JP2012150086A - Assembled battery system designing method and apparatus for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an assembled battery system designing method and an apparatus for implementing the system to be applied to an assembled battery system comprising a large number of battery cells, the method and the apparatus enabling a required utilization rate to be secured even if the battery cells include considerably deteriorated ones.SOLUTION: An assembled battery system comprising a large number of battery cells connected in series is provided with a main power unit made up of battery cells for normal use and a reserve power unit made up of battery cells to replace any of the battery cell running into trouble in the main power unit; the number of battery cells making up the reserve power unit, which is connected in series to the main power unit, is determined by the utilization rate required for the assembled battery system and by the probability of failure of each of the individual battery cells making up the assembled battery system.

Description

  The present invention relates to a design method and an apparatus for an assembled battery system including a large number of battery cells.

  The market for lithium-ion batteries is not limited to power sources for mobile terminals such as notebook personal computers and mobile phones, which are the main markets for the batteries, but the market is expanding to power sources such as electric cars and hybrid cars. is there.

  One electric vehicle uses several thousand lithium ion batteries in terms of a mobile phone. When a lithium ion battery is used as a power source for an electric vehicle, the lithium ion battery has a safety In terms of reliability, reliability and durability, it is necessary to meet stricter requirements than mobile terminals.

  For this reason, lithium-ion batteries used as power sources for electric vehicles still have sufficient performance in other applications even if they are judged to be unsuitable for reuse in electric vehicles due to deterioration over time. It may be determined that reuse is possible. Such reusable uses include, for example, storage batteries for stabilizing the output power of renewable power sources that use natural energy such as sunlight and wind power, and that have large fluctuations in power generation, and cheap nighttime power. A storage battery or the like can be considered. At present, sodium-sulfur batteries and the like are mainly used for such power storage batteries.

  The storage battery for power storage has less restrictions on installation space and weight than applications such as mobile terminals and electric vehicles. Therefore, the energy density / cost ratio is more important than simply high energy density. At present, it can be said that the lithium ion battery is inferior in energy density / cost ratio to the sodium sulfur battery. However, in the future, the cost of lithium-ion batteries will decrease as electric vehicles and the like become widespread, and reuse batteries are cheaper than new batteries, so there is a possibility that the reuse market for lithium-ion batteries will expand. .

  In addition, since the sodium-sulfur battery is handled as a dangerous substance in the Fire Service Act, it requires permission at the time of installation, and has a demerit that it is necessary to maintain it at a high temperature (about 300 ° C.) during operation. Therefore, in the future, cost-competitiveness can also be found in reusable lithium ion batteries as storage batteries for power storage.

  However, since the reusable lithium ion battery uses a battery cell that has been used for several years, it is considered that the deterioration is considerably advanced depending on the battery cell. In general, a large-scale assembled battery including a large number of battery cells is used as a storage battery for power storage. Therefore, when a lithium ion battery used in an electric vehicle or the like is collected and disassembled into battery cells to reconfigure the assembled battery, the assembled battery that satisfies the required performance requirements is included even if the battery cell has deteriorated. A technique for functioning as a battery and a technique for accurately predicting the operating rate of the assembled battery are required.

  When a lithium ion battery is normally overcharged or overdischarged, there is a risk that the electrode becomes unstable and generates heat, which may rupture or ignite. In particular, when using lithium ion batteries connected in series, not only a protection circuit that avoids overcharging and overdischarging, but also a cell balance circuit that handles non-uniform power storage for each battery cell is required. .

  The cell balance circuit maintains the balance of the amount of electricity stored between the battery cells by performing a process of not discharging the battery cell having too little charged amount during discharging and charging the battery cell having too much charged amount during charging. The assembled battery may use a booster circuit to obtain a necessary output voltage while balancing the amount of electricity stored between the battery cells.

  By the way, in an assembled battery system that reuses collected battery cells, it is considered that the use of battery cells that have deteriorated to a certain extent causes the capacity of the battery cells to drop rapidly during reuse, resulting in a failure state. It is done.

  For example, FIG. 1 of Non-Patent Document 1 shows a graph showing the relationship between the battery capacity and the number of charge / discharge cycles. In Non-Patent Document 1, battery characteristics whose capacity decreases as the number of charge / discharge cycles increases are classified into four regions (A)-(D).

  Region (A) is a region where the number of charge / discharge cycles is small and the battery capacity rapidly decreases.

  The area (B) is an area where the rate of decrease in battery capacity is moderate following the area (A).

  The region (C) is a region that follows the region (B), and the rate of decrease of the battery capacity becomes more gradual than the region (B).

  The area (D) is an area where the battery capacity rapidly decreases again following the area (C).

  A measurement example of such battery capacity-charge / discharge cycle characteristics is described in FIG.

  It can be determined that the triangles and rhombuses shown in FIG. 3 of Non-Patent Document 2 are in the region (A)-(C), and the quadrangle is in the region (A)-(D).

  Further, according to FIG. 8 of Non-Patent Document 3, a lithium ion battery using B-MCF (boron-doped mesophase pitch-based carbon fiber) as a negative electrode has a region (A)-(C It is recognized that the lithium ion battery using the negative electrode as the negative electrode reaches the region (D) before and after 200 charge / discharge cycles.

R. Spotniz, “Simulation of Capacity fade in Lithium-ion batteries”, Journal of Power Sources, Vol.113, p.72-80, 2003 Hiroe Nakagawa, 3 others, “Development of high-performance lithium-ion secondary battery using fluorinated alkyl group-containing phosphoric acid ester-added flame retardant electrolyte”, GS Yuasa Technical Report, Vol. 6, No. 2, p. . 7-13, 2009 Norio Takami, “Ultra-thin aluminum laminate exterior lithium ion battery”, Toshiba Review, Vol. 56, No. 2, p. 10-13, 2001

  In order to effectively use the collected battery cells, the assembled battery system includes battery cells in the area (D) from the latter half of the area (C), which are considered to have been considerably deteriorated in battery capacity. However, it is important to ensure the availability of the assembled battery system by some device.

  In the above-described cell balance circuit, if many such deteriorated battery cells are included, it is difficult to maintain the battery capacity and output voltage necessary for the assembled battery system.

  The present invention has been made to solve the above-described problems of the background art, and in an assembled battery system including a large number of battery cells, the required operation is performed even if battery cells that have deteriorated are included. It is an object of the present invention to provide an assembled battery system design method and apparatus capable of securing the rate.

In order to achieve the above object, a method for designing an assembled battery system of the present invention includes a main power supply unit that includes a plurality of battery cells connected in series, which are usually used.
A spare power supply unit comprising a plurality of battery cells connected in series, used in place of the failed battery cell when the battery cell of the main power supply unit fails,
A battery pack system design method comprising:
Measure the battery capacity for each battery cell, determine the failure probability for each battery cell based on the battery capacity for each battery cell,
From the failure probability for each battery cell, an operation rate that is a probability that the assembled battery system operates normally when the number of battery cells of the standby power supply unit is changed is obtained, and is required for the assembled battery system. This is a method of determining the number of battery cells of the standby power supply unit that satisfies the operating rate.

On the other hand, the assembled battery system design apparatus of the present invention is a main power supply unit including a plurality of battery cells connected in series, which are normally used,
A spare power supply unit comprising a plurality of battery cells connected in series, used in place of the failed battery cell when the battery cell of the main power supply unit fails,
An apparatus for designing an assembled battery system having
A failure probability calculation unit for obtaining a failure probability for each battery cell based on the battery capacity of each battery cell measured in advance;
From the failure probability for each battery cell, an operation rate that is a probability that the assembled battery system operates normally when the number of battery cells of the standby power supply unit is changed is obtained, and is required for the assembled battery system. An operation rate calculation unit that determines the number of battery cells of the standby power supply unit that satisfies the operation rate;
Have

  According to the present invention, in an assembled battery system including a large number of battery cells, an assembled battery system that can ensure a required operating rate is obtained even when the battery cells that have progressed deterioration are included.

It is a circuit diagram which shows one structural example of the assembled battery system of this invention. It is a graph which shows an example of the change of the operating rate of the assembled battery system, and the probability that the battery cell of a main power supply part and a backup power supply part will fail when the number of battery cells of the backup power supply part shown in FIG. 1 is changed. It is a block diagram which shows the example of 1 structure of the design apparatus of the assembled battery system of this invention. It is a flowchart which shows an example of the process sequence of the measurement data analysis part and system structure design part which were shown in FIG. It is a graph which shows an example of probability density distribution and a cumulative distribution when the dispersion | variation in the failure probability of a lithium ion battery is large, and a dispersion | distribution is small. It is a graph which shows a mode that the capacity | capacitance at the time of normalizing a cycle-capacity curve with an initial capacity | capacitance was fitted with the convex function and the concave function. It is a graph which shows an example of the probability density function and cumulative distribution of the failure of a battery cell.

  Next, the present invention will be described with reference to the drawings.

  As shown in FIG. 1, the assembled battery system of the present embodiment includes a main power supply unit 10 including M battery cells Ec connected in series and a failure of the battery cell Ec of the main power supply unit 10. A spare power supply unit 20 having m battery cells Ec connected in series, sometimes used in place of the failed battery cell Ec, and the main power supply unit 10 and the standby power supply unit 20 are connected in series. It is the structure which was made. The number of battery cells required by the standby power supply unit 20 is determined from the failure probability of each battery cell Ec constituting the assembled battery system. Thereby, the assembled battery system which can ensure the requested operation rate even if the battery cell Ec whose battery capacity has been deteriorated is included can be realized.

  Each battery cell Ec includes a first switch A (switch A) connected in series and a second switch B (switch B) connected in parallel. The assembled battery system according to the present embodiment determines whether or not the battery cell Ec included in the main power supply unit 10 or the standby power supply unit 20 has failed, and includes a first switch A and a second switch provided for each battery cell Ec. A control unit (not shown) for controlling the operations of B is provided.

  In such a configuration, when the first switch A corresponding to an arbitrary battery cell Ec is short-circuited and the second switch B is opened, the battery cell Ec is connected in series with the adjacent battery cell Ec. On the other hand, when the second switch B corresponding to an arbitrary battery cell Ec is short-circuited and the first switch A is opened, the battery cell Ec is bypassed.

  The control unit short-circuits the first switches A corresponding to the battery cells Ec of the main power supply unit 10 and opens the second switches B at the beginning of the assembled battery system (in an initial state), thereby opening each battery cell Ec. Are connected in series. On the other hand, the second switch B corresponding to each battery cell Ec of the standby power supply unit 20 is short-circuited, and the first switch A is opened to bypass each battery cell Ec.

  When the user uses the assembled battery system and detects a failure due to deterioration of an arbitrary battery cell Ec of the main power supply unit 10, the control unit detects the second switch B corresponding to the battery cell Ec determined to be a failure. Is short-circuited, the first switch A is opened, and the battery cell is bypassed. Further, the first switch A corresponding to the bypassed battery cell of the standby power supply unit 20 is short-circuited, the second switch B is opened, and the battery cell is connected to the main power supply unit 10 in series. Thus, by replacing the battery cell Ec of the main power supply unit 10 determined to be a failure with the battery cell Ec of the standby power supply unit 20, the output voltage and the battery capacity required for the battery system are maintained.

  Whether or not a battery cell has failed may be determined as a failure when, for example, the battery capacity of each battery cell Ec is measured every predetermined period and the battery capacity falls below a preset threshold value. . What is necessary is just to set the threshold value used for a failure determination to the value of 70-80% of the initial capacity of the battery cell Ec, for example. The battery capacity may be measured, for example, by actually measuring the amount of power accumulated by discharging the battery cell to the end. Moreover, as will be described later, the battery capacity can be estimated by measuring the internal resistance of the battery cell.

  The control unit includes, for example, a CPU that operates according to a program, a known capacity measurement circuit for measuring the battery capacity of each battery cell Ec, a memory for storing the battery capacity of each battery cell, various logic gates, This can be realized by an integrated circuit including a known driver circuit for driving the first switch and the second switch.

  Next, a method for designing the assembled battery system shown in FIG. 1 will be described.

First, the probability that an individual battery cell Ec included in the main power supply unit 10 will fail up to a certain point is defined as p _1, and the probability that an individual battery cell Ec included in the standby power supply unit 20 exists up to a certain point is defined as P _2. It is defined as

In order for the assembled battery system shown in FIG. 1 to operate normally, M battery cells Ec corresponding to the number of battery cells of the main power supply unit 10 must be unfailed. Here, the probability that the assembled battery system does not operate normally is represented by the sum of the following two probabilities Pr ₁ and Pr ₂ .

Pr ₁ is the probability that (m + 1) or more battery cells Ec will fail in the main power supply unit 10,
Pr ₁ = Σ _{k MC k} (1−p ₁ ) ^Mk p ₁ ^k (1)
It is represented by However, k is a natural number which changes from (m + 1) to M.

Pr ₂ is the main power supply unit 10.

When the number of cells has failed, the number n of battery cells Ec that have not failed in the standby power supply unit 20 is a probability of being (k−1) or less,
_{Pr 2 = Σ k M C k} (1-p 1) Mk p 1 k Σ n m C n (1-p 2) n p 2 mn ... (2)
It is represented by However, k is a natural number that varies from 0 to m, and n is a natural number that varies from 0 to (k−1).

The probability P _A that the assembled battery system shown in FIG. 1 operates normally is an excuse event of the probabilities Pr ₁ and Pr ₂ that do not normally operate.
P _A = 1-Pr ₁ -Pr ₂ (3)
Can be obtained. Hereinafter, in this specification, this probability P _A is referred to as “operating rate”.

For example, the M = 100, and p ₁ = p ₂ = 0.95, Figure 2 shows the change of Pr _1, Pr ₂ and P _A when changing the m from 0 to 20.

As shown in FIG. 2, when the standby power supply unit 20 is not (m = 0), operation rate P _A of the battery module system is 0.6%. On the other hand, it includes a standby power supply unit 20, increasing the number of battery cells m of the auxiliary power supply unit 20, the operation rate P _A of the battery pack system gradually increases, m = 12 in the operation rate P _A 99.5 %. Therefore, it is possible to realize a battery module system having a high operation rate P _A by providing the standby power supply unit 20.

  However, when such a method is used, it is necessary to calculate the probability of failure (failure probability) p by time t for each battery cell Ec. This failure probability p corresponds to a failure cumulative distribution probability F (t) described below.

  The following relational expression holds for the probability density function f (t) of the failure at time t, the instantaneous failure probability z (t), and the reliability R (t). The instantaneous failure probability is a conditional probability that a failure does not occur until time t and a failure occurs at time t, and the reliability is a probability that no failure occurs until time t.

z (t) = {− dR (t) / dt} / R (t) (4)
When this is integrated, R (t) = exp {−∫z (s) ds} (the integration interval is from 0 to t) (5)
It becomes.

Therefore, the cumulative distribution probability F (t) of the failure up to time t is
F (t) = 1−R (t) (6)
The probability density function f (t) of the failure at time t is
f (t) = dF (t) / dt = z (t) exp {−∫z (s) ds} (7)
It is represented by

By the way, in Example 2 of Non-Patent Document 4 (The Electropaedia, “Battery Reliability and How to Improve it”, Battery and Energy Technologies, Internet <URL: http://www.mpoweruk.com/reliability.htm>) It has been shown that the failure probability of many lithium ion batteries follows a Weibull distribution. The Weibull distribution uses two parameters α and β,
f (t) = αβ ⁻ αtα ⁻¹ exp {− (t / β) α} (8)
It can be expressed in the function form of

  Therefore, for example, by fitting the measurement data of the battery capacity to the Weibull function and determining the parameters α and β of the Weibull function using the least square method or the like, the failure probability of each battery cell Ec can be estimated. However, when the variation in the battery capacity of each battery cell is large and the failure probability for each battery cell cannot be said to follow the Weibull distribution, the cumulative distribution probability F (t) for each battery cell Ec is obtained.

  The cumulative distribution probability F (t) at which the battery cell Ec fails by a certain time t can be determined from the charge / discharge cycle characteristics of the lithium ion battery as follows.

First, the failure of the battery cell Ec is defined as a state in which the battery capacity of the battery cell Ec is equal to or less than a preset threshold value _QTH . The battery capacity Q of the lithium ion battery shows a decreasing curve proportional to the number of charge / discharge cycles or the square root of time t in the region where the remaining battery capacity is large (in this specification, the characteristic in the region where the remaining capacity is large is expressed as “route”. Called "regular behavior").

  Usually, since the initial capacity of a lithium ion battery is the maximum capacity, when k is used as a proportionality constant and normalized by the initial capacity, the battery capacity Q of the lithium ion battery in the root-law behavior region is expressed by the following formula (9 ).

Q = 1−k√t (9)
On the other hand, in a region where the number of charge / discharge cycles (number of cycles) is large, the battery capacity rapidly deteriorates. It is also shown in FIG. 8 of Non-Patent Document 3, for example, that the battery capacity is subject to root-law behavior or rapidly deteriorates depending on the number of charge / discharge cycles.

  As described above, if the battery capacity of the battery cell Ec is in the root law behavior region, the parameter is only k. If the battery capacity is actually measured about several times, the value of k is calculated by the least square method or the like. Can be determined.

In that case, the time t _{0 at} which the battery cell Ec can be regarded as malfunctioning can be obtained by the following equation.

_{t 0 = {(1-QT} H) / k} 2 ... (10)
On the other hand, the characteristic of the battery capacity Q in the region where the battery capacity rapidly deteriorates can be fitted to a concave function, for example, Q = (r−st) ^1/2 (11)
Can be expressed as Here, when fitting is performed from t = 0, r = 1.

  If the capacity of a large number of battery cells Ec is measured, probability density functions g (k) and g (s) of parameters k and s can be obtained. Therefore, the probability density function f (t) of the failure at time t is obtained from the relationship of the following equation (12) (probability density function g (k) of k) and equation (13), for example.

g (k) = g (k (t)) | dk / dt | dt = f (t) dt (12)
k = (1-Q _TH ) / √t (13)
FIG. 3 is a block diagram showing an example of the configuration of the design apparatus for the assembled battery system of the present invention, and FIG. 4 is a flowchart showing an example of the processing procedure of the measurement data analysis unit and the system configuration design unit shown in FIG. .

  As shown in FIG. 3, the assembled battery design support apparatus of the present invention includes a measurement data analysis unit 100, a system configuration design unit 200, and a display unit 300.

  The measurement data analysis unit 100 includes a data input unit 110, a W distribution fitting unit 120, and an R distribution fitting unit 130. The system configuration design unit 200 includes a data input unit 210, a failure probability calculation unit 220, and an operation rate calculation unit 230.

As shown in FIG. 4 (a), the measurement data analysis unit 100 uses the data input unit 110 to measure the number of charge / discharge cycles (time) t for each battery cell Ec measured by a known charge / discharge tester or the like. battery capacity in _n C (t _n) to collect (volume data).

  The W distribution fitting unit 120 fits the collected capacity data for each battery cell Ec to a Weibull function indicating a failure probability for each battery cell Ec, and obtains parameters α and β of the Weibull function using a least square method or the like. . At this time, if the fitting accuracy to the Weibull function satisfies a preset reference value, the parameters α and β of the obtained Weibull function are supplied to the data input unit 210 of the system configuration design unit 200. If the fitting accuracy does not satisfy the reference value, the process proceeds to processing by the R distribution fitting unit 130.

  The R distribution fitting unit 130 performs analysis assuming a root-law behavior region represented by a convex function represented by Expression (9) based on the capacity data for each battery cell Ec, or a concave function represented by Expression (11). An analysis is performed assuming a region where the battery capacity represented by

  The R distribution fitting unit 130 determines the values of the parameters k and s of the convex function or the concave function by the least square method or the like, and supplies the values of these parameters to the data input unit 210 of the system configuration design unit 200. In addition, if there is a battery cell Ec in the region that has already deteriorated rapidly in this analysis, the information on the battery cell Ec is mainly used in the display unit 300.

As shown in FIG. 4 (b), the system configuration design unit 200 is connected to the required operating rate (R) required for the design of the assembled battery system via the data input unit 210 in series in the main power supply unit 10. The number of cells (M), the maximum number of cells (m _max ) of the standby power supply unit 20, the number of charge / discharge cycles to be evaluated or the number of times (n) and the charge / discharge cycles or times (t ₁ , t ₂ ,... A value such as t _n ) is input by the user using an input device such as a keyboard. The data input unit 210 of the system configuration design unit 200 receives parameters α and β determined by the measurement data analysis unit 100 or parameters k and s.

The failure probabilities p ₁ and p ₂ of each battery cell Ec included in the main power supply unit 10 and the standby power supply unit 20 are endogenous variables determined from the parameters calculated by the measurement data analysis unit 100, but the failure probability p ₁ And p ₂ may be set as an exogenous variable by the user or the like using the data input unit 210.

  The failure probability calculation unit 220 uses the parameters α and β or the parameters k and s describing the failure distribution to calculate the time using the equations (4) to (13).

The failure probability p ₁ of each battery cell Ec included in the main power supply unit 10 and the failure probability p ₂ of each battery cell Ec included in the standby power supply unit 20 are calculated, and the failure probabilities p ₁ and p _{2 for} each obtained battery cell Ec are calculated. Is output to the operating rate calculation unit 230.

The operating rate calculation unit 230 calculates the Pr ₁ and Pr ₂ using the equations (1) and (2) from the failure probabilities p ₁ and p _{2 for} each battery cell Ec received from the failure probability calculation unit 220, from Pr ₁ and Pr ₂ obtained to calculate the operation rate P _a of the battery module system at time t _k using equation (3). Also, operating rate calculating unit 230, at time t _k, when changing the number m of cell Ec of the standby power supply section 20 from 0 to m _max, determined the respective operating ratio P _A, the battery pack system number of battery cells of the standby power supply section 20 to meet the required operating ratio P _a, i.e. determining the minimum cell number m _min should provide power backup unit 20. And time

In, and outputs the number m of cell Ec of the standby power supply section 20 from 0 at the time of changing up to m _max, to each of the operating rate display unit 300 P _A and m _min.

  The display unit 300 receives the time received from the system configuration design unit 200.

In, when changing the number m of cell Ec of the standby power supply section 20 from 0 to m _max, with representation of the value of each of the operating rate P _A and m _min, number of battery cells of the auxiliary power supply unit 20 m and the relationship between the operating ratio P _a, for example, to display a graph. Moreover, when the battery cell Ec in the rapid deterioration area is included in the assembled battery system, for example, the risk of incorporating the battery cell Ec is displayed and notified to the designer.

  The assembled battery design support apparatus shown in FIG. 3 may be realized by, for example, a computer including input devices such as a CPU, a memory, a display, and a keyboard that operate according to a program.

  In this embodiment, since it is important that the battery cell Ec which comprises an assembled battery system follows the independent same distribution in the failure probability, it is desirable to be manufactured by the same specification.

  For example, each battery cell Ec of the assembled battery system may be a general-purpose product having the same shape such as a 18650 type battery (a cylindrical battery having a diameter of 18 mm and a length of 65 mm, or a standard size of a lithium ion battery). Moreover, you may use the battery cell Ec which can be considered as the same failure probability distribution by each capacity | capacitance measurement for each battery cell Ec of an assembled battery system.

  In order to calculate the failure probability of each battery cell Ec, it is desirable to actually measure each battery capacity as described above. However, there is a correlation between battery capacity and impedance. For example, Non-Patent Document 5 (Kazuhiko Takeno, Tomomi Shirota, “Capacity Degradation Characteristics of Lithium Ion Battery for Mobile Terminals”, NTT Docomo Technical Journal, Vol. 13, Vol. 4, p.62-65). Therefore, you may estimate the battery capacity of each battery cell Ec by measuring each impedance.

  It is desirable to acquire the distribution of the proportionality constant k in the root-law behavior region of the battery capacity and the parameter s in the rapid deterioration region by performing a charge / discharge test of each battery cell Ec included in the assembled battery system.

  However, in order to determine k and s statistically, it is necessary to obtain a sufficient number of measurement values, which is often difficult to implement because the number of test steps increases. Therefore, the parameters k and s may be determined on the assumption that the distribution follows a normal distribution, for example.

  The above formulas (1) and (2) necessary for calculating the operating rate of the assembled battery system include factorial calculation. Therefore, for example, if the number of battery cells of the main power supply unit 10 is about 100, the above formula (1) and formula (2) can be used as they are, but the calculation accuracy is better when the factorial log is taken. This is desirable. Further, when the number of battery cells is about 1000, it may be calculated using a well-known Stirling formula.

  Note that different types of battery cells Ec (different failure probability distributions) may be used for the main power supply unit 10 and the standby power supply unit 20. However, even if battery cells Ec having a lower probability of failure than the main power supply unit 10 are used for the standby power supply unit 20 as shown in the first embodiment described later, the number of battery cells in the entire assembled battery system cannot be significantly reduced. . Therefore, it is preferable to use the same type of battery cell Ec for the main power supply unit 10 and the standby power supply unit 20.

  In the present invention, the minimum number of battery cells of the standby power supply unit 20 that satisfies the required operating rate can be obtained, but other constraints such as cost are set as long as the required operating rate is satisfied. The optimal number of battery cells may be determined by linear programming, integer programming, or the like.

  According to the present invention, it is possible to configure an assembled battery system that measures capacity data for each battery cell Ec, calculates a failure probability for each battery cell Ec from the capacity data, and satisfies the required operating rate using the value. As a result, the used secondary battery such as a lithium ion battery can be reused as an assembled battery system that satisfies the required performance, and an assembled battery system that contributes to resource saving and economic cost reduction is provided. can get.

Embodiments of the present invention will be described below with reference to the drawings.
(First embodiment) Determination of the configuration of the assembled battery system The failure probability of the battery cell Ec of the main power supply unit 10 is large (p ₁ = 0.3) and small (p ₂ = 0.1). The battery cell Ec of the standby power supply unit 20 has the same probability of failure as that of the main power supply unit 10 and a total of three cases where the failure probability is smaller than that of the main power supply unit 10.

The number of battery cells (M _min ) of the main power supply unit 10 is assumed to be 100, and the minimum number of battery cells (m _min ) of the standby power supply unit 20 that makes the operating rate (R) 99.9% or 99.99% The results obtained by the design method of the invention are listed in Table 1.

Even if the main power supply unit 10 is configured with battery cells Ec (p ₁ = p ₂ = 0.3) having a high failure probability, the spare power supply unit 20 is provided, so that even in a large-scale assembled battery system with M = 100 An operating rate of 99.99% can be achieved.

  Further, the minimum number of battery cells required for the standby power supply unit 20 may be significantly reduced when the failure probability of the battery cell Ec of the backup power supply unit 20 is low.

For example, when p ₁ = 0.3, the p ₂ drops from 0.3 to 0.1, when the R = 99.9%, m _min is reduced 27.5% from 70 to 51. However, if p ₂ with respect to p ₁ is sufficiently small, p ₂ is not further reduction of even m _min becomes a small value greater. For example, when p ₁ = 0.3, even if p ₂ is halved from 0.1 to 0.05, m _min is only reduced from 51 to 48.

Also, if the failure probability of the battery cell Ec of the main power source unit 10 is relatively small (e.g., p ₁ = 0.1), the value of m _min is half than in the case of p ₁ = 0.3 described above, but p ₂ is less reductions of even m _min decreased from 0.1 to 0.01.

Therefore, as a design method of the assembled battery system, it is conceivable that the battery cell Ec having the same failure probability is used for the main power supply unit 10 and the standby power supply unit 20 simply.
(Second embodiment) Determination of configuration of assembled battery system using Weibull distribution As described above, Non-Patent Document 4 reports an analysis example in which the failure probability of a lithium ion battery follows the Weibull distribution.

  Probability density distribution and accumulation when the variance is large (α, β) = (2, 8) and when the variance is small (α, β) = (8, 8), described in Example 2 of Non-Patent Document 4. The distribution is shown in FIG. 5 (in Non-Patent Document 4, the parameters α and β of the Weibull function are reversed from the normal notation, but here, according to the normal notation as shown in Expression (8)).

The average life of the battery cell Ec calculated as the average value of the probability density function is 7.09 years for the former and 7.53 years for the latter. Assuming that the same battery cell Ec is used in the main power supply unit 10 and the standby power supply unit 20 in these cases, the number M of battery cells in the main power supply unit 10 is 100, and the operating rate (R) satisfies 99.9%. Table 2 (when the variance of the distribution is large) and Table 3 (when the variance is large) show the results of obtaining the minimum number of battery cells (m _min ) of the reserve power supply unit 20 that satisfies the operating rate (R) of 99.99%. (When small).

When m _max = 100, as shown in Table 2, if the variance is large, a system with an operating rate of 99.9% and a system with an operating rate of 99.99% will be used for up to 5 years during reuse. Each can be configured. However, when the usage period is six years, the operation rate remains at 98.0% even if the number of battery cells of the standby power supply unit 20 is m _max .

On the other hand, when the variance is small, a system with an operating rate of 99.9% and a system with an operating rate of 99.99% can be configured up to a usage period of 7 years. However, since the cumulative distribution probability suddenly increases around 8 years, the operation rate is only 0.01% in the period 8 years.
(Third embodiment) Determination of configuration of assembled battery system using measured cycle-capacity curve In the third embodiment, the cycle-capacity curve described in FIG. The capacity when normalized by the initial capacity is fitted with a convex function (Q = 1−k√t) and a concave function (Q = (1−st) ^1/2 ).

  The parameter values determined by the method of least squares are k = 0.08027 and S = 0.00107. These graphs are shown in FIG.

  Since fitting to the concave function as a whole is closer to the measurement result, fitting to the concave function is performed below (however, since the value of S is small, the concave function looks like a straight line in FIG. 6).

  Assuming that the value of S described above is a normal distribution with a standard deviation of 0.2 S, which gives a variation in battery cell life, the failure of this battery cell Ec is calculated from the above equations (11)-(12). Probability density function and cumulative distribution are obtained. This is shown in FIG.

  Here, the initial capacity of the battery cell Ec was set to “1”, and when the battery capacity became 0.75 or less, it was set that the battery cell Ec can be regarded as having failed (battery life).

  It can be seen from the cumulative distribution that the number of cycles is about 400 and almost half of the cells have reached the “lifetime”.

Assuming that the same battery cell Ec is used for the main power supply unit 10 and the standby power supply unit 20, the number M of battery cells in the main power supply unit 10 is 100, and the operation rate R satisfies 99, 9%, or 99.99%. Table 4 shows the results obtained by the present invention for the minimum number of cells (m _min ) of the standby power supply unit 20 required for the above.

When m _max = 100 is set, a system with an operation probability of 99.9% and 99.99% can be configured when the number of charge / discharge cycles is 350.

On the other hand, when the number of charge / discharge cycles is 400, the operating rate remains 89.48% even if the number of battery cells of the standby power supply unit 20 is set to m _max .

DESCRIPTION OF SYMBOLS 10 Main power supply unit 20 Standby power supply unit 100 Measurement data analysis unit 110, 210 Data input unit 120 W distribution fitting unit 130 R distribution fitting unit 200 System configuration design unit 220 Failure probability calculation unit 230 Operation rate calculation unit 300 Display unit

Claims (7)

A main power supply unit comprising a plurality of battery cells connected in series, which is normally used;
A spare power supply unit comprising a plurality of battery cells connected in series, used in place of the failed battery cell when the battery cell of the main power supply unit fails,
A battery pack system design method comprising:
Measure the battery capacity for each battery cell, determine the failure probability for each battery cell based on the battery capacity for each battery cell,
From the failure probability for each battery cell, an operation rate that is a probability that the assembled battery system operates normally when the number of battery cells of the standby power supply unit is changed is obtained, and is required for the assembled battery system. A design method for an assembled battery system that determines the number of battery cells of the standby power supply unit that satisfies an operation rate.   The method for designing an assembled battery system according to claim 1, wherein the failure probability is determined as following a Weibull function.   The failure probability is obtained by following a convex function when the battery cell is in a region where the remaining battery capacity is large, and following a concave function when the battery cell is in a region where the number of charge / discharge cycles for the battery cell is large. The design method of the assembled battery system of Claim 1 or 2 calculated | required as. A main power supply unit comprising a plurality of battery cells connected in series, which is normally used;
A spare power supply unit comprising a plurality of battery cells connected in series, used in place of the failed battery cell when the battery cell of the main power supply unit fails,
An apparatus for designing an assembled battery system having
A failure probability calculation unit for obtaining a failure probability for each battery cell based on the battery capacity of each battery cell measured in advance;
From the failure probability for each battery cell, an operation rate that is a probability that the assembled battery system operates normally when the number of battery cells of the standby power supply unit is changed is obtained, and is required for the assembled battery system. An operation rate calculation unit that determines the number of battery cells of the standby power supply unit that satisfies the operation rate;
An apparatus for designing an assembled battery system.   The assembled battery system design device according to claim 4, further comprising a W distribution fitting unit that determines a parameter of the Weibull function and supplies the determined parameter to the failure probability calculation unit, assuming that the failure probability follows a Weibull function.   When the battery cell is in a region where the remaining battery capacity is large, assuming that the failure probability follows a convex function, the parameter of the convex function is determined, and the battery cell has a large number of charge / discharge cycles for the battery cell. 6. The group according to claim 4, further comprising an R distribution fitting unit that determines a parameter of the concave function and supplies the determined parameter to the failure probability calculation unit, assuming that the failure probability follows a concave function when in the region. Battery system design equipment. A main power supply unit comprising a plurality of battery cells connected in series, which is normally used;
When the battery cell of the main power supply unit fails, a spare power supply unit including a plurality of battery cells connected in series, used in place of the failed battery cell,
A plurality of first switches connected in series corresponding to the battery cells and a plurality of second switches connected in parallel corresponding to the battery cells;
In an initial state, the first switch corresponding to each battery cell of the main power supply unit is short-circuited, the second switch is opened, and the battery cells of the main power supply unit are connected in series, and the standby power supply Whether each of the second switches corresponding to each battery cell of the unit is short-circuited, and each of the battery cells of the standby power supply unit is bypassed by opening each of the first switches, and whether the battery cell has failed every predetermined period When the failure of the battery cell of the main power supply unit is detected, the second switch corresponding to the battery cell determined to be the failure is short-circuited, and the corresponding first switch corresponding to the battery cell is The battery cell is bypassed to open, the first switch corresponding to the bypassed battery cell of the standby power supply unit is short-circuited, and the second switch corresponding to the battery cell is opened. A control unit for connecting the battery cells in series with the main power supply unit,
An assembled battery system.

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