U.S. patent application number 14/068472 was filed with the patent office on 2014-02-27 for determination system and determination method for determining whether metal lithium is preciptated in a lithium ion secondary battery, and vehicle equipped with the determination system.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is Yusuke Ito, Ryo Mano. Invention is credited to Yusuke Ito, Ryo Mano.
Application Number | 20140055144 14/068472 |
Document ID | / |
Family ID | 43798304 |
Filed Date | 2014-02-27 |
United States Patent
Application |
20140055144 |
Kind Code |
A1 |
Mano; Ryo ; et al. |
February 27, 2014 |
DETERMINATION SYSTEM AND DETERMINATION METHOD FOR DETERMINING
WHETHER METAL LITHIUM IS PRECIPTATED IN A LITHIUM ION SECONDARY
BATTERY, AND VEHICLE EQUIPPED WITH THE DETERMINATION SYSTEM
Abstract
A determination system for determining whether metal lithium is
precipitated in a lithium ion secondary battery includes: a
discharging unit that causes the lithium ion secondary battery to
perform constant current discharge until a voltage of the lithium
ion secondary battery becomes a voltage corresponding to a
predetermined low state of charge; a natural increase acquisition
unit that acquires a natural increase in voltage of the lithium ion
secondary battery after the constant current discharge is
terminated; and a precipitation determining unit the compares the
acquired natural increase with a predetermined threshold, that
determines that the metal lithium is not precipitated when the
natural increase is larger than or equal to the threshold, and that
determines that the metal lithium is precipitated when the natural
increase is smaller than the threshold.
Inventors: |
Mano; Ryo; (Toyota-shi,
JP) ; Ito; Yusuke; (Okazaki-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Mano; Ryo
Ito; Yusuke |
Toyota-shi
Okazaki-shi |
|
JP
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
43798304 |
Appl. No.: |
14/068472 |
Filed: |
October 31, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13024380 |
Feb 10, 2011 |
|
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14068472 |
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Current U.S.
Class: |
324/433 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 10/0525 20130101; H01M 10/48 20130101; G01R 31/396 20190101;
Y02E 60/122 20130101; H01M 10/4285 20130101; Y02T 10/70
20130101 |
Class at
Publication: |
324/433 |
International
Class: |
G01R 31/36 20060101
G01R031/36 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2010 |
JP |
2010-035727 |
Claims
1-7. (canceled)
8. A determination method for determining whether metal lithium is
precipitated in a lithium ion secondary battery, comprising:
causing the lithium ion secondary battery to perform constant
current discharge; terminating the constant current discharge when
the lithium ion secondary battery has a predetermined low state of
charge through the constant current discharge; acquiring a natural
increase in voltage of the lithium ion secondary battery after the
constant current discharge is terminated; comparing the acquired
natural increase with a predetermined threshold; and determining
that the metal lithium is not precipitated when the natural
increase is larger than or equal to the threshold, and determining
that the metal lithium is precipitated when the natural increase is
smaller than the threshold.
9. The determination method according to claim 8, further
comprising increasing the threshold for comparison with the natural
increase as a temperature of the lithium ion secondary battery
decreases, and decreasing the threshold for comparison with the
natural increase as the temperature of the lithium ion secondary
battery increases.
10. The determination method according to claim 8, further
comprising: correcting the acquired natural increase to a smaller
value as a temperature of the lithium ion secondary battery
decreases, and correcting the acquired natural increase to a larger
value as the temperature of the lithium ion secondary battery
increases; and comparing the corrected natural increase with the
threshold.
11. The determination method according to claim 8, further
comprising: ahead of the constant current discharge, determining
whether a voltage of the lithium ion secondary battery is higher
than or equal to a predetermined determination start allowable
voltage; and when it is determined that the voltage of the lithium
ion secondary battery is higher than or equal to the determination
start allowable voltage, the constant current discharge is
immediately started; whereas, when it is determined that the
voltage of the lithium ion secondary battery is lower than the
determination start allowable voltage, the lithium ion secondary
battery is charged to a predetermined target charge voltage and
then the constant current discharge is started.
12. The determination method according to claim 11, wherein the
determination start allowable voltage corresponds to a 60 to 70
percent state of charge of the lithium ion secondary battery.
13. The determination method according to claim 8, wherein the
predetermined low state of charge is a 10 to 20 percent state of
charge of the lithium ion secondary battery.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2010-035727 filed on Feb. 22, 2010 including the specification,
drawings and abstract is incorporated herein by reference in its
entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a determination system and
determination method for determining whether metal lithium is
precipitated in a lithium ion secondary battery, and a vehicle
equipped with the determination system.
[0004] 2. Description of the Related Art
[0005] Generally, the internal state of a lithium ion secondary
battery is inspected. As one internal state of the lithium ion
secondary battery, it is intended to recognize through inspection
whether metal lithium is precipitated. For example, a phenomenon
called "dendrite precipitation" in Japanese Patent Application
Publication No. 8-190934 (JP-A-8-190934) is also the precipitation
of metal lithium. A lithium ion secondary battery originally does
not contain metal lithium; however, depending on usage, metal
lithium may precipitate on a surface of a negative electrode. A
lithium ion secondary battery that has reached such a state needs
to be replaced because deterioration in performance is remarkable.
Therefore, it is necessary to recognize whether metal lithium is
precipitated in a lithium ion secondary battery.
[0006] A method of detecting the internal state of a lithium ion
secondary battery is, for example, described in Japanese Patent
Application Publication No. 2003-59544 (JP-A-2003-59544). In the
method described in JP-A-2003-59544, the lithium ion secondary
battery first undergoes constant current charge and subsequently
undergoes constant voltage charge. Then, a difference in index,
such as charged capacity and internal resistance, between the
charged battery and a standard battery is obtained. The internal
state of the inspected battery is intended to be recognized on the
basis of the above difference.
[0007] However, in order to apply the above related art, a power
supply for charging an inspected battery needs to be compatible
with both constant current charge and constant voltage charge. For
this reason, a complex power supply system is required.
Particularly, for example, in the case of in-car application, it is
difficult to perform required charge by an electrical system of a
vehicle itself. Generally, the electrical system of a vehicle is
not designed for constant voltage control.
[0008] In addition, information about a decrease in capacity or an
increase in internal resistance may be acquired as a result of
inspection; however, it is impossible to determine what factor
gives the acquired information. That is, not only in the case of
precipitation of metal lithium but also in the case of normal usage
degradation (hereinafter, referred to as "cycle degradation") that
is not attended with precipitation of metal lithium, the tendency
of a decrease in capacity or an increase in internal resistance is
observed. Therefore, it has been difficult to determine whether
metal lithium is precipitated.
SUMMARY OF INVENTION
[0009] The invention provides a determination system and
determination method for determining whether metal lithium is
precipitated in a lithium ion secondary battery without using
constant voltage control, and a vehicle equipped with the
determination system.
[0010] A first aspect of the invention provides a determination
system for determining whether metal lithium is precipitated in a
lithium ion secondary battery. The determination system includes: a
discharging unit that causes the lithium ion secondary battery to
perform constant current discharge until a voltage of the lithium
ion secondary battery becomes a voltage corresponding to a
predetermined low state of charge; a natural increase acquisition
unit that acquires a natural increase in voltage of the lithium ion
secondary battery after the constant current discharge is
terminated; and a precipitation determining unit the compares the
acquired natural increase with a predetermined threshold, that
determines that the metal lithium is not precipitated when the
natural increase is larger than or equal to the threshold, and that
determines that the metal lithium is precipitated when the natural
increase is smaller than the threshold.
[0011] A second aspect of the invention provides a vehicle that
includes: a lithium ion secondary battery; and the determination
system according to the first aspect.
[0012] A third aspect of the invention provides a determination
method for determining whether metal lithium is precipitated in a
lithium ion secondary battery. The determination method includes:
causing the lithium ion secondary battery to perform constant
current discharge; terminating the constant current discharge when
the lithium ion secondary battery has a predetermined low state of
charge through the constant current discharge; acquiring a natural
increase in voltage of the lithium ion secondary battery after the
constant current discharge is terminated; comparing the acquired
natural increase with a predetermined threshold; and determining
that the metal lithium is not precipitated when the natural
increase is larger than or equal to the threshold, and determining
that the metal lithium is precipitated when the natural increase is
smaller than the threshold.
[0013] In a battery in which metal lithium is precipitated,
polarization of a negative electrode is remarkable when the SOC is
low, so polarization of the negative electrode becomes large while
polarization of a positive electrode is not so large. Therefore,
only the polarization of the negative electrode contributes to an
increase in internal resistance when the SOC is low. Therefore, in
comparison with a battery in which metal lithium is not
precipitated, the internal resistance is small when the SOC is low.
In the aspects of the invention, this situation is determined on
the basis of a natural increase in battery voltage after the
battery is subjected to constant current discharge.
[0014] According to the aspects of the invention, it is possible to
provide a determination system and determination method for
determining whether metal lithium is precipitated in a lithium ion
secondary battery without using constant voltage control, and a
vehicle equipped with the determination system.
BRIEF DESCRIPTION OF DRAWINGS
[0015] The features, advantages, and technical and industrial
significance of this invention will be described below with
reference to the accompanying drawings, in which like numerals
denote like elements, and wherein:
[0016] FIG. 1 is a block diagram of a lithium precipitation
determination system according to an embodiment of the
invention;
[0017] FIG. 2 is a flowchart of a determination procedure executed
by the lithium precipitation determination system according to the
embodiment of the invention;
[0018] FIG. 3 is a graph that shows changes in voltage of both ends
of a battery in the determination procedure according to the
embodiment of the invention;
[0019] FIG. 4 is a graph that shows the relationship between an SOC
(state of charge) and potentials of positive and negative
electrodes in a lithium ion secondary battery according to the
embodiment of the invention; and
[0020] FIG. 5 is a perspective view of a hybrid automobile equipped
with the lithium precipitation determination system shown in FIG.
1.
DETAILED DESCRIPTION OF EMBODIMENTS
[0021] Hereinafter, an embodiment of the invention will be
described in detail with reference to the accompanying drawings. A
lithium precipitation determination system 1 for a lithium ion
secondary battery according to the present embodiment is configured
as shown in FIG. 1. The lithium precipitation determination system
1 for a lithium ion secondary battery shown in FIG. 1 determines
whether metal lithium is precipitated in a battery group 5. The
battery group 5 is a battery pack in which a plurality of lithium
ion secondary batteries are serially connected.
[0022] The lithium precipitation determination system 1 includes an
ammeter 2, a voltmeter 3, a processing unit 4, a load 7 and a
thermometer 11. The ammeter 2 measures the magnitude of current
that flows through the battery group 5. The voltmeter 3 measures
the voltage between both ends of the battery group 5. The
processing unit 4 acquires the results measured by the voltmeter 3,
and the like, and then makes the above described determination or
performs processing necessary for the determination on the basis of
the measured results. The processing unit 4 also controls the load
7. The thermometer 11 acquires the temperature of the battery group
5.
[0023] The load 7 includes a charging unit 8, a discharging unit 9
and a DC/DC converter 10. The charging unit 8 functions to supply
charging current to the battery group 5. The charging unit 8 is,
for example, a generator. The discharging unit 9 receives
discharging current from the battery group 5 to operate in some
way. The discharging unit 9 is, for example, a motor. One device
may serve as both the charging unit 8 and the discharging unit 9.
The DC/DC converter 10 uses the discharging unit 9 to discharge
current as constant current discharge, which will be described
later. In addition, a relay 6 is arranged between the load 7 and
the battery group 5. The relay 6 is also operated by the processing
unit 4.
[0024] Determination is made by the lithium precipitation
determination system 1 shown in FIG. 1 in accordance with the
procedure shown in the flowchart of FIG. 2. First, constant current
discharge is started (S1). That is, the relay 6 is connected, and
the discharging unit 9 is operated by discharging current from the
battery group 5. At this time, the status of operation of the
discharging unit 9 is controlled while monitoring the ammeter 2 so
that the discharging current is constant.
[0025] The current value of the constant current discharge is set
to fall within the range that does not degrade the batteries of the
battery group 5. In order not to degrade the batteries of the
battery group 5 in constant current discharge, the current value
should be set to a value in ampere that is lower than or equal to
three times of the value that indicates the battery capacity of the
battery group 5 in Ah (ampere hour). For example, when the battery
capacity of the battery group 5 is 5 Ah, the current value of
constant current discharge should be lower than or equal to 15 A.
The current having the above current value almost does not
adversely influence the service life of the batteries. In addition,
the current value should be set to a value that is higher than or
equal to one-tenth of the value that indicates the battery capacity
of the battery group 5 in Ah (ampere hour). This is because an
extremely low current causes an increased period of time for
determination.
[0026] Note that, when the voltage between both ends of the battery
group 5 before constant current discharge is started in step S1 is
too low, the battery group 5 may be preliminary charged to some
degree ahead of constant current discharge. In this case, a
threshold for determining whether to perform preliminary charge
(preliminary charge determination threshold) is set for the
preliminary voltage of both ends of the battery group. For the
threshold, for example, it is conceivable that a voltage
corresponding to the 60 to 70 percent SOC (state of charge) is used
as a determination start allowable voltage. Then, when the
preliminary voltage of both ends of the battery group 5 is higher
than or equal to the determination start allowable voltage, the
constant current discharge is immediately started; whereas, when
the preliminary voltage of both ends of the battery group 5 is
lower than the determination start allowable voltage, charging is
performed. When the preliminary charge determination threshold is
too low, constant current discharge cannot be sufficiently
performed, and there is a possibility that the determination
accuracy for the lithium precipitation determination system 1
cannot be sufficiently obtained. On the other hand, when the
preliminary charge determination threshold is too high, an
unnecessary process is performed, and it takes an extra period of
time for determination.
[0027] In addition, a target charge voltage for preliminary charge
is desirably set to a voltage that ranges from a value that is
equal to the preliminary charge determination threshold (a voltage
corresponding to the 60 to 70 percent SOC (state of charge)) to a
value that is higher by the 10 percent SOC than the preliminary
charge determination threshold. This is because, as the target
charge voltage is too low, the voltage of the batteries at the time
of start of constant current discharge is insufficient and, as a
result, measurement accuracy deteriorates. In addition, this is
also because, as the target charge voltage is too high, it takes
time for charging and constant current discharge thereafter.
Charging is, of course, performed using the charging unit 8 of the
load 7.
[0028] While the constant current discharge started in step S1 is
being performed, the voltage between both ends of the battery group
5 is constantly monitored. This monitors whether the voltage
between both ends has decreased to a predetermined discharge
termination voltage VQ (S2). The discharge termination voltage VQ
at this time is desirably a voltage corresponding to the 10 to 20
percent low SOC. If the discharge termination voltage VQ is too
high, appropriate determination cannot be made as will be described
later. On the other hand, if the discharge termination voltage VQ
is too low, the battery group 5 is brought into an overdischarge
state. That is, the determination process itself degrades the
battery group 5.
[0029] When the voltage between both ends has decreased to the
discharge termination voltage VQ (S2: YES), the constant current
discharge is terminated, and the relay 6 is opened (S3). Then,
after that, the voltage between both ends of the battery group 5
naturally increases to some extent. Then, the amount of increase in
the voltage, that is, the voltage recovery amount VR, is acquired
(S4). The details will be described later. Then, the acquired
voltage recovery amount VR is compared with a threshold (lithium
precipitation determination threshold) predetermined therefor (S5).
When the acquired voltage recovery amount VR is lower than or equal
to the threshold (S5: YES), it is determined that metal lithium is
precipitated in the battery group 5 (S6). When the acquired voltage
recovery amount VR is higher than the threshold (S5: NO), it is
determined that metal lithium is not precipitated in the battery
group 5 (S7). Up to this point, the basic procedure of
determination according to the present embodiment is described.
[0030] Next, the process shown in the flowchart of FIG. 2 will be
described in further details with reference to FIG. 3. FIG. 3 is a
graph that shows changes in voltage of the battery group 5 in
process of executing the process shown in FIG. 2. The ordinate axis
represents a cell voltage (voltage between both ends of each
battery) [V], and the abscissa axis represents time [s]. The graph
of FIG. 3 shows a curve X and a curve Y. The curve X is an example
of a voltage change obtained when metal lithium is not precipitated
in the battery group 5. The curve Y is an example of a voltage
change obtained when metal lithium is precipitated in the battery
group 5. In FIG. 3, time Z at which the voltage is the lowest
corresponds to the time at which the constant current discharge is
terminated in step S3 in FIG. 2. In FIG. 3, the curve X and the
curve Y are overlappingly shown so that the time at which the
constant current discharge is terminated coincides with each other
with respect to the abscissa axis. Then, the zero point of the
abscissa axis is set at the time at which the constant current
discharge is started (corresponding to S1 in FIG. 2) in the curve X
for the sake of convenience.
[0031] In FIG. 3, a portion before (on the left side of) time Z is
a period of time during which the constant current discharge is
performed from step S1 to step S3 in FIG. 2. In FIG. 3, the cell
voltage at the time when the constant current discharge is started
is set at 3.6 V, and the discharge termination voltage VQ is set at
2.3 V in cell voltage. During the constant current discharge, the
cell voltage gradually decreases. The curve X and the curve Y
slightly differ from each other in the shape of the graph during
the constant current discharge; however, this difference falls
within the range of individual difference, and is not a significant
difference.
[0032] When the constant current discharge is terminated at time Z,
the cell voltage steeply increases thereafter. However, the cell
voltage does not increase without limit, the curve X converges to
about 3.3 to 3.4 V, and the curve Y converges to about 2.7 to 2.8
V. A difference between the converged cell voltage and 2.3 V that
is the cell voltage VQ at the time of termination of discharge is
the voltage recovery amount VR acquired in step S4 in FIG. 2. That
is, the voltage recovery amount VR of the curve X is about 1.0 to
1.1 V, and the voltage recovery amount VR of the curve Y is about
0.4 to 0.5 V. A specific manner of determining the voltage recovery
amount VR in FIG. 3 will be described later.
[0033] The voltage recovery amount VR obtained here is compared
with the lithium precipitation determination threshold. In FIG. 3,
the threshold is set at 0.8 V (3.1 V in cell voltage); however,
this is just an example. In the curve X, the voltage recovery
amount VR is higher than the threshold, so it may be determined
that metal lithium is not precipitated in the battery group 5. In
the curve Y, the voltage recovery amount VR is lower than or equal
to the threshold, so it may be determined that metal lithium is
precipitated in the battery group 5. Thus, it is possible to
determine whether metal lithium is precipitated in the battery
group 5.
[0034] Note that the determination that metal lithium is
precipitated does not mean that metal lithium is precipitated
because of the above described constant current discharge; the
determination means that metal lithium has already been
precipitated before the constant current discharge (or preceding
preliminary charge) is started. In addition, as is apparent from
FIG. 3, determination as to whether metal lithium is precipitated
in the present embodiment may be adequately carried out about 30
minutes later after the constant current discharge is started.
[0035] Next, the reason why it may be determined whether metal
lithium is precipitated through the above described procedure will
be described with reference to the graph of FIG. 4. FIG. 4 is a
graph that shows the relationship between an SOC (state of charge)
and potentials of positive and negative electrodes in the lithium
ion secondary battery. In the graph of FIG. 4, the ordinate axis
represents the potentials of electrodes, and the abscissa axis
represents the SOC of the batteries. The graph of FIG. 4 shows a
curve D, a curve E and a curve F. The curve D is a curve for the
positive electrode potential. The curve E is a curve for the
negative electrode potential in the negative electrode in which
metal lithium is not precipitated. The curve F is a curve for the
negative electrode potential in the negative electrode in which
metal lithium is precipitated. Note that the positive electrode
potential is not differentiated depending on whether metal lithium
is precipitated. This is because, as described above, metal lithium
precipitates in the negative electrode.
[0036] The curve D of the positive electrode potential in FIG. 4
slopes upward as a whole. Within the curve D, particularly, a slope
in a low SOC range is steep. The slope corresponds to the magnitude
of polarization in the positive electrode of the lithium ion
secondary battery. That is, polarization in the positive electrode
of the lithium ion secondary battery is substantially constant in
the range other than the low SOC range, and is higher in the low
SOC range than in the other range.
[0037] On the other hand, the curve E of the negative electrode
potential (no metal lithium precipitation) slopes downward as a
whole. With in the curve E as well, particularly, a slope in a low
SOC range is steep. The absolute value of the slope corresponds to
the magnitude of polarization in the negative electrode of the
lithium ion secondary battery. That is, polarization in the
negative electrode of the lithium ion secondary battery is
substantially constant in the range other than the low SOC range,
and is higher in the low SOC range than in the other range. In
addition, in the range in which the curve D has large polarization,
the curve E also has large polarization. That is, in the lithium
ion secondary battery in which metal lithium is not precipitated,
as the polarization of the positive electrode increases, the
polarization of the negative electrode also increases. Furthermore,
the curve E in FIG. 4 is located below the curve D as a whole. That
is, the curve E is located below the curve D at the left end in
FIG. 4 where the SOC is lowest.
[0038] The curve F in the case where metal lithium is precipitated
may be presumed as the one that is obtained by substantially
shifting the curve E slightly rightward in parallel as a whole.
Thus, in the range in which the curve F has large polarization, the
curve D has small polarization. In addition, the curve F
substantially overlaps with the curve E in FIG. 4 except the range
in which the polarization is large.
[0039] Voltage measurement described with reference to FIG. 2 and
FIG. 3 according to the present embodiment is as follows when
applied to FIG. 4. First, the cell voltage of the lithium ion
secondary battery corresponds to a difference between the positive
electrode potential and the negative electrode potential at the
same SOC in FIG. 4. For example, G-G' is an example of that. Here,
it is assumed that G-G' is a voltage (3.6 V in FIG. 3) at the time
when the constant current discharge is started (S1 in FIG. 2).
Then, when metal lithium is not precipitated, H-H' corresponds to
the voltage VQ (2.3 V at time Z in FIG. 3) at the time when the
constant current discharge is terminated (S3 in FIG. 2); whereas,
when metal lithium is precipitated, J-J' corresponds to the voltage
VQ. The length of H-H' is equal to the length of J-J', and the
length of G-G' is larger than the length of H-H' or the length of
J-J'. In addition, three points G; G' and J fall within the small
polarization range in the respective curves; however, three points
H, H' and J' fall within the large polarization range in the
respective curves.
[0040] Here, focusing on the time at which the constant current
discharge is terminated when metal lithium is not precipitated, the
points H and H' both fall within the large polarization range as
described above. That is, both the positive electrode and the
negative electrode are greatly polarized. Large polarization means
that the internal resistance of the lithium ion secondary battery
is large. In this way, discharge is terminated in a state where the
internal resistance is large, so the voltage recovery amount VR
thereafter is large. This is because the voltage recovery amount VR
is substantially proportional to the product of the internal
resistance [.OMEGA.] of the lithium ion secondary battery at the
time of termination of discharge and the current value [A]
immediately before the termination of discharge.
[0041] On the other hand, when metal lithium is precipitated, as
described above, the point J' falls within the large polarization
range; however, the point J falls within the small polarization
range. That is, the negative electrode is greatly polarized, but
the positive electrode is not polarized so much. Therefore, the
internal resistance of the lithium ion secondary battery at the
time of termination of discharge is smaller than that when metal
lithium is not precipitated. Thus, the voltage recovery amount VR
after termination of discharge is small by that much. This is the
reason why it may be determined whether metal lithium is
precipitated on the basis of the voltage recovery amount VR.
[0042] Here, the role of the constant current discharge before the
voltage recovery amount VR is acquired is to bring the objective
lithium ion secondary battery into a determinable state. That is,
when the SOC of the lithium ion secondary battery is high (around
the points G and G' in FIG. 4), there is almost no difference of
polarized state depending on whether metal lithium is precipitated.
This is because the curve E and the curve F in FIG. 4 almost
overlap each other in this range. Therefore, it is difficult to
make determination in this state.
[0043] The SOC of the lithium ion secondary battery is decreased by
constant current discharge to around the points H, H', J and J' in
FIG. 4. By so doing, there appears a difference of polarized state
depending on whether metal lithium is precipitated. This is because
the curve E and the curve F in FIG. 4 do not overlap each other in
this range. The voltage recovery amount VR is acquired in this
state, so it is possible to determine whether metal lithium is
precipitated. In other words, the role of the constant current
discharge is to bring the lithium ion secondary battery into a low
state of charge to an extent such that there appears a difference
of polarized state depending on whether metal lithium is
precipitated.
[0044] Note that the internal resistance of the lithium ion
secondary battery has temperature dependency. That is, the internal
resistance tends to be larger at low temperatures than at high
temperatures. This means that the above described voltage recovery
amount VR tends to increase at low temperatures as compared with at
high temperatures. Therefore, the lithium precipitation
determination threshold to be compared with the voltage recovery
amount VR desirably has temperature dependency. That is, a large
threshold is used at low temperatures as compared with at high
temperatures. Conversely, a small threshold is used at high
temperatures as compared with at low temperatures. In order to
implement the above configuration, it is only necessary that a map
that defines a threshold applied at each temperature of the battery
group 5 is stored in the processing unit 4. Of course, in the map,
thresholds specified for low temperatures should be larger than
thresholds specified for high temperatures. Then, an appropriate
threshold is selected from among the thresholds in the map on the
basis of the temperature acquired by the thermometer 11.
[0045] Alternatively, instead of selecting the threshold to be
compared with the voltage recovery amount VR from the map on the
basis of the temperature of the batteries, the acquired voltage
recovery amount VR itself may be corrected on the basis of the
temperature of the battery group 5. That is, the voltage recovery
amount VR is corrected to a smaller value as the battery
temperature decreases; whereas the voltage recovery amount VR is
corrected to a larger value as the battery temperature increases.
After that, the corrected voltage recovery amount VR is compared
with the threshold. Through the above manner as well, it is
possible to handle the temperature dependency of the internal
resistance of the lithium ion secondary battery.
[0046] Next, a method of determining the voltage recovery amount VR
in FIG. 3 will be described. Some of determining methods are
conceivable; however, any determining method may be used as long as
the same method is used each time. The simplest method is based on
a fixed waiting time. That is, a waiting time for sampling the
voltage between both ends of the battery group 5 after time Z is
determined in advance in order to acquire the voltage recovery
amount VR. By subtracting the discharge termination voltage VQ per
cell from the cell voltage after a lapse of the waiting time, the
voltage recovery amount VR may be determined. In FIG. 3, a period
of time during which the voltage steeply increases after time Z is
slightly shorter than 100 seconds, so it is only necessary that the
waiting time is set to a period of time longer than or equal to 100
seconds.
[0047] Another determining method focuses on an increase in cell
voltage. That is, after time Z, the voltage between both ends of
the battery group 5 is periodically repeatedly sampled. Then, the
voltage between both ends of the battery group 5 increases each
time it is sampled; however, an increase in the voltage becomes
extremely small as the curve after time Z in FIG. 3 becomes closer
to a horizontal line. Then, a threshold (voltage recovery amount
determination threshold) is set for an increase between adjacent
voltage values sampled at a constant time interval. The voltage
recovery amount VR may be determined on the basis of the cell
voltage at which the increase in voltage is smaller than or equal
to the threshold.
[0048] Another conceivable method is, for example, a determining
method in which the curve of a change in voltage value after time Z
is approximated to a function that converges to a limit value and
then the voltage recovery amount VR is determined on the basis of
the limit value. Any of these generally known methods is
applicable.
[0049] A lithium ion secondary battery equipped for a vehicle is
conceivable as a major application of the method of determining
whether metal lithium is precipitated in a lithium ion secondary
battery according to the present embodiment. The vehicle may be any
vehicle that entirely or partially uses electric energy from a
lithium ion secondary battery as the power source. The vehicle may
be, for example, an electric automobile, a hybrid automobile, a
plug-in hybrid automobile, a hybrid railroad vehicle, a forklift,
an electric wheelchair, an electric power-assisted bicycle, an
electric scooter, or the like.
[0050] That is, determination may be made in such a manner that the
lithium precipitation determination system 1 shown in FIG. 1 is
externally connected to the in-vehicle lithium ion secondary
battery. Alternatively, determination may be made by the vehicle by
itself in such a manner that a control unit of the vehicle
incorporates the function of the lithium precipitation
determination system 1 shown in FIG. 1.
[0051] An example of such a vehicle is shown in FIG. 5. The vehicle
400 is a hybrid automobile that is driven by using an engine 440
and a motor 420 in combination. The vehicle 400 includes a vehicle
body 490, the engine 440, the motor 420 assembled to the engine
440, a cable 450, a control unit 430 and a battery pack 401 that
contains a plurality of batteries inside. The control unit 430
incorporates not only an inverter, or the like, for driving the
motor 420 but also the function of the lithium precipitation
determination system 1 shown in FIG. 1. However, the motor 420 in
the vehicle 400 serves as the charging unit 8 and the discharging
unit 9 among the elements of FIG. 1.
[0052] In the case of the vehicle 400, the voltage recovery amount
VR in FIG. 3 is desirably about 1 [V]. This is to reliably start
the engine 440. However, when the battery group 5 degrades because
of precipitation of metal lithium, it may result in a situation
that the voltage recovery amount VR becomes only about 0.5 [V].
When such a state is detected by the determining method according
to the present embodiment, it is possible to prompt a user to take
appropriate measures, such as battery replacement.
[0053] Here, in the vehicle 400, usually, it is not assumed to
drive the motor 420 through constant voltage control. Therefore,
the control unit 430 mostly does not have a constant voltage
control function. However, it is not inconvenient because of that.
This is because, in the method according to the present embodiment,
constant current control is used but constant voltage control is
not used. Of course, for some other reasons, the control unit 430
may have a constant voltage control function.
[0054] In addition, a lithium ion secondary battery equipped for a
device, other than a vehicle, that uses a battery as at least one
of energy sources may be set as a determination object. Such a
device may be, for example, various household electrical
appliances, office equipment and industrial equipment, such as a
personal computer, a cellular phone, a battery-powered electric
tool and an uninterruptible power supply. In addition, an electric
cell that is not formed into a battery pack may be set as a
determination object.
[0055] As described in detail above, according to the present
embodiment, the lithium ion secondary battery performs constant
current discharge until the battery voltage has decreased to the
discharge termination voltage VQ and then acquires the voltage
recovery amount VR after termination of the discharge. The voltage
recovery amount VR depends on the internal resistance of the
lithium ion secondary battery at the time of termination of
discharge as described above, and the internal resistance varies on
the basis of whether metal lithium is precipitated in the lithium
ion battery. Therefore, it may be determined whether metal lithium
is precipitated in the lithium ion battery on the basis of the
voltage recovery amount VR. Thus, the system and method that are
able to determine whether metal lithium is precipitated in the
lithium ion secondary battery and the vehicle equipped with the
system are implemented. Here, determination according to the
temperature of the battery is possible.
[0056] The invention has been described with reference to example
embodiments for illustrative purposes only. It should be understood
that the description is not intended to be exhaustive or to limit
form of the invention and that the invention may be adapted for use
in other systems and applications. The scope of the invention
embraces various modifications and equivalent arrangements that may
be conceived by one skilled in the art.
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