Apparatus for charging a battery for a charge time determined based on the depth of discharge

A charging apparatus for a rechargeable storage battery is disclosed which charges the battery at a constant current. The charging apparatus monitors a variation in voltage of the battery during charge and finds a minimum value thereof to estimate the amount of capacity of the battery consumed before charge for determining the charge time required for charging the battery fully based on the estimate of the amount of capacity and the temperature of the battery.

BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates generally to a charging apparatus for use 
with a rechargeable battery such as a nickel cadmium (Ni--Cd) storage 
battery or a nickel-hydrogen storage battery, and more particularly to a 
charging apparatus designed to charge a battery for a charge time 
determined based on the depth of discharge, i.e., the amount of capacity 
of the battery consumed before charge. 
2. Background of Related Art 
Nickel cadmium storage batteries are in widespread use as a secondary 
battery which may be recharged many times. A system monitoring a battery 
temperature at completion of a charging operation and a system monitoring 
a maximum value of a battery voltage are known in the art as techniques 
for determining a time when a charging operation of a battery is 
completed. These systems also measure the elapsed time from start of the 
charging operation using a charging timer and terminate the charging 
operation forcibly after a given period of time for avoiding the 
over-charge. 
Japanese Patent First Publication No. 6-290816 teaches a charging system 
which monitors a battery temperature for determining completion of charge 
of a battery. FIG. 10 represents typical temperature characteristics of a 
nickel-hydrogen storage battery and shows that when a battery temperature 
is high, for example, above 40.degree. C. a voltage drop from a peak 
voltage hardly occurs. It is thus impossible to detect the completion of 
charge by monitoring a maximum value of a battery voltage. The system 
taught in the above publication terminates the charging operation using a 
charge timer regardless of the voltage drop. However, the termination of 
the charging operation after a given period of time regardless of a 
completion time of charge of the battery dependent upon a charging 
condition of the battery or environmental conditions may lead to the lack 
of charge. To avoid this drawback, Japanese Patent First Publication No. 
6-267592 teaches a charging system which estimates the remaining capacity 
in a storage battery based on a discharged current, a battery voltage, and 
a battery temperature immediately before a load is activated and 
determines a charge time based on the estimated remaining capacity. 
The above prior art charging system however encounters the drawback in that 
if there is a time interval between the termination of an operation of the 
load and the start of the charging operation, the storage battery may 
experience the lack of charge since the amount of capacity lost by the 
self-discharge of the nickel storage battery is usually great. The 
determination of the amount of capacity lost by the self-discharge based 
on the time interval between the termination of the operation of the load 
and the start of the charging operation and environmental conditions of 
the battery requires complex mathematical operations, and it is difficult 
to find it with high accuracy. Additionally, there is a problem of 
over-charge since a battery temperature during the charging operation is 
not taken into account in determination of the amount of capacity lost by 
the self-discharge. This is due to the fact that an allowable charge 
capacity of the battery is undesirably decreased when a battery 
temperature during charge is greater than or equal to 20.degree. C. 
SUMMARY OF THE INVENTION 
It is therefore a principal object of the present invention to avoid the 
disadvantages of the prior art. 
It is another object of the present invention to provide a charging 
apparatus designed to charge a secondary battery such as a nickel-hydrogen 
storage battery safely and precisely. 
According to one aspect of the present invention, there is provided a 
charging apparatus for a rechargeable storage battery which comprises a 
current regulating circuit for regulating a charging current supplied to 
the battery from a power circuit to a given constant level; a voltage 
detecting means for detecting a voltage of the battery; a temperature 
detecting means for detecting a temperature of the battery; and a charge 
time determining means for determining a charge time required for charging 
the battery fully at the charging current regulated in level by the 
current regulating circuit, the charge time determining means monitoring a 
variation in voltage detected by the voltage detecting means during charge 
and finding a minimum value of the variation in voltage to estimate the 
amount of capacity of the battery consumed before the charge for 
determining the charge time based on the amount of capacity estimated and 
the temperature of the battery detected by the temperature detecting 
means. 
In the preferred mode of the invention, the charge time determining means 
corrects the charge time using an allowable charge capacity dependent upon 
the temperature of the battery detected by the temperature detecting 
means. 
When the temperature of the battery detected by the temperature detecting 
means is smaller than a given value, the charge time determining means 
estimates a full charge capacity of the battery based on the minimum value 
of the variation in voltage and a time from start of charge until a 
maximum value of the variation in voltage detected by the voltage 
detecting means is reached to correct the charge time based on the 
estimated full charge capacity of the battery. 
A display means is further provided for displaying information about a 
charged condition of the battery after the charge time expires. 
The charge time determining means estimates the depth of discharge defined 
by a ratio of the amount of capacity of the battery consumed before the 
charge and a fully charged capacity of the battery based on the minimum 
value of the variation in voltage of the battery to determine the charge 
time. 
The charge time is given by the following relation: 
EQU Charge time=depth of discharge/the charging current 
A charge terminating means is further provided for terminating the charge 
of the battery after the charge time expires.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIG. 1, there is shown a 
charging apparatus according to the present invention which may be used 
for charging a rechargeable battery mounted in an electric vehicle. 
The charging apparatus generally includes a power circuit 2, an electronic 
control unit (ECU) 3, and a display 10. 
The power circuit 2 is connected to both terminals of a battery 1 to supply 
the current to the battery 1 during charge. The ECU 3 includes a current 
control circuit 4 and a microcomputer 5. The current control circuit 4 
adjusts the current from the power circuit 2 to a given constant value. 
The microcomputer 5 includes a CPU 5a and a memory 5b. 
The charging apparatus further includes a current detector 7, a voltage 
detector 8, and a temperature detector 9. The current detector 7 measures 
the current flowing from the power circuit 2 to the battery 1 and provides 
a signal indicative thereof to the current control circuit 4. The voltage 
detector 8 measures the voltage developed across the terminals of the 
battery 1 and provides a signal indicative thereof to the microcomputer 5. 
The temperature detector 9 includes a thermocouple 9a attached to a casing 
of the battery 1 to measure the temperature of the battery 1 and to 
provide a signal indicative thereof to the microcomputer 5. The constant 
current value set by the current control circuit 4 is also inputted to the 
microcomputer 5. 
The CPU 5a of the microcomputer 5 includes, as shown in FIG. 2, a charge 
time determining circuit 51 and a charge terminating circuit 52. The 
charge time determining circuit 51 determines a charge time for which the 
battery 1 is to be charged at the current value set by the current control 
circuit 4 based on parameters measured by the detectors 8 and 9. The 
charge terminating circuit 52 provides a control signal to the current 
control circuit 4 to terminate a charging operation thereof after the 
charge time determined by the charge time determining circuit 51 expires. 
For the determination of the charge time, the charge time determining 
circuit 51 first calculates a rate of change in voltage of the battery 1 
detected by the voltage detector 8, and estimates the depth of discharge 
of the battery 1 before charging according to a minimum value of the rate 
of change in voltage to determine a time required for the battery 1 to be 
fully charged at a constant current set by the current control circuit 4. 
The memory 5b stores therein data for determining the charge time based on 
a charging current value supplied to the battery 1 and a minimum value of 
the rate of change in voltage and correction data for correcting the 
charge time according to a battery temperature and a full charge capacity 
of the battery 1. The memory 5b also stores results of operations 
performed by the CPU 5a temporarily. 
The display 10 indicates information about the battery 1 during charging. 
For example, the display 10 indicates warning information when the battery 
temperature becomes undesirably high, the remainder of the charge time, 
and a full charge capacity of the battery 1. 
Hereinbelow, the correction data stored in the memory 5b and the charge 
control by the CPU 5a will be discussed in detail. 
FIG. 3 shows an example of the relation between the charge rate and the 
voltage of the battery 1 when the battery 1 is discharged from a fully 
charged condition at a constant current of 0.1 CA to depths of discharge 
of 10%, 20%, and 100%, after which the battery 1 is charged at a constant 
current of 0.1 CA. FIG. 4 shows the relation between the charge rate and 
variations in voltage .DELTA.V of the battery 1 at the depths of discharge 
of 10%, 20%, and 100% as shown in FIG. 3. 
The depth of discharge represents the ratio of the amount of capacity 
consumed or discharged from the battery 1 to a full charge capacity of the 
battery 1. The charge rate is given by the following relation. 
EQU Charge rate(%)={full charge capacity (Ah)-discharged capacity (Ah)+charged 
capacity (Ah)}/full charge capacity (Ah)!.times.100 (1) 
As can be seen from FIG. 4, minimum values .DELTA.Vmin of the variations in 
voltage, as indicated at a, b, and c, are different according to the depth 
of discharge before the battery 1 is charged. 
FIG. 5 shows the relation between the charge rate and the variations in 
voltage .DELTA.V when the battery 1 is fully discharged and then charged 
at a constant current of 0.1 CA at battery temperatures of -20.degree. C., 
30.degree. C., and 60.degree. C. As can be seen in the drawing, the 
relation between the charge rate and the variations in voltage hardly 
depends upon the battery temperature. 
FIG. 6 shows an example of the relation between a minimum value .DELTA.Vmin 
of the variation in voltage, as shown in FIG. 5, and the depth of 
discharge of the battery 1 before charge. The memory 5b stores therein 
this relation. The depth of discharge in FIG. 6 includes a loss of 
capacity caused by the self-discharge of the battery 1 after an operation 
of a load connected to the battery 1 is terminated. The charge time Cend' 
required for charging the battery 1 fully from the start of the charging 
operation is represented by the following relation. 
EQU Cend' (h)={depth of discharge(%)/100}/charging current value (CA)(2) 
The memory 5b also stores therein the relation between an allowable charge 
capacity and a battery temperature, as shown in FIG. 9, as correction data 
for the charge time Cend' and a current full charge capacity of the 
battery 1 (Usually, the full charge capacity of a battery deteriorates to 
the capacity of actual use, and an initial value thereof is a rated 
capacity). The full charge capacity is, as will be described in detail, 
determined based on a charge time required for the battery 1 to be charged 
below 20.degree. C. to reach a maximum value of a variation in voltage. If 
the battery 1 consists of a single battery pack made up of a collection of 
same type batteries, the memory 5b may store a full charge capacity of 
each of batteries. 
FIGS. 7 and 8 show a flowchart of a charge control program executed by the 
CPU 5a. This program is initiated upon connection of the battery 1 to the 
charging apparatus and turning on of the power circuit 2 to start the 
charging operation and repeated every program cycle of 1 min. 
After entering the program, the routine proceeds to step 100 wherein an 
initializing operation is performed. Specifically, count values n and m 
are cleared, the minimum value .DELTA.Vmin of a variation in voltage of 
the battery 1 and the maximum value .DELTA.Vmax thereof are reset to "100" 
and "0", respectively, and a timer built in the CPU 5a is started. 
The routine then proceeds to step 101 wherein a voltage variation .DELTA.V 
(mV/min/cell) of the battery 1 is determined in the following manner. The 
battery temperature T is monitored every 0.1 sec. through the temperature 
detector 9 to determine an average temperature value thereof for 1 min. 
Similarly, the voltage V of the battery 1 is monitored every 0.1 sec. 
through the voltage detector 8 to determine an average voltage value 
thereof for 1 min. Next, a difference between the average voltage value 
derived in this program cycle and that derived in a previous program cycle 
is determined to find the voltage variation .DELTA.V of the battery 1. 
The routine then proceeds to step 102 wherein it is determined whether the 
battery temperature T (i.e., the average temperature value derived in step 
101) lies within a range from 0.degree. C. to 45.degree. C. or not. If a 
NO answer is obtained, then the routine proceeds to step 103. 
Alternatively, if a YES answer is obtained, then the routine proceeds to 
step 104. 
In step 103, a warning signal is provided to the display 10 to inform that 
the battery temperature T is not within an effective range because it is 
difficult to charge the battery 1 completely when the battery temperature 
T is out of the temperature range in step 102. In step 103, the charging 
operation may be stopped until the battery temperature T is decreased to 
fall within the effective range. For example, when the battery temperature 
T is increased above 45.degree. C., the charging operation is stopped. 
Afterward, when it reaches 30.degree. C., the charging operation is 
resumed. 
In step 104, it is determined whether the voltage variation .DELTA.V is 
smaller than three (3) or not. If a NO answer is obtained, then the 
routine returns back to step 101. Alternatively, if a YES answer is 
obtained, then the routine proceeds to step 105 wherein it is determined 
whether the voltage variation .DELTA.V is smaller than the minimum value 
.DELTA.Vmin of variations in voltage or not. If a YES answer is obtained 
(.DELTA.V &lt;.DELTA.Vmin), then the routine proceeds to step 106 wherein the 
minimum value .DELTA.Vmin is set to the voltage variation .DELTA.V, and 
the count value n is reset to zero (0). Alternatively, if a NO answer is 
obtained, then the routine proceeds to step 107 wherein the count value n 
is incremented by one. 
After step 106 or 107, the routine proceeds to step 108 wherein it is 
determined whether the count value n is greater than ten (10) or not. The 
provision of step 108 eliminates the need for steps after step 109 to be 
performed repeatedly each time the minimum value .DELTA.Vmin is set to the 
voltage variation .DELTA.V in step 106. 
If a NO answer is obtained in step 108, then the routine returns back to 
step 101. Alternatively, if a YES answer is obtained, then the routine 
proceeds to step 109 wherein it is determined whether the battery 
temperature T is within a range from 0.degree. C. to 20.degree. C. or not. 
If a YES answer is obtained, the routine proceeds to step 110 wherein it 
is determined whether the voltage variation .DELTA.V is greater than the 
maximum value .DELTA.Vmax or not since the maximum value .DELTA.Vmax of 
variations in voltage of the battery 1 can be detected when the battery 
temperature T is within a range from 0.degree. C. to 20.degree. C. If a 
YES answer is obtained, then the routine proceeds to step 111. 
Alternatively, if a NO answer is obtained, then the routine proceeds to 
step 112. 
In step 111, the maximum value .DELTA.Vmax is set to the voltage variation 
.DELTA.V, and the count value m is reset to zero (0). In step 112, the 
count value m is incremented by one. 
After step 111 or 112, the routine proceeds to step 113 wherein it is 
determined whether the count value m is greater than five (5) or not. If a 
NO answer is obtained, then the routine returns back to step 101. 
Alternatively, if a YES answer is obtained, then the routine proceeds to 
step 114 wherein a full charge capacity of the battery 1 is determined 
according to the following equation. 
EQU Full charge capacity (Ah)={charge time (h) until the maximum value 
.DELTA.Vmax is reached.times.charging current (CA).times.rated capacity 
(Ah)}/{100-depth of discharge(%)}.times.100 (3) 
Note that the depth of discharge of the battery 1 is determined, for 
example, using the map shown in FIG. 6 based on the minimum value 
.DELTA.Vmin. 
If a NO answer is obtained in step 109 meaning that the battery temperature 
T is out of the range from 0.degree. C. to 20.degree. C., then the routine 
proceeds to step 115 wherein the depth of discharge of the battery 1 
before charge is estimated using the map shown in FIG. 6 to determine the 
charge time Cend according to the following equation which corresponds to 
the equation (2) corrected using the relation between an allowable charge 
capacity and a battery temperature, as shown in FIG. 9, and the full 
charge capacity. 
EQU Cend (h)=Cend'(h).times.{allowable charge capacity(%)/100}.times.{full 
charge capacity (Ah)/rated capacity (Ah)} (4) 
This eliminates the need for the mount of capacity lost by the 
self-discharge of the battery 1 for the interval from the termination of 
operation of the load to the start of the charging operation to be 
determined, thereby allowing the charge time to be determined with high 
accuracy. 
After step 115, the routine proceeds to step 116 wherein it is determined 
whether the charge time has expired or not after the start of the charging 
operation. If a NO answer is obtained, then the routine returns back to 
step 101. Alternatively, if a YES answer is obtained, then the routine 
proceeds to step 117 wherein the amount of capacity stored in the battery 
1 or a charge rate (whose maximum value is 100%) is determined according 
to the following equation. 
EQU Charge rate(%)={100-depth of discharge(%)+charge time (h).times.charging 
current (CA).times.rated capacity (Ah)}/full charge capacity 
(Ah)!.times.100 (5) 
Next, the CPU 5a indicates the charge rate and the fully charged capacity 
of the battery 1 through the display 10, and the routine then terminates. 
Alternatively, the routine may terminate after the battery 1 is further 
charged at a given lower current for a preselected period of time. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate a better understanding thereof, it 
should be appreciated that the invention can be embodied in various ways 
without departing from the principle of the invention. Therefore, the 
invention should be understood to include all possible embodiments and 
modification to the shown embodiments which can be embodied without 
departing from the principle of the invention as set forth in the appended 
claims. For example, a charging current provided by the power circuit 2 to 
the battery 1 is not limited to the constant current of 0.1 A. The present 
invention may applied to all manner of secondary batteries which assume 
the above characteristics of the voltage variation during charge at a 
constant current.