharging cycle life by a method wherein a time differential value calculated from the charging current value of a charging circuit is compared with a predetermined comparing value and a charging operation is discontinued in accordance with the comparing and judging result. CONSTITUTION:A power is supplied to a battery 15 composed of lithium cells in which organic electrolyte is employed to charge the battery from an AC power supply 11 through a rectifier 12 and by a charging circuit 21. At that time, a charging current is measured periodically at certain time intervals by a current measuring circuit 22 and a clock circuit 24. The differential value of the charging current is calculated by a processing part 25 from the difference between two successive measured values or from the difference between two predetermined measured values. The calculated differential values of the charging current are successively compared with a predetermined comparing value by a comparing/judging circuit 26 and, when the differential value of the charging current exceeds the comparing value, a charging operation is discontinued by a switch 16. With this constitution, a full-charge state can be accurately detected and overcharge can be avoided, so that the decomposition of the organic electrolyte can be suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary battery system and a charging method, and more particularly to a secondary battery system and a charging method having a charging circuit connected to a plurality of batteries.

[0002]

2. Description of the Related Art Applications of large secondary batteries include transportation-related power sources such as electric vehicles, emergency power sources such as uninterruptible power sources (abbreviated UPS), and power storage batteries for leveling power loads. . Of these, the power storage battery stores (charges) surplus power at midnight in the secondary battery and discharges (discharges) it at the peak of daytime power demand. Similarly, in an electric vehicle, the battery is charged by using the electric power at midnight.

In the operation of these batteries, for example, as illustrated in the distributed power leveling system described in Japanese Patent Application Laid-Open No. 3-226233, a peak cut mode is set for daytime power demand. drive.

[0004]

When operating in the peak cut mode, if the power consumption in the daytime is small, the depth of discharge is not 100%, and the battery is charged at night with some battery capacity left. Migrate in some cases. In such a case, in a conventional secondary battery using an aqueous electrolyte such as a lead or nickel-cadmium battery, a mechanism for absorbing and recombining gas generated in an overcharged state works. Therefore, even if the charging current is continuously supplied in the fully charged state, the adverse effect on the battery is small. Charge a shallow discharge depth battery (when the remaining battery capacity is relatively large) and a deep discharge depth battery (where the remaining battery capacity is relatively small, the battery capacity is almost exhausted) under the same charging conditions. However, no inconvenience occurs.

On the other hand, when an organic electrolyte type lithium secondary battery is used as a driving power source for these systems,
Since the energy density is high, there is an advantage that the battery weight and volume can be reduced. However, in an electrolytic solution using an organic compound as a solvent, if charging in an overcharged state is continued for a long time, decomposition of the organic compound as a solvent occurs, and various hydrocarbons are generated by fragmentation (cleavage reaction) of the molecule of the organic compound. , Carbon monoxide, carbon dioxide, hydrogen, etc. are generated, but unlike the case of the aqueous electrolyte solution, it is difficult to recombine these gases with the original organic compound. The organic electrolytic solution has a unique stable potential width depending on its composition, that is, a potential range in which the electrolytic solution can exist stably. When constructing an organic electrolyte type lithium secondary battery, it is desirable that the operating potentials of the positive electrode and the negative electrode are included within this stable potential width. During charging, the potential of the positive electrode rises with the progress of charging, so the charging voltage should not exceed the upper limit of the stable potential width.

Therefore, as a method of charging an organic electrolyte type lithium secondary battery, a constant current constant voltage charging method (constant current charging is performed at the initial stage of charging and constant charging is performed after the charging voltage reaches a set value). The value of the charging current decreases as the charging progresses and the charging progresses.) Is suitable. This is a charging method in which the upper limit of the charging voltage is suppressed below the upper limit of the stable potential range of the organic electrolyte to be used in order to suppress the decomposition of the organic electrolyte.

Conventionally, a method has been mainly used in which a charging time from the start of charging is set, constant current and constant voltage charging is performed, and the charging is terminated when the total time reaches a set value. In this method, the fully charged state is indirectly known based on the charging time. Therefore, when charging is performed under the same conditions regardless of the depth of discharge of the battery before shifting to charging, an overcharged state is reached when the depth of discharge is shallow, and the decomposition reaction of the electrolytic solution proceeds. Therefore, the amount of the electrolytic solution is reduced to cause liquid dripping, the internal resistance of the battery is increased, the battery capacity is reduced, and the battery life is shortened.

FIG. 4 shows how the charging current changes in the next charging depending on whether the discharging depth in the immediately preceding discharging is shallow or deep. As in the conventional case, when charging is performed by setting a constant charging time, when constant current constant voltage charging is performed in a state where the depth of discharge is deep, the time required for constant current charging becomes long, and the constant voltage charging period is It gets shorter. However, when constant-current constant-voltage charging is performed in a state where the depth of discharge is shallow, the constant-voltage charging starts in a short time after the start of charging, the constant-voltage charging period becomes longer, and a constant current continues to flow afterwards. The state appears. When the floating state continues, the charging current flows for a long time, so that the decomposition of the electrolytic solution becomes remarkable and the battery performance may deteriorate.

An object of the present invention is to connect a plurality of batteries,
In a secondary battery system and a charging method provided with a charging circuit, a secondary battery system and a charging method are provided in which a full charge is accurately detected to prevent overcharging and the charge / discharge cycle life is extended.

[0010]

The present invention comprises means for comparing and determining a time differential value calculated from a charging current value of a charging circuit and a preset value, and stopping charging according to the determination result. Characterize. Further, while charging with a predetermined voltage, the charging current flowing in the charging circuit is measured to calculate a time differential value, and the time differential value and a preset value (preferably a value smaller than 0). And when the time differential value exceeds the set value, it is detected that the battery is fully charged, or the charging is terminated.

For example, in the case of a battery in a distributed power leveling system, the upper limit of the charging current is preferably about the current (0.125 C) equivalent to 8 hours of constant current charging. This is effective in effectively utilizing the surplus power at midnight by making the current almost flat during the charging period. In applications that require charging in a shorter time, it is necessary to change the current setting. It is desirable to set the upper limit of the charging voltage to a value lower than the upper limit of the stable potential width of the organic electrolyte used.

[0012]

[Operation] FIG. 3 shows how the charging voltage and the charging current change. After the transition from constant-current charging to constant-voltage charging, the charging current continues to decrease as the charging progresses, and becomes a float state (a state in which a substantially constant charging current flows and a substantially constant charging current flows).

When the time differential value of this charging current is calculated by the calculating means, it becomes as shown in FIG. The differential value becomes a negative value at the time of shifting to the constant voltage charging state, and then continues to rise and becomes close to 0 in the float state. When the differential value exceeds the set value by comparing the differential value with the set comparative value by the comparison / determination means, it can be easily detected that the battery is fully charged, and the charging can be terminated. It is desirable to set the comparison value in consideration of preventing the battery performance from deteriorating due to the overcharged state after the full charge.
For example, it may be set to a value smaller than 0 and a little smaller than the value at which it is considered that the floating state is caused by the change with time of the differential value. In the present invention, when it is determined that charging is completed, the charging current has become sufficiently small and the difference between the charging voltage and the open circuit voltage can be almost ignored, so it can be accurately determined that the battery is fully charged, and It is possible to prevent overcharging and shifting to an overcharged state.

[0014]

EXAMPLES The present invention will be described more specifically by way of examples. An example of the charging circuit according to the present invention is shown in FIG. By the charging circuit 21 from the AC power source 11 through the rectifier circuit 12, the battery group 15 composed of a lithium battery using an organic electrolytic solution is supplied with electric power to be charged. At this time, the charging current is measured by the current measuring circuit 22. The timing of charging current measurement is set by the clock 24, the charging current is measured at constant time intervals, and the differential value of the charging current is calculated by the calculation unit 25 from the difference between two consecutive measurement values or the difference between predetermined measurement values. To do. Separately, the timer 27 controls the charging time. FIG. 2 shows an example of changes with time of the differential value of the charging current calculated by the calculation unit 25. The differential value is 0 during constant current charging, and the charging voltage rises during this period, and when the preset charging upper limit voltage is reached, it shifts to the constant voltage charging state. At this time, the differential value of the charging current becomes a finite negative value. At this point, the charging control is shifted to the control based on the differential value of the charging current. The derivative value begins to increase with the passage of time,
Finally, it becomes almost 0 in the float charge state. The differential value of the calculated charging current is sequentially compared and judged by the comparison / determination circuit 26 with a preset value, and when the differential value of the charging current exceeds the set value, charging is stopped by the switch 16. If no change is detected in the differential value of the charging current during the charging period, the control by the time is continued as it is, the charging is continued for a predetermined time, and the charging is finished.

According to this method, it is possible to estimate the capacity of the battery and perform charging by the change in the differential value of the charging current, and it is possible to detect full charge. Therefore, regardless of the size of the remaining capacity of the battery before charging, it is possible to accurately charge the battery to the predetermined amount of electricity defined as the nominal capacity.

[0016]

According to the present invention, the full charge can be accurately detected, and the battery can be charged without exceeding the full charge and generating an overcharged state, so that the decomposition of the organic electrolyte is suppressed, It is possible to extend the charge / discharge cycle life of the battery.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a configuration of a charge control circuit in a secondary battery system.

FIG. 2 is a schematic diagram of changes with time of a differential value of a charging current in constant-current constant-voltage charging.

FIG. 3 is a schematic diagram of changes with time of charging current and voltage in constant current voltage charging.

FIG. 4 is a schematic diagram showing the influence of the depth of discharge on the changes over time of the charging current and voltage in constant current constant voltage charging.

[Explanation of symbols]

1 ... Constant current charging state, 2 ... Constant voltage charging state, 3 ... Charging current change with time, 4 ... Battery voltage change with time, 5 ... Deep discharge depth of battery, 6 ... Shallow discharge depth of battery When charged, 11 ... AC power supply, 12 ... Rectifier circuit, 15
... battery group, 16 ... switch, 21 ... charging circuit, 22 ... current measuring circuit, 24 ... clock, 25 ... arithmetic unit, 26 ... comparison / determination circuit, 27 ... timer.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Akihiro Goto 7-1-1 Omika-cho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Tatsuo Horiba 7-chome, Omika-cho, Hitachi-shi, Ibaraki No. 1 Hitachi Ltd. Hitachi Research Laboratory

Claims (4)

[Claims] 1. A secondary battery system including a battery group in which a plurality of lithium secondary batteries using an organic electrolytic solution are connected, and a charging circuit for charging the battery group, and calculated from a charging current value of the charging circuit. A secondary battery system comprising means for stopping charging according to a comparison determination result of the time differential value and a preset comparison value. 2. In a secondary battery system including a battery group in which a plurality of lithium secondary batteries using an organic electrolyte are connected, and a charging circuit for charging the battery group, the charging current of the charging circuit is measured. A current measuring unit, a calculating unit that calculates a time differential value from the charging current, a comparison determining unit that compares and determines the time differential value and a preset comparison value, and a unit that stops charging according to the determination result. A secondary battery system characterized by being provided. 3. A charging method for a secondary battery system comprising a charging circuit, wherein a plurality of lithium secondary batteries using an organic electrolytic solution are connected, and a charging current flowing through the charging circuit while charging at a predetermined voltage. Is calculated to calculate a time differential value, and the time differential value is compared with a preset comparison value,
A method for charging a secondary battery system, comprising detecting that the battery is fully charged when the time differential value exceeds a set value. 4. A charging method for a secondary battery system comprising a charging circuit, wherein a plurality of lithium secondary batteries using an organic electrolyte are connected, and a charging current flowing through the charging circuit while charging at a predetermined voltage. Is calculated to calculate a time differential value, and the time differential value is compared with a preset comparison value,
A charging method for a secondary battery system, wherein charging is terminated when the time differential value exceeds a set value.