U.S. patent application number 09/802267 was filed with the patent office on 2001-11-01 for device and method for controlling charging of secondary battery.
Invention is credited to Katsuda, Toshihiro, Kikuchi, Yoshiaki, Kimura, Tadao, Murakami, Yusai, Nakanishi, Toshiaki.
Application Number | 20010035741 09/802267 |
Document ID | / |
Family ID | 18584896 |
Filed Date | 2001-11-01 |
United States Patent
Application |
20010035741 |
Kind Code |
A1 |
Murakami, Yusai ; et
al. |
November 1, 2001 |
Device and method for controlling charging of secondary battery
Abstract
A device and method is provided that can detect overcharge
reliably by separating heat generated by internal resistance and
heat chemically generated upon overcharging in a secondary battery.
Based on the temperature of the secondary battery detected every
first predetermined time by a battery temperature detection
section, a temperature gradient operation section calculates a
temperature gradient indicating a temperature increase per unit
time for every second predetermined time that is longer than the
first predetermined time. When a temperature gradient determination
section determines that the temperature gradient is larger than a
predetermined temperature gradient threshold, which is preset
according to the charged state of the secondary battery, N times in
a row (where N is a natural number), a fully charged state of the
secondary battery is detected.
Inventors: |
Murakami, Yusai; (Shizuoka,
JP) ; Kimura, Tadao; (Hyogo, JP) ; Nakanishi,
Toshiaki; (Aichi, JP) ; Katsuda, Toshihiro;
(Aichi, JP) ; Kikuchi, Yoshiaki; (Aichi,
JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Family ID: |
18584896 |
Appl. No.: |
09/802267 |
Filed: |
March 8, 2001 |
Current U.S.
Class: |
320/150 |
Current CPC
Class: |
H01M 10/443 20130101;
H02J 7/0091 20130101; H02J 7/0049 20200101; H02J 7/007194 20200101;
H01M 10/448 20130101; Y02E 60/10 20130101 |
Class at
Publication: |
320/150 |
International
Class: |
H02J 007/04; H02J
007/16 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 9, 2000 |
JP |
2000-065352 |
Claims
What is claimed is:
1. A charging control device for controlling charging of a
secondary battery used for applications in which the secondary
battery is charged and discharged repeatedly in a charged but not
fully charged state, comprising: a battery temperature detecting
section for detecting a temperature of the secondary battery every
first predetermined time; a temperature gradient operation section
for calculating a temperature gradient, indicating a temperature
increase per unit time, every second predetermined time that is
longer than the first predetermined time, based on the temperature
detected by the battery temperature detecting section; and a
temperature gradient determination section for determining whether
the temperature gradient calculated by the temperature gradient
operation section is larger than a predetermined temperature
gradient threshold preset according to a charged state of the
secondary battery, wherein when the temperature gradient
determination section determines that the temperature gradient
calculated by the temperature gradient operation section is larger
than the predetermined temperature gradient threshold N times in a
row (where N is a natural number), a fully charged state of the
secondary battery is detected.
2. The charging control device according to claim 1, wherein the
temperature gradient operation section takes an operation result as
being valid when an average load current of the secondary battery
indicates a direction of charging the secondary battery.
3. The charging control device according to claim 1, wherein a
temperature gradient threshold preset according to at least one of
a magnitude of an average load current of the secondary battery and
cooling power of the secondary battery is used as the predetermined
temperature gradient threshold.
4. The charging control device according to claim 2, wherein a
value of the average load current is an average value of load
currents detected every first predetermined time during every
second predetermined time.
5. The charging control device according to claim 1, wherein the
second predetermined time is longer than a time, detected every
first predetermined time, of variations in load current due to
charge and discharge of the secondary battery.
6. The charging control device according to claim 1, wherein the
temperature gradient operation section determines the temperature
gradient by a moving average operation.
7. A charging control method for controlling charging of a
secondary battery used for applications in which the secondary
battery is charged and discharged repeatedly in a charged but not
fully charged state, comprising: detecting a temperature of the
secondary battery every first predetermined time; calculating a
temperature gradient indicating a temperature increase per unit
time for every second predetermined time that is longer than the
first predetermined time, based on the temperature detected;
determining whether the temperature gradient thus calculated is
larger than a predetermined temperature gradient threshold preset
according to a charged state of the secondary battery; and
detecting a fully charged state of the secondary battery when it is
determined that the temperature gradient calculated is larger than
the predetermined temperature gradient threshold N times in a row
(where N is a natural number).
8. The charging control method according to claim 7, wherein an
operation result of the temperature gradient is taken as being
valid when an average load current of the secondary battery
indicates a direction of charging the secondary battery.
9. The charging control method according to claim 7, wherein a
temperature gradient threshold preset according to at least one of
a magnitude of an average load current of the secondary battery and
cooling power of the secondary battery is used as the predetermined
temperature gradient threshold.
10. The charging control method according to claim 8, wherein a
value of the average load current is an average value of load
currents detected every first predetermined time during every
second predetermined time.
11. The charging control method according to claim 7, wherein the
second predetermined time is longer than a time, detected every
first predetermined time, of variations in load current due to
charge and discharge of the secondary battery.
12. The charging control method according to claim 7, wherein the
temperature gradient is determined by a moving average operation.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a technique of
controlling the charging of a secondary battery, particularly to a
technique of preventing a secondary battery, which is used for
applications in which the secondary battery is charged and
discharged repeatedly in a charged but not fully charged state,
from being overcharged by controlling the charging of the secondary
battery.
[0003] 2. Related Background Art
[0004] Examples of the secondary battery include lead batteries,
nickel-cadmium (Ni--Cd) batteries, nickel metal-hydride (Ni--MH)
batteries, and lithium ion batteries. Such batteries have a
characteristic allowing them to be connected to an external power
source to be charged with a predetermined current supplied from the
power source after consumption of their electric power. These
batteries have been used for various equipment, making good use of
such a characteristic. For instance, such a battery may be mounted
on a vehicle to supply electric power to an ignition plug of its
engine. Recently, such a battery also is used as a main power
source for driving an electric motor in a so-called hybrid electric
vehicle (HEV) provided with an engine and the electric motor.
[0005] Every battery has its own limit in terms of the chargeable
amount of electric power. Therefore, it is necessary to control the
charging amount so that the battery is charged in a range not
exceeding the chargeable capacity. When the battery is charged with
an amount of electric power exceeding the limit (overcharged),
there has been a possibility that the lifetime of the battery may
be shortened. In addition, the detection of overcharge is
indispensable for securing driving ability while a vehicle is
driven.
[0006] Conventionally, a general method as a method of detecting
overcharge of a battery includes detecting temperature variations
in a battery during charging and determining that the battery has
reached a fully charged state at a time a temperature variation per
unit time, i.e. a temperature gradient, increases abruptly.
[0007] In the case of a battery mounted on, for example, a hybrid
electric vehicle, however, the battery may be charged and
discharged repeatedly while the vehicle is driven. In the hybrid
electric vehicle, when the output of its engine is higher than the
required power for driving, a generator is driven with the surplus
power to charge the battery. On the contrary, when the output of
the engine is lower than the required power, an electric motor is
driven with the electric power of the battery to output the power
covering a shortage of the required power. In this case, the
battery is discharged. Such repeated charge and discharge are
carried out according to the driving state of the vehicle, the
charged state of the battery, and an operation of a driver.
[0008] Generally, a battery generates the Joule heat due to the
loss caused by internal resistance (DC-IR) of the battery when the
battery is charged and discharged. The amount of the heat thus
generated is proportional to the square of an effective value of a
current. Therefore, when charge and discharge are repeated as in
the case where the battery is mounted on a hybrid electric vehicle,
the temperature gradient abruptly changes even when the battery has
not reached a fully charged state. Therefore, a wrong determination
that the battery has reached the fully charged state may be made to
stop the charging. As a result, the battery cannot be charged
sufficiently and there is a possibility of troubles such that the
battery goes flat.
[0009] As a method of solving the above-mentioned problem, for
example, JP 11(1999)-299124 discloses a method including detecting
a temperature gradient during charging caused in connection with
the charging from the temperature gradient indicating a temperature
increase per unit time or a temperature gradient during charging,
calculated by subtracting a corrected amount of the temperature
gradient caused by discharge from the temperature gradient
indicating a temperature increase per unit time, and terminating
the charging of a secondary battery when the temperature gradient
is larger than a predetermined temperature gradient preset
according to a charged state of the secondary battery.
[0010] However, in a working environment of a hybrid electric
vehicle or an electric vehicle, the current values in charging and
discharging the secondary battery are large and therefore the
amount of heat generated by the internal resistance (DC-IR) of the
secondary battery is large. In addition, the charging current and
discharged current change instantaneously and heat is generated
with a time difference. Therefore, the cause of the heat generation
cannot be specified from instantaneous values of the currents. For
such reasons, it has been difficult to detect overcharge by
separating the heat generated by the DC-IR to determine the
temperature gradient caused by a chemical heat-generating reaction
alone upon overcharging.
SUMMARY OF THE INVENTION
[0011] With the foregoing in mind, it is an object of the present
invention to provide a charging control device and method that
reliably can detect overcharge by separating heat generated by
internal resistance and heat chemically generated upon overcharging
in a secondary battery.
[0012] In order to achieve the above-mentioned object, a charging
control device according to the present invention is a device for
controlling the charging of a secondary battery used for
applications in which the secondary battery is charged and
discharged repeatedly in a charged but not fully charged state. The
charging control device includes a battery temperature detecting
section, a temperature gradient operation section, and a
temperature gradient determination section. The battery temperature
detecting section detects the temperature of the secondary battery
every first predetermined time. The temperature gradient operation
section calculates a temperature gradient indicating a temperature
increase per unit time for every second predetermined time that is
longer than the first predetermined time, based on the temperature
detected by the battery temperature detecting section. The
temperature gradient determination section determines whether the
temperature gradient calculated by the temperature gradient
operation section is larger than a predetermined temperature
gradient threshold preset according to a charged state of the
secondary battery. When the temperature gradient determination
section determines that the temperature gradient calculated by the
temperature gradient operation section is larger than the
predetermined temperature gradient threshold N times in a row
(where N is a natural number), a fully charged state of the
secondary battery is detected.
[0013] According to this charging control device, based on the
temperature detected every first predetermined time, the
temperature gradient operation section calculates the temperature
gradient every second predetermined time that is longer than the
first predetermined time, and then a fully charged state of the
secondary battery is detected when the temperature gradient
determination section determines that the temperature gradient
calculated exceeds the predetermined temperature gradient threshold
N times in a row (where N is a natural number). Thus, the influence
of the heat instantaneously generated due to the repeated charge
and discharge of the secondary battery while a hybrid electric
vehicle or the like is driven is removed and the heat generated by
the internal resistance and the heat chemically generated upon
overcharging in the secondary battery are separated. Consequently,
the overcharge can be detected reliably.
[0014] This can prevent the deterioration and life-shortening of
the secondary battery due to the overcharge. In addition, the
secondary battery can be used up to a range in which a high level
of charging is required, and therefore the driving ability can be
secured easily while a vehicle is driven.
[0015] In the charging control device, it is preferable that the
temperature gradient operation section takes an operation result as
being valid when an average load current of the secondary battery
indicates a direction of charging the secondary battery.
[0016] According to this configuration, the temperature gradient
calculated is used for overcharge detection when the average load
current of the secondary battery indicates the charging direction,
so that overcharge further can be detected reliably.
[0017] In the charging control device, it is preferable that a
temperature gradient threshold preset according to at least one of
the magnitude of an average load current of the secondary battery
and cooling power of the secondary battery is used as the
predetermined temperature gradient threshold.
[0018] According to this configuration, the temperature gradient
threshold is set according to at least one of the magnitude of the
average load current and the cooling power of the secondary
battery, so that overcharge further can be detected accurately.
[0019] Preferably, a value of the average load current is an
average value of load currents detected every first predetermined
time during every second predetermined time.
[0020] In the charging control device, it also is preferable that
the second predetermined time is longer than a time, detected every
first predetermined time, of variations in load current due to
charge and discharge of the secondary battery.
[0021] According to this configuration, the influence of the heat
instantaneously generated due to the repeated charge and discharge
of the secondary battery while a hybrid electric vehicle or the
like is driven is removed, and the heat generated by the internal
resistance and the heat chemically generated upon overcharging in
the secondary battery are separated. Consequently, the overcharge
can be detected reliably.
[0022] Furthermore, in the charging control device, preferably, the
temperature gradient operation section determines the temperature
gradient by a moving average operation.
[0023] According to this configuration, the temperature gradient is
determined not with a simple average but with a moving average, so
that overcharge can be detected further accurately.
[0024] In order to achieve the above-mentioned object, a charging
control method of the present invention is a method for controlling
the charging of a secondary battery used for applications in which
the secondary battery is charged and discharged repeatedly in a
charged but not fully charged state. The method includes detecting
the temperature of the secondary battery every first predetermined
time, calculating a temperature gradient indicating a temperature
increase per unit time for every second predetermined time that is
longer than the first predetermined time, based on the temperature
detected, determining whether the temperature gradient thus
calculated is larger than a predetermined temperature gradient
threshold preset according to a charged state of the secondary
battery, and detecting a fully charged state of the secondary
battery when it is determined that the temperature gradient
calculated is larger than the predetermined temperature gradient
threshold N times in a row (where N is a natural number).
[0025] According to this charging control method, based on the
temperature detected every first predetermined time, the
temperature gradient is calculated every second predetermined time
that is longer than the first predetermined time. When the
temperature gradient thus calculated exceeds the predetermined
temperature gradient threshold N times in a row (where N is a
natural number), a fully charged state of the secondary battery is
detected. Thus, the influence of the heat instantaneously generated
due to the repeated charge and discharge of the secondary battery
while a hybrid electric vehicle or the like is driven is removed
and the heat generated by the internal resistance and the heat
chemically generated upon overcharging in the secondary battery are
separated. Consequently, the overcharge can be detected
reliably.
[0026] This can prevent the deterioration and life-shortening of
the secondary battery due to overcharge. In addition, the secondary
battery can be used in a range up to a range in which a high level
of charging is required, and therefore the driving ability can be
secured easily while a vehicle is driven.
[0027] In the charging control method, it is preferable that an
operation result of the temperature gradient is taken as being
valid when an average load current of the secondary battery
indicates a direction of charging the secondary battery.
[0028] According to this method, the temperature gradient
calculated is used for the overcharge detection when the average
load current of the secondary battery indicates the charging
direction, so that overcharge further can be detected reliably.
[0029] In the charging control method, it also is preferable that a
temperature gradient threshold preset according to at least one of
the magnitude of an average load current of the secondary battery
and cooling power of the secondary battery is used as the
predetermined temperature gradient threshold.
[0030] According to this method, the temperature gradient threshold
is set according to at least one of the magnitude of the average
load current and the cooling power of the secondary battery, so
that overcharge further can be detected accurately.
[0031] Preferably, a value of the average load current is an
average value of load currents detected every first predetermined
time during every second predetermined time.
[0032] In the charging control method, it is preferable that the
second predetermined time is longer than a time, detected every
first predetermined time, of variations in load current due to
charge and discharge of the secondary battery.
[0033] According to this method, the influence of the heat
instantaneously generated due to the repeated charge and discharge
of the secondary battery while a hybrid electric vehicle or the
like is driven is removed and the heat generated by the internal
resistance and the heat chemically generated upon overcharging in
the secondary battery are separated. Consequently, the overcharge
can be detected reliably.
[0034] Moreover, in the charging control method, it is preferable
that the temperature gradient is determined by a moving average
operation.
[0035] According to this method, the temperature gradient is
determined not with a simple average but with a moving average, so
that overcharge further can be detected accurately.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a block diagram showing a configuration of a
charging control device according to a first embodiment of the
present invention.
[0037] FIG. 2 is a flow chart showing a secondary battery charging
control routine according to the first embodiment of the present
invention.
[0038] FIG. 3 is a block diagram showing a configuration of a
charging control device according to a second embodiment of the
present invention.
[0039] FIG. 4 is a table showing a temperature gradient threshold
TH to be set with respect to an average load current Iave and
cooling power CP.
[0040] FIG. 5 is a graph showing curves of battery temperature and
temperature gradients with respect to a charged amount in the cases
of a low charging rate (IL) and a high charging rate (IH) of the
average load current Iave.
[0041] FIG. 6 is a graph showing curves of temperature gradients
with respect to elapsed time in the cases of low, medium, and high
cooling power of a secondary battery.
[0042] FIG. 7 is a flow chart showing a secondary battery charging
control routine according to the second embodiment of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0043] Embodiments of the present invention are described using
drawings as follows.
[0044] First Embodiment
[0045] FIG. 1 is a block diagram showing a configuration of a
charging control device according to a first embodiment of the
present invention.
[0046] In FIG. 1, numeral 10 indicates a secondary battery, for
example, a nickel-hydrogen battery to be mounted on a hybrid
electric vehicle or the like. Generally, this secondary battery 10
is made of a battery pack including a combination battery with a
number of cells combined to obtain a predetermined output power.
Numeral 12 denotes a battery temperature detecting section, in
which the battery temperature measured by a temperature sensor (not
shown in the figure) placed in a predetermined position inside the
secondary battery 10 is sampled every first predetermined time t1,
for example every 0.1 second, and thus a battery temperature sample
T(n) is obtained.
[0047] Numeral 14 indicates a temperature gradient operation
section that calculates a temperature gradient dT(n)/dt (=K)
indicating a temperature increase per unit time set to be a second
predetermined time t2, for instance, 20 seconds, with respect to
the battery temperature sample T(n) output from the battery
temperature detecting section 12. This second predetermined time t2
is set to be longer than the first predetermined time t1 as the
time for sampling the battery temperature and also may be longer
than the time of variation in load current due to the repeated
charge and discharge of the secondary battery 10 while a hybrid
electric vehicle or the like is driven.
[0048] Furthermore, in the temperature gradient operation section
14, not an operation for obtaining a simple average but that for
obtaining a moving average is employed for determining the
temperature gradient. The moving average is obtained as follows.
For example, a temperature gradient K.sub.i is obtained between a
certain time ti and a time ti+20 after a second predetermined time
(20 seconds) from the time ti and then a temperature gradient
K.sub.i+ is obtained between a time ti+0.1 after a first
predetermined time (0.1 second) from the time ti and a time ti+20.1
after the second predetermined time (20 seconds) from the time
ti+0.1. Then the average of K.sub.i and K.sub.i+1 is calculated.
This procedure is repeated sequentially.
[0049] In this case, as the second predetermined time t2 increases,
the influence of the heat generated by the internal resistance
(DC-IR) in the secondary battery 10 can be removed increasingly,
but the operation becomes slower and thus the secondary battery 10
tends to come into an overcharged state.
[0050] Numeral 16 denotes a temperature gradient determination
section. The determination section 16 determines whether the
temperature gradients dT(n)/dt sequentially output from the
temperature gradient operation section 14 are larger than the
predetermined temperature gradient threshold TH preset according to
the charged state of the secondary battery, determines that the
secondary battery 10 is in a fully charged state when the
temperature gradient dT(n)/dt exceeds the temperature gradient
threshold TH N times, for example three times, in a row, and
outputs a signal FC for notifying of the fully charged state.
[0051] Next, the following description is directed to a control
process in the first embodiment configured as described above.
[0052] FIG. 2 is a flow chart showing a secondary battery charging
control routine according to the first embodiment of the present
invention.
[0053] First, a battery temperature T(n) is obtained by being
sampled every first predetermined time t1 (S20). Based on the
battery temperature sample T(n) thus obtained, the temperature
gradient dT(n)/dt during the second predetermined time t2 is
determined by a moving average operation (S21).
[0054] Next, it is determined whether the temperature gradient
dT(n)/dt determined at step S21 exceeds the predetermined
temperature gradient threshold TH (S22). As a result of the
determination at step S22, when the temperature gradient dT(n)/dt
is not larger than the temperature gradient threshold TH, the
procedure is returned to step S20, and obtaining the battery
temperature sample T(n) continues.
[0055] As a result of the determination at step S22, when the
temperature gradient dT(n)/dt exceeds the temperature gradient
threshold TH, the procedure is advanced to step S23 and it is
determined whether the temperature gradient dT(n)/dt exceeds the
temperature gradient threshold TH N times in a row. At step S23,
when the temperature gradient dT(n)/dt does not exceed the
temperature gradient threshold TH N times in a row, it is
determined that the temperature increase is not caused by the heat
chemically generated in the fully charged state and the procedure
also is returned to step S20 and obtaining the battery temperature
sample T(n) continues.
[0056] As a result of the determination at step S23, when the
temperature gradient dT(n)/dt exceeds the temperature gradient
threshold TH N times in a row, it is determined that the
temperature increase is caused by the heat chemically generated in
the fully charged state and detects that the battery is currently
in the fully charged state and further charging results in
overcharge (S24). Thus, the secondary battery charging control
routine is ended.
[0057] Second Embodiment
[0058] FIG. 3 is a block diagram showing a configuration of a
charging control device according to a second embodiment of the
present invention. The present embodiment is different from the
first embodiment in that the temperature gradient threshold is set
variably depending on the average load current and cooling power of
a secondary battery. Configurations and functions of a battery
temperature detecting section 12, a temperature gradient operation
section 14, and a temperature gradient determination section 16 are
identical with those in the first embodiment. Therefore, their
descriptions are not repeated.
[0059] In FIG. 3, numeral 32 is a current sensor for detecting a
load current caused by the charge and discharge of a secondary
battery. The load current detected by the current sensor 32 is
supplied to an average load current measurement section 34. The
measurement section 34 samples a load current (I(n)) every first
predetermined time, for example, every 0.1 second, for sampling the
battery temperature described in the first embodiment, obtains the
average during the second predetermined time, for example, during
20 seconds, and outputs it as an average load current Iave.
[0060] Numeral 36 is a secondary battery cooling means such as a
cooling fan or the like. A cooling power setting section 38 sets
the cooling power CP, for instance, the rotational speed of the
cooling fan, according to the temperature of the secondary battery
to prevent the secondary battery performance from
deteriorating.
[0061] The average load current Iave output from the measurement
section 34 and the cooling power CP output from the cooling power
setting section 38 are supplied to a temperature gradient threshold
setting section 30. The setting section 30 determines a temperature
gradient threshold TH according to the magnitude of the average
load current Iave and the cooling power CP input thereto and sets
the temperature gradient threshold TH for the temperature gradient
determination section 16.
[0062] FIG. 4 is a diagram showing a table of the temperature
gradient threshold TH set with respect to the average load current
Iave and the cooling power CP. In FIG. 4, since the temperature
gradient in a fully charged state decreases with the increase in
the cooling power CP, the value of the temperature gradient
threshold TH is set to be smaller accordingly. In addition, since
the temperature gradient in a fully charged state increases with
the increase in charging rate of the average load current Iave, the
value of the temperature gradient threshold TH is set to be larger
accordingly.
[0063] FIG. 5 is a graph showing curves of battery temperature and
temperature gradients with respect to a charged amount in the cases
of a low charging rate (IL) and a high charging rate (IH) of the
average load current Iave.
[0064] FIG. 6 is a graph showing curves of temperature gradients
with respect to elapsed time in the cases of low, medium, and high
cooling power of a secondary battery.
[0065] Next, the following description is directed to a control
process in the present embodiment configured as described
above.
[0066] FIG. 7 is a flow chart showing a secondary battery charging
control routine according to the second embodiment of the present
invention.
[0067] Initially, a battery temperature T(n) and a load current
I(n) are obtained by being sampled every first predetermined time
t1 (S70). Based on the battery temperature sample T(n) thus
obtained, the temperature gradient dT(n)/dt during a second
predetermined time t2 is determined by a moving average operation.
In addition, based on the load current sample I (n) thus obtained,
the average load current Iave during the second predetermined time
t2 is determined (S71).
[0068] Next, it is determined whether the average load current Iave
determined at step S71 indicates the charging direction (whether
the average load current Iave is larger than zero). As a result of
the determination at step S72, when the average load current Iave
does not indicate the charging direction (i.e. indicates a
discharging direction), the procedure is returned to step S70 and
obtaining the battery temperature sample T(n) and the load current
sample I(n) continues.
[0069] As a result of the determination at step S72, when the
average load current Iave indicates the charging direction (when
the average load current Iave is larger than zero), the procedure
is advanced to step S73 and based on the magnitude of the average
load current Iave and the cooling power, an optimum temperature
gradient threshold TH is set.
[0070] Next, it is determined whether the temperature gradient
dT(n)/dt determined at step S71 exceeds the temperature gradient
threshold TH set at step S73 (S74). As a result of the
determination at step S74, when the temperature gradient dT(n)/dt
is not more than the temperature gradient threshold TH, the
procedure is returned to step S70 and obtaining the battery
temperature sample T(n) and the load current sample I(n)
continues.
[0071] As a result of the determination at step S74, when the
temperature gradient dT(n)/dt exceeds the temperature gradient
threshold TH, the procedure is advanced to step S75 and it is
determined whether the temperature gradient dT(n)/dt exceeds the
temperature gradient threshold TH N times in a row. At step S75,
when the temperature gradient dT(n)/dt does not exceed the
temperature gradient threshold TH N times in a row, it is
determined that the temperature increase is not caused by the heat
chemically generated in the fully charged state and the procedure
also is returned to step S70 and obtaining the battery temperature
sample T(n) and the load current sample I(n) continues.
[0072] As a result of the determination at step S75, when the
temperature gradient dT(n)/dt exceeds the temperature gradient
threshold TH N times in a row, it is determined that the
temperature increase is caused by the heat chemically generated in
the fully charged state. It is detected that the secondary battery
is currently in the fully charged state and further charging
results in overcharge (S76), and thus the secondary battery
charging control routine is ended.
[0073] As described above, according to the present invention, the
temperature gradient in the case where the average load current
Iave indicates the charging direction is compared with the optimum
temperature gradient threshold TH preset according to the magnitude
of the average load current Iave and the cooling power of the
secondary battery. Therefore, a fully charged state can be detected
with higher accuracy than that in the first embodiment.
[0074] Moreover, in the embodiments of the present invention, the
first predetermined time t1 for sampling the battery temperature
was set to be 0.1 second, the second predetermined time t2 for the
calculation of the temperature gradient to be 20 seconds, and the
number of times N the temperature gradient exceeds the threshold to
be 3. However, the present invention is not limited to those
numerical values. For instance, those numerical values can be
changed according to the specification of the secondary battery,
vehicle load characteristics, a driving pattern of a vehicle, or
the like.
[0075] The invention may be embodied in other forms without
departing from the spirit or essential characteristics thereof The
embodiments disclosed in this application are to be considered in
all respects as illustrative and not limiting. The scope of the
invention is indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of equivalency of the claims are intended to be
embraced therein.
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