JP2008204750A - Battery device with abnormality judging means - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a battery device which has a highly sensitive abnormality judging means capable of easily and surely identifying an abnormal cell or a unit containing an abnormal cell among a number of cells, thus eliminating a need of a means that monitors the operation of each of unit cells making up a battery pack. <P>SOLUTION: The battery device includes a plurality of serial connection units each consisting of a plurality of secondary cells connected in series. The battery device is provided with the abnormality judging means which monitors each serial connection unit to watch a rate of voltage change occurring along with the progress of charging, detect the presence of a maximum point on a curve of the rate of voltage change occurring along with the progress of charging in a deep charging area, and judge that the serial connection unit contains an abnormal cell. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a battery device including secondary battery abnormality determination means.

  In recent years, electric vehicles (EV), hybrid electric vehicles (HEV), and the like have attracted attention from the viewpoint of protecting the global environment. Batteries used in EVs and HEVs are mainly used as motor drives and backup power supplies, and are required to have high voltages, so they are configured as assembled batteries in which multiple batteries (unit batteries) are connected in series. The At present, replacement of these devices with lithium secondary batteries having high energy density (high voltage) from conventional lead, nickel-cadmium or nickel metal hydride batteries, capacitors, etc. is being studied.

  In conventional lead, nickel-cadmium or nickel metal hydride batteries, even if some of the batteries (unit batteries) that make up the assembled battery are overdischarged or overcharged, the overall assembled battery performance is only slightly degraded. Since it is rare that it becomes impossible, it is sufficient to monitor the operating state of the assembled battery as a whole when operating and using a battery device equipped with the assembled battery on a daily basis. There was no need to monitor the condition of a single battery.

  However, lithium secondary batteries are vulnerable to overcharge and overdischarge due to the difference in electrode reaction mechanism, and if not used within the specified voltage range, there may be significant capacity reduction or abnormal heat generation, In order to use a lithium secondary battery, it is necessary to strictly control the upper limit voltage and the lower limit voltage.

  Therefore, there are many assembled batteries in which a plurality of lithium secondary batteries are connected, because even if some of the batteries (unit batteries) constituting the assembled battery are overdischarged or overcharged, the entire assembled battery is affected. In daily operation and use of a battery device having an assembled battery to which a single lithium secondary battery is connected, it is not sufficient to monitor the operating state of the entire assembled battery. When an abnormality such as an internal micro short circuit occurs in a battery, it is necessary to identify such an abnormal battery from a large number of lithium secondary batteries constituting the assembled battery at an early stage and replace it with a normal battery. Therefore, in a battery device having an assembled battery in which a large number of lithium secondary batteries are connected, it is necessary to provide means for monitoring the operation of each unit cell, and the cost of the device is enormous. There was a problem of becoming.

  Patent Document 1 describes a method of detecting the state of a secondary battery based on the voltage between terminals of the secondary battery, the value of current flowing between the terminals, and the like.

  In Patent Document 2, in the rechargeable lithium battery, the terminal voltage is disturbed or the terminal voltage at a preset time becomes smaller than the preset voltage. Alternatively, a method is described in which a minute short circuit is detected by making the terminal voltage during charging smaller than the voltage determined by learning from the conventional charging behavior.

  By the way, in order to reduce the number of operation monitoring means in a battery device having an assembled battery to which a large number of lithium secondary batteries are connected, the entire assembled battery is divided into several blocks, and the operation is monitored for each block. A method of providing means is conceivable. For example, if an assembled battery in which 5 lithium secondary batteries are connected in series is one unit and a battery device in which 10 units are connected in parallel is provided with an operation monitoring unit for each unit, one abnormal battery is provided. Even if it cannot be specified, it is possible to narrow down the units in which abnormal batteries exist, so that the detection of abnormal batteries can be accelerated and the number of operation monitoring means can be reduced to 1/10. . However, even if the method described in Patent Documents 1 and 2 is applied to an assembled battery, for example, when an abnormality occurs in one unit battery in one unit, the behavior is diluted for the other nine batteries. As a result, the sensitivity for detecting an abnormality is greatly reduced.

  According to Patent Document 3, “If the variation rate of the OCV with respect to the SOC is large, it is possible to accurately detect the SOC of the unit battery even if the accuracy and resolution of the voltage detection circuit of the unit battery are lowered. The change rate of the open circuit voltage of each unit battery relative to the change in the SOC of the entire assembled battery is detected, and the opening of each unit cell is within a range where the change rate exceeds a predetermined value. A method is described in which the variation in SOC between the unit cells is determined based on the circuit voltage.

  However, the above-described dilution problem still remains even with the method described in Patent Document 3, and in the method described in Patent Document 3, which is determined based on the magnitude of the change rate, the value of the change rate changes continuously. The criteria for determining whether the value of a value is significant is arbitrary, and it is difficult to select a parameter value to be set in the determination means in order to accurately perform the determination as intended. There was a problem such as.

In Patent Document 4, when a battery is formed by using spinel type lithium titanium oxide as a negative electrode and combining with the positive electrode, the electric capacity of the reversible region of the negative electrode is set to the electric capacity of the reversible region of the positive electrode. A lithium secondary battery smaller than the capacity is described.
JP 2000-323183 A Japanese Patent Laid-Open No. 09-17458 JP 2000-92732 A JP-A-10-69922

  The present invention has been made in view of the above problems, and it is not necessary to provide a means for monitoring the operation of each unit battery, and an abnormal battery or a unit including an abnormal battery is selected from a large number of batteries. It is an object of the present invention to provide a battery device including an abnormality determining means that can be easily identified with high sensitivity and reliability.

  In order to solve the above problems, the present invention provides an abnormality determination unit for a secondary battery that detects the presence of a maximum point in the rate-of-change curve of the battery voltage as charging proceeds in a deep charge region and determines that there is an abnormality. The battery device provided.

  Further, the present invention provides a battery device including a plurality of units connected in series, each of which has a plurality of secondary batteries connected in series, and the battery is provided with a plurality of the units. Monitoring the rate of change of the accompanying voltage, detecting that there is a maximum point in the rate of change curve of the voltage accompanying the progress of charging in the deep charge region, and determining that the series connection unit includes an abnormal battery; The battery device includes an abnormality determination unit.

  Moreover, in the battery device provided with the abnormality determining means of the present invention, the secondary battery is characterized in that lithium titanate is used for the negative electrode.

FIG. 3 is a diagram for explaining the principle of the present invention. LiNi _1/6 Mn _1/6 Co _2/3 O ₂ which is a transition metal composite oxide having a lithium titanate as a negative electrode and a layered structure as a positive electrode. 2 is a charging curve (a) of a battery using. FIG. 3 shows that an inflection point exists in the vicinity of the battery voltage of 3.3V. The graph also plots the rate of change in battery voltage over time (dV / dt) as charging progresses. It can be seen that there is a maximum value in the dV / dt curve corresponding to the inflection point, that is, a peak is formed. The inflection point observed on the charging curve is a reflection of a phenomenon observed at a potential in the vicinity of 0.5 V (vs. Li / Li ⁺ ) in the lithium titanate negative electrode. Thus, the present invention can be achieved by assuming that the secondary battery provided in the battery device of the present invention is a secondary battery in which lithium titanate is used as the negative electrode active material for the negative electrode.

  Here, when there is no abnormal battery in the battery device of the present invention, the end-of-charge state of the battery device is shallower than the state where the inflection point appears in the high SOC region (deep charge region) of the secondary battery. It is assumed that it is designed to correspond to.

  In addition, when using a potential inflection point peculiar to lithium titanate that appears when charging is performed deeper than a predetermined end-of-charge state, the secondary battery included in the battery device of the present invention has a positive / negative capacity balance. In this respect, it is necessary that the negative electrode is designed to be restricted, that is, the negative electrode has a smaller electric capacity than the positive electrode.

In FIG. 3, a battery using lithium titanate as a negative electrode has been described as an example. However, the use of lithium titanate as a negative electrode is not an essential requirement for carrying out the present invention. The phenomenon in which the inflection point appears in the charge curve in the deep charge region is not limited to lithium titanate. For example, in LiCoO ₂ used as a positive electrode active material, 4.3 V (vs. Li ^/ Li ⁺ ) Since an inflection point is observed in the vicinity, the principle of the present invention can be applied as it is to a battery using LiCoO ₂ for the positive electrode. In this case, it goes without saying that the positive electrode is required to be limited in terms of positive and negative electrode capacity balance. In this case, the negative electrode active material may be a carbonaceous material such as graphite.

  However, it is preferable to employ a configuration in which lithium titanate is used for the negative electrode because the behavior of the lithium titanate negative electrode accompanied by a sudden increase in potential at the end of charging can be used.

  According to the present invention, the abnormality determination is performed based on the presence / absence of the maximum point in the change rate curve of the battery voltage as the charging progresses, so the determination criterion is clear and the determination can be made with high accuracy.

  In addition, according to the present invention, when an abnormality occurs in a part of directly connected batteries, the influence is amplified and reflected in the change rate curve of the battery voltage as the charging progresses, so that the sensitivity is high. Judgment is possible.

  EXAMPLES Hereinafter, although this invention is demonstrated in more detail based on an Example, these description does not limit this invention at all.

The secondary battery (unit battery) according to the following examples uses lithium titanate for the negative electrode, LiNi _1/6 Mn _1/6 Co _2/3 O ₂ for the positive electrode, and the electric capacity of the negative electrode is higher than that of the positive electrode. Is also designed to be smaller.

  One unit assembled battery 1 was prepared by connecting in series the five unit batteries (cells A to E) in which all the unit batteries were previously adjusted to SOC (charge depth) 50%. Lead wires from the positive and negative electrodes connected to both ends were connected to a charging / discharging device, and charging / discharging of the assembled battery 1 was performed. The charging / discharging conditions of the assembled battery were a constant current and a constant voltage charge up to a charge upper limit voltage of 16 V after a constant current discharge to a discharge lower limit voltage of 7.5 V at a current value corresponding to 1 ItA at 25 ° C. During this time, the behavior of individual unit cells was also monitored.

  FIG. 2 shows a charging curve of the assembled battery 1 and a charging curve of each unit battery constituting the assembled battery 1. Moreover, the value of dV / dt with respect to the assembled battery 1 output from the charging / discharging apparatus was plotted together in FIG. From FIG. 2, a rapid increase in the dV / dt value is observed corresponding to a voltage increase portion near 13 V seen at the end of charging of the assembled battery. In each unit battery (cells A to E), an increase in the battery voltage is observed almost evenly. When the voltage across the assembled battery is 16 V, the voltage of each unit battery is about 3.2 V. ing.

  Next, assuming that one of the five unit batteries has a slight internal short circuit, in order to reproduce this state in a pseudo manner, only cell A is adjusted to SOC 40%, and cells B to E are adjusted. Was adjusted to 50% SOC. Thereafter, five unit batteries (cells A to E) including the cell A were connected in series to produce the assembled battery 2. Lead wires from the positive and negative electrodes connected to both ends were connected to a charging / discharging device, and charging / discharging of these assembled batteries 2 was performed. The charging / discharging conditions of the assembled battery are the same as those of the assembled battery 1 described above. After performing constant-current discharge up to a discharge lower limit voltage of 7.5 V at a current value corresponding to 25 ° C. and 1 ItA, Current constant voltage charging was performed. During this time, the behavior of individual unit cells was also monitored.

  FIG. 1 shows a charging curve of the assembled battery 2 and a charging curve of each unit battery constituting the assembled battery 2. Moreover, the value of dV / dt with respect to the assembled battery 2 output from the charging / discharging apparatus was plotted together in FIG. A rapid increase in the dV / dt value is observed corresponding to a voltage increase portion near 13 V seen at the end of charging of the assembled battery, but the dV / dt curve accompanying the progress of charging decreases after the increase. That is, a maximum point (peak) clearly exists in the dV / dt curve. When the voltage of each unit battery (cells A to E) when the voltage of the entire assembled battery 2 reached the charging upper limit voltage 16V was examined, the cell A that reproduced the pseudo internal micro short circuit state reached 3V. On the other hand, the cells B to E reached 3.3 V or higher.

  This is because the SOC at the start of charging for cell A was lower than that for other cells B to E, so that even if the other cells B to E reached 3.2 V, which is the end-of-charge voltage originally designed. Since the cell A does not reach the originally designed voltage at the end of charging, charging was continued until the voltage of the assembled battery 2 reached the charging upper limit voltage of 16 V. As a result, the cells B to E were overcharged. It is because.

  As can be seen from this result, the dV / dt curve for the assembled battery is only one of the n unit batteries connected in series as the abnormal battery having a shallower charging depth than the other batteries. By using the presence / absence of the local maximum value as a criterion, it can be seen that the influence of one abnormal battery is amplified to (n-1) times, rather than being diluted to 1 / n. Therefore, according to the present invention, determination with high detection sensitivity can be performed.

  When constant current / constant voltage charging is adopted as the charging mode, the dV / dt value naturally drops sharply at the part where the constant current mode is switched to the constant voltage mode. In order to avoid this, the determination of the presence / absence of the maximum value in the dV / dt curve must be made for the portion of the constant current charging mode prior to the time of switching to the constant voltage mode.

  The dV / dt curves illustrated in FIGS. 1 and 2 include discontinuous portions called so-called peak peaks or noise. For this, selection of data sampling conditions and general waveform processing devices are used. By setting the peak recognition condition that is normally performed in step 1, the possibility of erroneous determination due to the presence of the discontinuous portion can be eliminated. Note that the peak recognition conditions normally performed in a general waveform processing apparatus are: minimal width (recognized as a peak only when the peak width is greater than a certain value), minimal height (peak width is greater than a certain value) Only when it is recognized as a peak). Note that a commercially available product for chromatographic analysis may be used as the waveform processing apparatus. For reference, the data sampling condition used in the above example is that the time elapsed from the start of charging when the time has elapsed for 1 second or the voltage has changed more than 5 mV from the previous recording time. A data pair of (t) and voltage (V) was recorded.

  In the above embodiment, the change in the battery voltage with the progress of charging was captured by observing the change in the battery voltage with the elapsed time of constant current charging. You may employ | adopt the method by observing.

It is a figure which shows the charge behavior of the assembled battery which concerns on Example 2. FIG. It is a figure which shows the charge behavior of the assembled battery which concerns on Example 1. FIG. It is a figure for demonstrating the charge behavior of a cell.

Claims (3)

A battery device comprising secondary battery abnormality determination means for detecting the presence of a maximal point in a change rate curve of battery voltage as charging proceeds in a deep charge region and determining an abnormality. A battery unit including a plurality of units connected in series, each of which has a plurality of secondary batteries connected in series, and having a voltage change rate as the charging progresses with respect to the units connected in series. An abnormality determination means is provided for monitoring and detecting that a maximum point is present in the rate-of-change curve of the voltage along with the progress of charging in a deep charging region, and determining that the series connection unit includes an abnormal battery. Battery device. The battery device having an abnormality determining means according to claim 1 or 2, wherein the secondary battery uses lithium titanate as a negative electrode.

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