U.S. patent application number 09/727588 was filed with the patent office on 2001-06-28 for battery charger and method of detecting a fully charged condition of a secondary battery.
Invention is credited to Moriyama, Shigeru, Takano, Nobuhiro.
Application Number | 20010005127 09/727588 |
Document ID | / |
Family ID | 18368173 |
Filed Date | 2001-06-28 |
United States Patent
Application |
20010005127 |
Kind Code |
A1 |
Takano, Nobuhiro ; et
al. |
June 28, 2001 |
Battery charger and method of detecting a fully charged condition
of a secondary battery
Abstract
A battery charger is capable of accurately determining that a
battery has reached a fully charged condition regardless of the
kind of the batteries to be charged, the condition of the battery,
battery temperature at the time when charging starts, charge
current, and ambient temperature. A battery temperature is sampled
at every predetermined timing, and a change in battery temperature
rise gradient is computed each time the battery temperature is
sampled. It is determined that the battery has reached the fully
charged condition based on a transition changing from increment to
decrement of the change in battery temperature rise gradient.
Inventors: |
Takano, Nobuhiro;
(Hitachinaka-shi, JP) ; Moriyama, Shigeru;
(Kumamoto-shi, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN, MACPEAK & SEAS, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3202
US
|
Family ID: |
18368173 |
Appl. No.: |
09/727588 |
Filed: |
December 4, 2000 |
Current U.S.
Class: |
320/150 |
Current CPC
Class: |
H02J 7/0049 20200101;
H02J 7/0047 20130101 |
Class at
Publication: |
320/150 |
International
Class: |
H02J 007/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 1999 |
JP |
11-344299 |
Claims
What is claimed is:
1. A controlling method of a battery charger, comprising the steps
of: a) starting charging a battery; b) sampling a battery
temperature at every predetermined timing; c) computing a change in
battery temperature rise gradient each time the battery temperature
is sampled; and d) determining that the battery has reached a fully
charged condition based on a transition changing from increment to
decrement of the change in battery temperature rise gradient.
2. The controlling method according to claim 1, wherein step d)
comprises the steps of: d1) obtaining a maximum value of the change
in battery temperature rise gradient at every sampling of the
battery temperature; and d2) determining that the battery has
reached the fully charged condition when an updated value of the
change in battery temperature rise gradient falls a predetermined
value from the maximum value.
3. The controlling method according to claim 1, wherein step d)
comprises the steps of: d3) detecting that the change in battery
temperature rise gradient exceeds a first predetermined value; d4)
after step d3), detecting that the change in battery temperature
rise gradient falls below a second predetermined value; and d5)
after step d4), determining that the battery has reached the fully
charged condition.
4. A battery charger comprising: a battery temperature sensing
device for sensing a battery temperature and outputting a battery
temperature signal indicative of the battery temperature; sampling
means for sampling the battery temperature signal at every
predetermined timing; computing means for computing a change in
battery temperature rise gradient and outputting an updated value
of the change in battery temperature rise gradient each time the
battery temperature is sampled; determining means for determining
that the battery has reached a fully charged condition based on a
transition changing from increment to decrement of the change in
battery temperature rise gradient.
5. The battery charger according to claim 4, wherein said
determining means designates a maximum value of the change in
battery temperature rise gradient at every sampling of the battery
temperature, and determines that the battery has reached the fully
charged condition when the updated value of the change in battery
temperature rise gradient falls a predetermined value from the
maximum value.
6. The battery charger according to claim 4, wherein said
determining means detects that the change in battery temperature
rise gradient exceeds a first predetermined value and thereafter
detects that the change in battery temperature rise gradient falls
below a second predetermined value, whereupon said determining
means determines that the battery has reached the fully charged
condition.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a battery charger and a
method of detecting a fully charged condition of a secondary
battery such as a nickel-cadmium battery and a nickel-hydrogen
battery.
[0003] 2. Description of the Related Art
[0004] Various methods have been proposed in the art to detect the
fully charged condition of a secondary battery. One is to sample
battery voltage at every predetermined timing. When the peal
voltage appearing at the charge termination period is detected, it
is determined that the battery has reached a fully charged
condition. This method will hereinafter be referred to as "peak
voltage detection method". Another method is to detect battery
temperature at every predetermined timing and compute a rate of
temperature rise, that is, a temperature rise gradient. When the
temperature rise gradient has exceeded a predetermined value, the
battery is determined to be fully charged. This method will
hereinafter referred to as "dT/dt detection method".
[0005] The peak voltage detection method is not suitable for the
batteries which exhibit battery charge characteristic with no clear
peak voltage. Such batteries include a nickel-hydrogen battery.
[0006] The dT/dt detection method, on the other hand, may fail to
detect the fully charged condition of the battery. In the dT/dt
detection method, the temperature rise gradient is compared with a
fixed critical value. As such, detection of the fully charged
condition of the battery is made based, among other things, only on
the temperature rise gradient. Other factors, such as the kind of
the battery to be charged, the condition of the battery, battery
temperature at the time when charging starts, charge current, or
ambient temperature, are not considered for determining the fully
charged condition. Those unconsidered factors may increase the
battery temperature rise gradient more than the fixed critical
value despite the fact that the battery has not yet reached the
fully charged condition. In such a case, charging is stopped before
the battery is fully charged, so the battery is undercharged. On
the other hand, the battery temperature rise gradient may not
increase more than the fixed critical value despite the fact that
the battery has reached the fully charged condition. In this case,
the battery is overcharged because charging will not stop even if
the battery is fully charged. Overcharging the battery may cause
electrolyte to leak out from the battery attendant to gas
generation occurring at the charge termination period. This
shortens a cycle lifetime of the battery.
[0007] If with the dT/dt detection method, the critical value used
for evaluating the temperature rise gradient is varied depending on
the kind of the battery to be charged, the condition of the
battery, battery temperature at the time when charging starts,
charge current, or ambient temperature, the battery charger
employing the dT/dt detection method and the control of the battery
charger will become complicated.
SUMMARY OF THE INVENTION
[0008] Accordingly, it is an object of the present invention to
provide a battery charger and a method of accurately detecting a
fully charged condition of a secondary battery regardless of the
kind of the battery to be charged, the condition of the battery,
battery temperature at the time when charging starts, charge
current, or ambient temperature.
[0009] To achieve the above and other object, there is provided,
according to one aspect of the present invention, a controlling
method of a battery charger, including the steps of:
[0010] a) starting charging a battery;
[0011] b) sampling a battery temperature at every predetermined
timing;
[0012] c) computing a change in battery temperature rise gradient
each time the battery temperature is sampled; and
[0013] d) determining that the battery has reached a fully charged
condition based on a transition changing from increment to
decrement of the change in battery temperature rise gradient.
[0014] Step d) may include the steps of:
[0015] d1) obtaining a maximum value of the change in battery
temperature rise gradient at every sampling of the battery
temperature; and
[0016] d2) determining that the battery has reached the fully
charged condition when an updated value of the change in battery
temperature rise gradient falls a predetermined value from the
maximum value.
[0017] Step d) may include the steps of:
[0018] d3) detecting that the change in battery temperature rise
gradient exceeds a first predetermined value;
[0019] d4) after step d3), detecting that the change in battery
temperature rise gradient falls below a second predetermined value;
and
[0020] d5) after step d4), determining that the battery has reached
the fully charged condition.
[0021] According to another aspect of the invention, there is
provided a battery charger which includes: a battery temperature
sensing device for sensing a battery temperature and outputting a
battery temperature signal indicative of the battery temperature;
sampling means for sampling the battery temperature signal at every
predetermined timing; computing means for computing a change in
battery temperature rise gradient and outputting an updated value
of the change in battery temperature rise gradient each time the
battery temperature is sampled; determining means for determining
that the battery has reached a fully charged condition based on a
transition changing from increment to decrement of the change in
battery temperature rise gradient.
[0022] In one embodiment, the determining means designates a
maximum value of the change in battery temperature rise gradient at
every sampling of the battery temperature, and determines that the
battery has reached the fully charged condition when the updated
value of the change in battery temperature rise gradient falls a
predetermined value from the maximum value.
[0023] In another embodiment, the determining means detects that
the change in battery temperature rise gradient exceeds a first
predetermined value and thereafter detects that the change in
battery temperature rise gradient falls below a second
predetermined value, whereupon the determining means determines
that the battery has reached the fully charged condition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The particular features and advantages of the invention as
well as other objects will become apparent from the following
description taken in connection with the accompanying drawings, in
which:
[0025] FIG. 1 is a circuit diagram showing a structure of the
battery charger according to a preferred embodiment of the present
invention;
[0026] FIG. 2 is a flowchart illustrating the operation of the
battery charger shown in FIG. 1; and
[0027] FIG. 3 is a graphical representation showing battery
temperature, battery temperature rise gradient, and change in the
battery temperature rise gradient during charging of the
battery.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0028] A battery charger according to a preferred embodiment of the
present invention will be described while referring to the
accompanying drawings.
[0029] To charge a battery 2 with the battery charger shown in FIG.
1 the battery 2 is connected between a rectifying/smoothing circuit
30 (to be described later) and ground. The battery 2 consists of a
plurality of cells connected in series. A temperature detecting
device 2A, such as a thermistor, is provided in contact with or in
proximity to the battery 2 for detecting the temperature of the
battery 2.
[0030] The battery charger includes a resistor 3 serving as a
current detector for detecting a charge current flowing in the
battery 2. A rectifying/smoothing circuit 10 is connected to an
A.C. power source 1 for converting the A.C. voltage to D.C.
voltage. The circuit 10 includes a full-wave rectifier 11 and a
smoothing capacitor 12. A switching circuit 20 is connected to the
output of the rectifying/smoothing circuit 10 and includes a high
frequency transformer 21, a MOSFET 22, and a PWM (pulse width
modulation) controlling IC 23. The PWM controlling IC 23 regulates
the output of the rectifying/smoothing circuit 10 by changing the
width of driving pulses applied to the MOSFET 22. Another
rectifying/smoothing circuit 30 is connected to the output of the
switching circuit 20. The circuit 30 includes diodes 31, 32, a
choke coil 33, and a smoothing capacitor 34. A battery voltage
detection section 40 is connected in parallel to the battery 2,
which includes two resistors 41 and 42 connected in series. The
voltage across the battery 2 is divided with a ratio of resistances
of the two resistors 41 and 42. The output of the battery voltage
detection section 40 is taken out from the junction of the
resistors 41 and 42.
[0031] The battery charger further includes a microcomputer 50
having a CPU 51, a ROM 52, a RAM 53, a timer 54, an A/D converter
55, an output port 56, a reset input port 57, and an input port 58
which are mutually connected by a bus. As will be described later,
the CPU 51 implements various jobs in accordance with programs
stored in the ROM 52. Specifically, the CPU 51 detects the battery
temperature at every sampling. The RAM 53 stores the latest
predetermined number of battery temperature values, for example,
the latest six battery temperature values. Each time the battery
temperature is detected, the battery temperature data stored in the
RAM 53 is shifted in a manner to expel the oldest battery
temperature data and to join the newest data, yet storing the
latest six battery temperature values.
[0032] The CPU 51 computes battery temperature rise gradient each
time the battery temperature is sampled. The battery temperature
rise gradient is computed by subtracting the oldest battery
temperature value from the newest battery temperature value while
referring to the data stored in the RAM 53. The RAM 53 stores the
latest predetermined number of battery temperature rise gradients,
for example, the latest six battery temperature rise gradients.
Each time the battery temperature rise gradient is computed, the
battery temperature rise gradient data stored in the RAM 53 is
shifted in a manner to expel the oldest battery temperature rise
gradient data and to join the newest data, yet storing the latest
six battery temperature rise gradient values.
[0033] The CPU 51 further computes a change in battery temperature
rise gradient each time the battery temperature is sampled. The
change in battery temperature rise gradient is computed by
subtracting the oldest battery temperature rise gradient value from
the newest battery temperature rise gradient value while referring
to the data stored in the RAM 53. Each time the change in battery
temperature rise gradient is computed, the CPU 51 checks if the
thus computed change in battery temperature exceeded the maximum
value ever recorded. If so, the maximum value of the change in
battery temperature rise gradient stored in the RAM 53 is
updated.
[0034] A charge current control section 60 is connected between the
current detector (resistor) 3 and the switching circuit 20 to
maintain the charge current at a predetermined level. The charge
current control section 60 includes cascade-connected operational
amplifiers 61 and 62, and resistors 63 through 66.
[0035] A constant voltage power supply 70 is provided for supplying
constant voltages to the microcomputer 50 and the charge current
control section 60. The constant voltage power supply 70 includes a
transformer 71, a full-wave rectifier 72, a smoothing capacitor 73,
a three-terminal voltage regulator 74, and a reset IC 75. The reset
IC 75 issues a reset signal to the reset input port 57 of the
microcomputer 50 to reset the same. A charge current setting
section 80 is connected between the output port 56 of the
microcomputer 50 and the inverting input terminal of the
operational amplifier 62. The charge current setting section 80 is
responsive to the signal output from the microcomputer 50 and sets
the charge current by changing the voltage applied to the inverting
input terminal of the operational amplifier 62.
[0036] A photo-coupler 4 is connected between the output port 56 of
the microcomputer 50 and the PWM controlling IC 23 of the switching
circuit 20. The photo-coupler 4 transmits signals from the
microcomputer 50 to the PWM controlling IC 23 to control start and
stop of charging Another photo-coupler 5 is connected between the
output of the charge current setting section 60 and the PWM
controlling IC 23. The photo-coupler 5 feeds back the charge
current signal to the PWM controlling IC 23.
[0037] A battery temperature detecting section 90 is connected
between the battery temperature detecting device 2A and the A/D
converter 55 of the microcomputer 50. The battery temperature
detecting section 90 includes resistors 91 and 92 connected in
series. The series-connected resistors 91 and 92 are connected
between the constant voltage source of 5V and ground. The battery
temperature detecting device 2A is connected between the junction
of the resistors 91, 92 and ground. That is, the battery
temperature detecting device 2A and the resistor 92 are connected
in parallel between the resistor 91 and ground. The resistance of
the battery temperature detecting device 2A changes depending on
the temperature of the battery 2. As a result, the voltage
developed across the resistor 92 changes depending on the
temperature of the battery 2 and is applied to the A/D converter
55.
[0038] Next, a description of operations of the battery charger
will be given while referring to the flowchart shown in FIG. 2.
Hereinafter individual steps will be referred to with an "S"
followed by the step number.
[0039] When power is turned ON, the microcomputer 50 prompts the
operator to load a battery 2 in the charger (S101). When, by
referring to the signal output from the battery voltage detection
section 40, the microcomputer 50 determines that the battery 2 is
loaded (S101 YES), the microcomputer 50 outputs a charge start
signal from the output port 56 to the PWM control IC 23 via the
photo coupler 4. Also, the microcomputer 50 applies a charge
current setting reference voltage Vi0 to the operational amplifier
62 via the charge current setting section 80, to thereby start
charging with a charge current I0 (S102).
[0040] During charging the battery 2, an actual charge current
flowing through the battery 2 is detected at the resistor 3. A
reference voltage corresponding to a target charge current is
subtracted from the voltage corresponding to the actual charge
current detected at the resistor 3, and the resultant difference
signal is fed back to the PWM control IC 23 via the photo-coupler
5. More specifically, the width of the pulse applied to the high
frequency transformer 21 is reduced when the actual charge current
is greater than the target charge current whereas the width of the
pulse applied to the high frequency transformer 21 is increased
when the actual charge current is less than the target charge
current. The output from the secondary winding of the high
frequency transformer 21 is subjected to rectification and
smoothing by the rectifying/smoothing circuit 30. In this way, the
charge current is substantially maintained at a predetermined
value, i.e., the target charge current I0.
[0041] Next, it is detected that the loaded battery has reached a
fully charged condition. To this effect, the RAM 53 is reset
(S103). The RAM 53 stores the latest six battery temperature values
Ti-06, Ti-05, . . . , Ti-01 detected through the latest six
samplings, and the latest six battery temperature rise gradient
values dT/dt(i-06), dT/dt(i-05, . . . , dT/dt(i-01). The RAM 53
further stores the maximum value of the change in temperature rise
gradient values d.sup.2T/dt.sup.2(MAX). Each time the battery
temperature is sampled, the battery temperature rise gradient is
computed by subtracting the oldest battery temperature value (as
detected at the time six samplings ahead of the current sampling)
from the newest temperature value, and also a change in temperature
rise gradient is computed by subtracting the oldest battery
temperature rise gradient value (as computed at the time six
samplings ahead of the current sampling) from the newest battery
temperature rise gradient value while referring to the data stored
in the RAM 53. It is to be noted that in the flowchart, the symbol
(infinite) indicates the maximum digital value of the A/D
conversion.
[0042] In S104, the timer 54 is started to measure the sampling
time. When t has elapsed from the start of the timer 54 (S105). the
timer 54 is restarted (S106).
[0043] Next, the voltage developed across the resistor 92 of the
battery temperature detecting section 90 is applied to the A/D
converter 55 where the applied voltage is converted to a digital
signal Tin (S107) which will be referred to as a battery
temperature signal. The battery temperature signal Tin is
indicative of the battery temperature detected by the battery
temperature detecting device 2A. In S108, a battery temperature
rise gradient is computed by the CPU 51 of the microcomputer 50
based on the updated battery temperature signal Tin and the battery
temperature signal Ti-06 obtained at the time of six samplings
ahead of the present sampling. Specifically, the battery
temperature rise gradient dT/dt(in) for the updated battery
temperature Ti is computed in accordance with the following
equation:
dT/dt(in)=Tin-(Ti-06)
[0044] Next, it is determined that the battery temperature rise
gradient dT/dt(in) is negative (S109). When the battery temperature
rise gradient dt/dt(in) is negative (S109: YES), the value of
dT/dt(in) is replaced by 0 (zero) (S110). On the other hand, when
the value of dT/dt(in) is positive (S109: NO), the routine skips
S110 and proceeds to S111 where a change in battery temperature
rise gradient corresponding to the updated battery temperature Tin
is computed based on the updated battery temperature rise gradient
dT/dt(in) and the battery temperature rise gradient dT/dt(in-06)
corresponding to the battery temperature detected at the time six
samplings ahead of the present sampling. Specifically, the change
in the battery temperature rise gradient corresponding to the
updated battery temperature Tin is computed in accordance with the
following equation (S111).
dT.sup.2/dt.sup.2(in)=dT/dt(in)-dT/dt(i-06)
[0045] Next, it is determined that the change in battery
temperature rise gradient dT.sup.2/dt.sup.2(in) thus computed is
negative (S112). When the change in battery temperature rise
gradient dT.sup.2/dt.sup.2(in) is negative (S112: YES), the value
of dT.sup.2/dt.sup.2(in) is replaced by 0 (zero) (S113). On the
other hand, when the change in battery temperature rise gradient
dT.sup.2/dt.sup.3(in) is positive (S112: NO), the routine skips
S113 and proceeds to S114. In S114, the CPU 51 compares the value
of dT.sup.2/dt.sup.2(in) with the maximum value of the change in
battery temperature rise gradient dT.sup.2/dt.sup.2(MAX) and
determined that the latter is greater than the former by a
predetermined constant K or more, i.e.,
dT.sup.2/dt.sup.2(MAX)-dT.sup.2/dt.sup.2(in).gtoreq.K. When this
condition is met, the microcomputer 50 issues a charge stop signal
from the output port 56 to the PWM control IC 23 via the
photo-coupler 4 to stop charging (S118). Thereafter, it is
determined that the battery 2 has been unloaded (S119). When it is
determined that the battery 2 has been unloaded, the routine
returns to S101 and waits for loading of another battery 2.
[0046] When determination made in S114 is negative, i a., when the
condition dT.sup.2/dt.sup.3(MAX)-dT.sup.2/dt.sup.2(in).gtoreq.K is
not met (S114: NO), then the routine proceeds to S115 where
comparison of dT.sup.2/dt.sup.2(in) to dT.sup.2/dt.sup.2(MAX) is
made. When the former is greater than the latter (S115: YES), the
value of dT.sup.2/dt.sup.2(MAX) is updated by substituting the
value of dT.sup.2/dt.sup.2(in) into dT.sup.2/dt.sup.2(MAX) (S116).
When the former is less than the latter (S115: NO), the routine
skips S116 and proceeds to S117 where the latest six sampling data
stored in the memory 53 are shifted to update the stored data.
Specifically, the battery temperature values Ti-06, Ti-05, . . . ,
Ti-01 stored in the memory 53 are shifted in such a manner that the
value T0-06 is expelled, and the values Ti-05, . . . , Ti-01 are
shifted to Ti-06, . . . , Ti-02, respectively. The battery
temperature Tin detected in S107 is stored in the memory 53 as
Ti-01. The battery temperature rise gradient values for the latest
six samplings are also shifted in the same manner. That is, the
stored dT/dt(i-06) indicative of the battery temperature rise
gradient computed at the time six samplings ahead of the present
sampling is expelled, and the gradient values of dT/dt(i-05), . . .
, dT/dt(i-01) are shifted to dT/dt(i-06), . . . , dT/dt(i-02),
respectively. The gradient value newly computed at S108 is stored
as dT/dt(i-01). Upon completion of rewriting of the data in S117,
the routine returns to S105 and repeats the sampling and computing
processes in S105 at seq.
[0047] Typically, the battery temperature gradually increases as
shown in FIG. 3. Therefore, the determination made in S114 is "NO"
and the determination made in S115 is "YES", resulting in updating
the dT.sup.2/dt.sup.2(MAX). However, immediately before the battery
is fully charged, the change in the battery temperature rise
gradient does no longer increase and shows the absolute maximum as
shown in FIG. 3. The charging of the battery is continued until the
difference between the absolute maximum and the updated
dT.sup.2/dt.sup.2(in) is equal to or greater than the predetermined
constant (S114: YES). The predetermined constant is determined to
meet the fully charged condition of the battery. In this manner, it
is determined that the battery has reached a fully charged
condition when a transit from increment to decrement of the change
in battery temperature rise gradient is detected.
[0048] While the invention has been described in detail with
reference to a specific embodiment thereof, it would be apparent to
those skilled in the art that various changes and modifications may
be made therein without departing from the spirit of the invention.
For example, determination that the battery has reached the fully
charged condition may be made when the change in battery
temperature rise gradient exceeds a first predetermined value and
then falls below a second predetermined value. With this detection,
the absolute maximum of the change in the battery temperature rise
gradient resides in between two time instances when the battery
temperature rise gradient exceeds the first predetermined value and
then falls below the second predetermined value.
* * * * *