JP2015230742A - Lithium ion battery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique that enhances the detection precision of deposition of lithium.SOLUTION: In a lithium ion battery system, the absolute humidity in a battery case in which a battery cell is mounted is calculated from the temperature and humidity in the battery case by temperature and humidity measuring processing S2 and absolute humidity calculation processing S4 in battery deterioration monitoring processing executed by a battery monitoring ECU, and the integration amount of a water amount infiltrated into the battery case is calculated on the basis of predetermined relationship between the water amount infiltrated into the battery case per unit time and the absolute humidity in the battery case by water infiltration amount calculation processing S6 and water infiltration amount integration processing S8. Determination processing S10 based on the infiltration amount integration value compares the integration amount of the infiltrated water amount with a permissible limit amount over which lithium will deposit, and estimates that lithium deposits when the integration amount exceeds the permissible limit amount.

Description

  The technology disclosed in this specification relates to a battery system that estimates lithium deposition of a lithium ion battery cell.

  In an electric vehicle including a hybrid vehicle, a battery (secondary battery) for driving force source is mounted in order to supply electric power to a traveling motor. The output power of the battery is converted into electric power suitable for motor driving by the power conversion device and supplied to the traveling motor, and the traveling motor generates driving force. Further, at the time of braking, the battery is charged by the power converter upon receiving regenerative power from a traveling motor that functions as a generator. The battery is repeatedly charged and discharged in this manner, and deteriorates accordingly. As a system for managing a battery, Patent Document 1 below discloses a technique for acquiring a battery parameter from a management unit connected to a battery cell and calculating a deterioration state value of the battery cell using the acquired parameter. ing.

JP 2011-67047 A

  By the way, at present, lithium ion batteries are widely used as batteries mounted on electric vehicles. Lithium ion batteries may deposit metallic lithium on the surface of the negative electrode (graphite) due to repeated charge and discharge. When metallic lithium is deposited, lithium ions used during charging and discharging are reduced, and battery performance (for example, cycle characteristics) is lowered. Therefore, in a battery system that manages lithium ion batteries, lithium deposition should be managed, but the above-mentioned Patent Document 1 does not mention lithium deposition. The present specification provides a technique for accurately estimating that lithium deposition has occurred.

  The lithium ion battery system disclosed in this specification includes an absolute humidity calculation unit, a moisture intrusion amount calculation unit, and a lithium deposition estimation unit. Then, the absolute humidity calculation unit calculates the absolute humidity in the battery pack from the temperature and humidity in the battery pack housing the lithium ion battery cell. Next, from the absolute humidity calculated by the moisture intrusion amount calculation unit based on a predetermined relationship between the absolute humidity in the battery pack and the amount of moisture that enters the battery pack per unit time (moisture intrusion amount). The integrated amount of moisture that has entered the battery pack is calculated. Finally, the lithium deposition estimation unit compares the accumulated amount of moisture that has entered the battery pack with the allowable limit amount that would lead to lithium deposition, and if the integrated amount exceeds the allowable limit amount, Presumed to have precipitated. The relationship between the absolute humidity in the battery pack and the amount of moisture that permeates into the battery pack per unit time, and the “allowable limit amount that leads to lithium deposition beyond that” have been determined in advance. It is stored in the management controller of the ion battery system.

  Lithium ion batteries are known to be liable to deposit lithium as the moisture content increases and the temperature decreases. Accordingly, it is estimated that lithium is deposited when the integrated amount exceeds the allowable limit amount based on the amount of water in the battery pack (water intrusion amount). Therefore, the detection accuracy of lithium deposition is improved as compared with detection based on parameters such as current and voltage.

  The technology disclosed in the present specification provides a lithium ion battery system with high accuracy for estimating the occurrence of lithium deposition. Details of the technology disclosed in this specification and further improvements will be described in the embodiments of the present invention.

It is a block diagram which shows the structural example of a hybrid vehicle. It is a perspective view which shows the structural example of a battery. It is a flowchart of a battery deterioration monitoring process. It is explanatory drawing which shows the example of a relationship between temperature / humidity and absolute humidity. (A) is explanatory drawing which shows the example of a relationship of the water permeation amount with respect to absolute humidity, (B) is explanatory drawing which shows the example of the relationship between the absolute humidity per unit time and a water permeation amount.

  A lithium ion battery system according to an embodiment will be described with reference to the drawings. In this embodiment, the battery system is applied to a battery monitoring system mounted on an electric vehicle. The electric vehicle includes an electric vehicle including a traveling motor but not including an engine, and a hybrid vehicle including both a traveling motor and an engine. In this embodiment, a hybrid vehicle will be described as an example.

  First, the configuration of the hybrid vehicle 1 will be briefly described with reference to FIG. FIG. 1 is a block diagram illustrating a configuration example of the hybrid vehicle 1. The hybrid vehicle 1 includes a motor 8 and an engine 6 as a driving source for traveling. The output torque of the motor 8 and the output torque of the engine are appropriately distributed / combined by the power distribution mechanism 7 and transmitted to the axle 9 (that is, the wheel). It should be noted that FIG. 1 shows only parts necessary for the description of the present specification, and some parts not related to the description are not shown.

  Electric power for driving the motor 8 is supplied from the battery 2 via a power control unit (hereinafter referred to as PCU) 5. That is, the DCU supplied from the battery 2 is converted into AC power suitable for driving the motor by the PCU 5 provided with a voltage converter, an inverter and the like, and supplied to the motor 8. Further, the ACU of the motor 8 generated by the braking force at the time of deceleration or the driving force of the engine 6 is converted into DC power suitable for charging the battery 2 and supplied to the battery 2. That is, the battery 2 is charged with regenerative power. The voltage converter and the inverter are controlled by a dedicated controller provided in the PCU 5.

  The battery 2 is a secondary battery in which a plurality of lithium ion type battery cells 10 are connected in series, and is also referred to as a battery pack or a battery stack. In the present embodiment, for example, the output voltage (nominal voltage) of the battery 2 is set to 300 volts. The battery 2 is connected to the PCU 5 via the system main relay 4. The system main relay 4 is connected to the vehicle control ECU 30 and is on / off controlled. In the present embodiment, the output current of the battery 2 is measured by, for example, a current sensor not shown. Further, the temperature and humidity in the battery case 3 (described later) of the battery 2 are measured by the temperature / humidity sensor 21. These sensors are connected to a battery monitoring ECU 20 described later.

  The temperature / humidity sensor 21 is a combined sensor combining a temperature sensor such as a thermocouple or a thermistor mount and a humidity sensor such as a capacitance change type or a resistance change type, and the measured temperature data and humidity data are monitored by a battery. It outputs to ECU20. Temperature data is expressed in degrees Celsius [° C.], and humidity data is expressed in relative humidity [%]. In the present embodiment, a composite type in which the temperature sensor and the humidity sensor are integrated into one chip is used. However, they may be individually provided in the battery case 3 by using separate components.

  The battery monitoring ECU 20 and the vehicle control ECU 30 are controllers composed of electronic components such as a memory and an input / output interface with a microcomputer as a center. These ECUs 20 and 30 are connected to each other. The vehicle control ECU 30 is connected to the system main relay 4 and the PCU 5. As a result, the battery monitoring ECU 20 outputs diagnostic information (diagnosis information) of the battery 2 to the vehicle control ECU 30. Further, the vehicle control ECU 30 outputs data corresponding to the driving situation (for example, the amount of depression of an accelerator pedal or a brake pedal) to the controller of the PCU 5. Although not shown, the hybrid vehicle 1 includes an auxiliary battery (for example, 12 volts) in addition to the battery 2, and the battery monitoring ECU 20 and the vehicle control ECU 30 are supplied with driving power from the auxiliary battery. .

  Next, the configuration of the battery 2 will be described with reference to FIG. FIG. 2 is a perspective view illustrating a configuration example of the battery 2. The battery 2 includes a prismatic battery case 3 in which a plurality of battery cells 10 are stacked and accommodated. In this embodiment, a vent 3a is formed on one end side of the battery case 3 in the longitudinal direction (Y-axis direction of the coordinate system shown in FIG. 1). Outside air is taken in from the vent 3a to cool the internal battery cell 10. In the present embodiment, a temperature / humidity sensor 21 is mounted in the vicinity of the vent 3 a in the battery case 3. The temperature / humidity sensor 21 measures the temperature and humidity of the inside air of the battery case 3 and outputs the measured data to the battery monitoring ECU 20. Thereby, from the temperature / humidity data obtained from the temperature / humidity sensor 21, the amount of moisture permeating into the battery case 3 (hereinafter, simply referred to as “water permeation amount”) is calculated in the battery deterioration monitoring process described later.

  The battery cell 10 is a square sealed lithium ion battery. The battery cell 10 is provided with a positive electrode terminal and a negative electrode terminal, and the positive electrode terminal of one adjacent battery cell 10 and the negative electrode terminal of the other adjacent battery cell 10 are electrically connected by a bus bar (not shown). Has been. Thereby, all the battery cells 10 are connected in series to obtain an output voltage of 300 volts. The battery cell 10 includes, for example, a laminated wound body in which a laminated body of a positive electrode sheet and a negative electrode sheet with a separator interposed therebetween is wound as a power generation element, that is, an electrode body. Note that the two-dot chain line portion shown in FIG. 2 is a simplified representation of a part of the plurality of stacked battery cells 10.

  By the way, a lithium ion battery may deposit metal lithium on the surface of a negative electrode by repetition of charging / discharging, for example. As described in the section “Problems to be Solved by the Invention”, it is known that precipitation of metallic lithium is more likely to occur as the moisture content increases and the temperature decreases. The deposition of metallic lithium directly decreases the battery performance (for example, cycle characteristics) by reducing lithium ions. Therefore, it is necessary to replace the battery 2 on which metallic lithium is deposited.

  Therefore, in this embodiment, the battery deterioration monitoring process shown in FIG. 3 is performed in order to detect a state where metallic lithium is estimated to be deposited in the battery 2. This battery deterioration monitoring process is realized by, for example, the battery monitoring ECU 20 executing a battery deterioration monitoring program at a predetermined cycle. For example, the battery monitoring ECU 20 executes a battery deterioration monitoring process every hour (in one hour cycle). FIG. 3 shows a flowchart of the battery deterioration monitoring process.

As shown in FIG. 3, in the battery deterioration monitoring process, first, a temperature / humidity measurement process is performed in step S2, and then an absolute humidity calculation process is performed in step S4. In the temperature / humidity measurement process (S2), temperature data and humidity data in the battery case 3 of the battery 2 sent from the temperature / humidity sensor 21 described above are acquired, and in the subsequent absolute humidity calculation process (S4), these data are acquired. Calculate the absolute humidity based on The absolute humidity referred to here is absolute volumetric humidity, and represents the amount of water vapor contained in the unit volume of the atmosphere by weight (unit: g / m ³ ).

For example, in the present embodiment, as shown in FIG. 4, the absolute humidity is calculated from an absolute humidity calculation map in which the relationship between the absolute humidity [g / m ³ ] and the temperature [° C.] is expressed by the ratio of the relative humidity [%]. To do. FIG. 4 is an explanatory diagram showing an example of the relationship between temperature and humidity and absolute humidity. In this embodiment, such an absolute humidity calculation map is obtained in advance and stored in the memory or the like of the battery monitoring ECU 20. In the example of FIG. 4, it is shown that the absolute humidity is 25.6 g / m ³ when the temperature is 40 ° C. and the humidity is 50%. The absolute humidity may be calculated using a predetermined arithmetic expression stored in advance in the battery monitoring ECU 20 without using such a calculation map. For example, the saturated water vapor pressure is obtained from the temperature, the saturated water vapor pressure is multiplied by the relative humidity to obtain the water vapor pressure of the measurement air, and the volumetric absolute humidity is further calculated based on this.

In the subsequent step S6, a moisture intrusion amount calculation process is performed. This process mainly calculates the amount of water contained in the air that has entered the battery case 3 from the vent 3a, and this also uses a moisture intrusion amount calculation map that is stored in advance in the memory of the battery monitoring ECU 20 or the like. Use to find. For example, as shown in FIG. 5A, the amount of moisture (water intrusion amount) [mg] entering the battery case 3 increases proportionally with time with a different slope for each absolute humidity. It is revealed by their experiments and computer simulations. Therefore, it can be seen that the moisture intrusion amount [mg] per unit time (for example, 1 hour) with respect to the absolute humidity [g / m ³ ] has a relationship as shown in FIG.

Therefore, in step S6, the battery deterioration monitoring process is executed every hour, and based on the absolute humidity [g / m ³ ] calculated in step S4, the battery entered after 1 hour from the previous execution. The moisture intrusion amount [mg] is obtained from the relationship map (moisture intrusion amount calculation map) shown in FIG. In addition, when the relational expression showing the relation of the moisture intrusion amount [mg] per unit time with respect to the absolute humidity [g / m ³ ] shown in FIG. 5B is obtained, this relational expression is previously stored in the battery monitoring ECU 20. You may memorize | store and use it, and you may calculate the amount of water penetration [mg].

  In the next step S8, moisture intrusion amount integration processing is performed. That is, in step S8, an integrated value of the moisture intrusion amount [mg] calculated so far is calculated. For example, when the battery deterioration monitoring process has been executed from 5 hours ago, the value calculated this time is further added to the values calculated from 5 hours ago to the most recent 1 hour ago, and all are totaled. The integrated value thus obtained is the total amount of moisture that has entered the battery case 3 so far. Note that the integrated value calculated this time may be stored in the memory or the like of the battery monitoring ECU 20, and may be read out at the next execution and used for the calculation of the integrated value.

  In step S10, based on the water intrusion amount integrated value calculated in step S8 (total amount of water intruded into the battery case 3), it is determined whether or not there is a possibility that lithium has precipitated in the battery cell 10 of the battery 2. Determine. That is, the deposition of lithium in the battery 2 is estimated. For example, the allowable moisture amount X is set to a moisture amount slightly lower than the minimum moisture amount assumed to be lithium precipitation with a low probability, and the limit moisture amount assumed to be lithium precipitation is set to the allowable moisture amount X. For example, it is set to 1.1 times (= 1.1X). This magnification is set to an optimum value for the battery 2 based on the results of experiments and computer simulations. In this case, when the moisture penetration amount integrated value is equal to or less than the allowable moisture amount X (S10: “allowable moisture amount or less”), the total amount of moisture that has entered the battery case 3 has not yet reached an amount that leads to precipitation of lithium. Therefore, it is estimated that lithium deposition has not occurred, and the battery deterioration monitoring process is terminated (end). And it prepares for execution of the next processing.

  On the other hand, when the integrated value of the water intrusion amount exceeds the allowable water amount X and falls below the limit water amount (= 1.1X) (S10: “exceeding the allowable water amount / less than the limit water amount”), Lithium precipitation is assumed with low probability. Therefore, it is estimated that lithium deposition may or may have occurred, and the process proceeds to the next step S12. In step S12, temperature measurement processing is performed. And the temperature data sent from the temperature / humidity sensor 21 is acquired, and it is determined by the following step S14 whether the temperature is -30 degrees C or less. In general, a lithium ion battery is liable to deposit lithium in an environment of −30 ° C. or lower. Therefore, the total amount of moisture that has entered the battery case 3 exceeds the allowable moisture amount X, and the battery This is because when the temperature in the case 3 is −30 ° C. or lower, there is a high possibility that lithium deposition will occur in the battery cell 10.

  When it is determined that the temperature is −30 ° C. or lower by this process (S14: YES), it is estimated that lithium has precipitated, so that the battery 2 is in a deteriorated state by the warning lamp lighting process in step S16. Is notified to the driver via the instrument panel (instrument panel) of the hybrid vehicle 1. On the other hand, when it is determined that the temperature is not −30 ° C. or lower (S14: NO), it is presumed that lithium is not deposited, and thus the battery deterioration monitoring process is terminated (end). And it prepares for execution of the next processing.

  On the other hand, when the water penetration amount integrated value is not less than the limit moisture amount (S10: “not less than the limit moisture amount”), lithium deposition is assumed with high probability. That is, when it is determined in step S10 that the integrated value of the water intrusion amount is equal to or greater than the limit water amount, the battery 2 is in a deteriorated state when the battery deterioration monitoring process is executed last time (one hour ago). Is informed to the driver through the instrument panel in step S16, and it is expected that the hybrid vehicle 1 is still running. In this case, since there is a high possibility that lithium deposition has occurred, temperature measurement processing is performed in the next step S21, and it is determined whether or not the temperature is −20 ° C. or lower in the subsequent step S23. This is because the lithium ion battery is unlikely to deposit lithium in a temperature environment exceeding −20 ° C., and the temperature is examined regardless of the total amount of moisture that has entered the battery case 3.

  And when it determines with temperature being -20 degrees C or less by this process (S23: YES), the driving | running state of the engine 6 is acquired by subsequent step S25. For example, the operating state of the engine 6 is obtained from the host system via the vehicle control ECU 30. If it is determined in step S27 that the engine 6 is stopped (S27: YES), the diagnosis information indicating that it is time to replace the battery 2 due to lithium deposition by the diagnosis output process in step S29. Is output to the vehicle control ECU 30. Accordingly, the vehicle control ECU 30 notifies the driver of information indicating that the battery 2 needs to be replaced, for example, via the instrument panel of the hybrid vehicle 1 based on the diagnosis information. On the other hand, if it is determined that the engine 6 has not stopped, that is, is in operation (S27: NO), the deposition of lithium does not affect the operation of the engine 6, and thus the battery deterioration monitoring process is terminated. (End). And it prepares for execution of the next processing.

  As described above, in the battery deterioration monitoring process executed by the battery monitoring ECU 20, the battery is detected from the temperature and humidity in the battery case 3 (battery 2) housing the battery cell 10 by the temperature / humidity measurement process S2 and the absolute humidity calculation process S4. The absolute humidity in the case 3 is calculated, and the amount of moisture entering the battery 2 per unit time and the absolute humidity in the battery 2 are obtained in advance by the moisture intrusion amount calculation process S6 and the moisture intrusion amount integration process S8. Based on the relationship, the integrated amount of moisture that has entered the battery 2 is calculated, and if the integrated amount of moisture that has entered the battery 2 is exceeded by the determination processing S10 based on the integrated amount of penetration, lithium deposition occurs. It is estimated that lithium was deposited when the integrated amount exceeded the allowable limit amount compared to the allowable limit amount. Thereby, the battery monitoring ECU 20 estimates that lithium is deposited when the integrated amount exceeds the allowable limit amount based on the moisture content in the battery 2. Therefore, it is possible to estimate with high accuracy that lithium deposition has occurred, compared to detection based on parameters such as current and voltage.

  Note that the battery deterioration monitoring process described above is executed in one-hour cycles, but the execution cycle is appropriately set according to the results of experiments and computer simulations. In the battery 2 described above, the temperature / humidity sensor 21 is mounted in the vicinity of the vent 3a of the battery case 3 to measure the inside air temperature and humidity in the vicinity of the vent 3a. Another temperature / humidity sensor 21 may be mounted in the battery case 3 away from the vent 3a so as to measure the inside air temperature and humidity. In this case, by applying the battery deterioration monitoring process described above for each temperature / humidity sensor 21, the estimation accuracy of lithium deposition is further improved. Moreover, each temperature (-30 degreeC, -20 degreeC) mentioned above is only an example, and an optimal value is set based on the result of the individual specific characteristic of the battery cell 10 which comprises the battery 2, or experiment or computer simulation. The

  Points to be noted regarding the example technology will be described. The battery 2 corresponds to an example of a battery pack. The battery monitoring ECU 20 corresponds to an example of an absolute humidity calculation unit, a moisture intrusion amount calculation unit, and a lithium deposition estimation unit. The temperature / humidity measurement process S2 and the absolute humidity calculation process S4 correspond to an example of an absolute humidity calculation unit. The moisture intrusion amount calculation process S6 and the moisture intrusion amount integration process S8 correspond to an example of a moisture intrusion amount calculation unit. The determination process S10 based on the penetration amount integrated value corresponds to an example of a lithium deposition estimation unit. The allowable moisture amount or the limit moisture amount corresponds to an example of the allowable limit amount.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. Further, the technical elements described in the present specification or drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Moreover, the technique illustrated in this specification or the drawings achieves a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

1: Hybrid vehicle 2: Battery 3: Battery case 3a: Vent 4: System main relay 5: PCU
6: Engine 8: Motor 9: Axle 10: Battery cell 20: Battery monitoring ECU
21: Temperature / humidity sensor 30: Vehicle control ECU

Claims (1)

It is a lithium ion battery system that estimates lithium deposition in lithium ion battery cells,
An absolute humidity calculator that calculates the absolute humidity in the battery pack from the temperature and humidity in the battery pack that houses the lithium ion battery cell;
Based on a predetermined relationship between the absolute humidity in the battery pack and the amount of moisture that permeates into the battery pack per unit time, the integrated amount of moisture that has entered the battery pack from the calculated absolute humidity A moisture intrusion amount calculating unit for calculating
Lithium that estimates the deposition of lithium when the cumulative amount exceeds the allowable limit amount by comparing the cumulative amount of moisture that has penetrated into the battery pack with the allowable limit amount that would lead to lithium deposition. A precipitation estimator;
A lithium ion battery system comprising:

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