Battery capacity calculating method and device therefor

Details Battery capacity calculating method and device therefor Battery capacity calculating method and device therefor

2001-10-09

2003-12-09

Tso, Edward H. (Department: 2838)

Electricity: measuring and testing

Electrolyte properties

Using a battery testing device

C320S132000

Reexamination Certificate

active

06661231

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for computing a charging capacity of a battery, which supplies the electric power to loads in a vehicle, and an apparatus for use in such method. More specifically, the present invention relates to a method and an apparatus for computing a charging capacity of a battery, in which a voltage-current characteristic derived from a periodically measured terminal voltage and discharge current of the battery is used to estimate an estimated voltage that is an estimated terminal voltage of the battery in its state of constant-load discharge, thereby the charging capacity of the battery is computed from the estimated voltage.
BACKGROUND ART
So far, a driving source of a vehicle has been mainly an engine, in which gasoline or gas oil is employed as the fuel, but in recent years a vehicle employing an electromotive motor, which does not directly discharge the combustion gas, as the only or the supplementary driving source has appeared. As to a vehicle loading the electromotive motor, grasping a charging capacity of a battery, which supplies the electric power to the electromotive motor, is important for computing a possible traveling distance and so on.
So far, an integration method of current or electric power has been employed, in which an integrated consumed electric power computed by using an integrated value of the discharge current is subtracted from a full charging capacity so as to compute the present charging capacity. However, in such a method, an original full charging capacity is changed depending upon individual differences among batteries, deterioration rates of the batteries and so on, therefore the present charging capacity of the battery cannot be accurately computed.
A state of charge of the battery can be known by measuring the density of the electrolyte of the battery since there is a certain linear relationship between the density of the electrolyte and the state of charge. However, actually in a battery during charging or discharging and a battery right after the completion of charge or discharge thereof, chemical reactions occurring between the electrolyte and the electrodes make the density of the electrolyte non-uniform, therefore the state of charge of the battery cannot be known accurately by measuring the density of the electrolyte.
Besides, the charging capacity of the battery may be known by measuring the terminal voltage of the battery. But the terminal voltage is not stable unless the discharge current is stabilized, therefore actually the terminal voltage correlating with the state of charge of the battery cannot be obtained by the measurement.
As shown in characteristic graphs in
FIG. 11
, in which the battery is subjected to the discharge with each constant current ranging from 10 to 80 A in units of 10 A, the discharging time (horizontal axis) increases with decreasing the discharge current while the terminal voltage (vertical axis) of the battery drastically decreases with the discharging time.
Here, the horizontal axis of the characteristic graphs in
FIG. 11
is the time, however, since the discharge is carried out with the constant current and the battery capacity is expressed by electrical quantity (Ah), this horizontal axis can be regarded as the battery capacity.
Then, the characteristic graphs in
FIG. 11
reveals that smaller the discharge current, higher the electric power to be obtained and that the capacity drop near the state of full charge of the battery is slow while the capacity drop near the state of full discharge is rapid.
As described above, even if the discharge current can be stabilized, since there is no linear correlation between the charging capacity of the battery and the terminal voltage thereof, the charging capacity cannot be derived from the terminal voltage of the battery.
Thus, appears to be reasonable is a method for computing the capacity employing a relationship between the state of charge of the battery and the open circuit voltage, which is possibly a linear relationship since there is about a linear relation between the electrolyte density of the battery and the open circuit voltage and since there is a linear relation between the electrolyte density of the battery and the state of charge of the battery.
However, the sole weak point of this method for computing the capacity is that the open circuit voltage can be measured during non-discharging period of time when the state of charge of the battery does not change, except the self-discharging. In other words, the open circuit voltage cannot be measured during discharging when the state of charge of the battery changes.
Consequently, a point of the above method for computing the capacity is how to find out the open circuit voltage during the discharge of the battery.
The terminal voltage and the discharge current can be measured during the discharge of the battery. As shown in
FIG. 11
, since the terminal voltage appears to decrease with increasing the discharge current even when the state of charge of the battery does not change, there is a voltage-current characteristic (I-V characteristic) showing a negative correlation between the terminal voltage and the discharge current, which changes with changing the state of charge of the battery.
Thus, in order to know a plurality of the voltage-current characteristics of the battery in response to the state of charge of the battery, the following measurement is carried out.
First, a discharge is continuously carried out by using an impulse current, in which a current I
a
and a current I
b
smaller than I
a
are periodically mutually appear, and then the predetermined number of the sets (for example, 100 sets) of the terminal voltage having a reverse phase with respect to the discharge current and the discharge current, i.e. (I
a
, V
I
), (I
b
, V
2
), (I
a
, V
3
), (I
b
, V
4
),—are continuously sampled at the same period of time with the impulse cycle (for example, 1 millisecond) of the discharge current.
Then, from thus sampled sets of the terminal voltage and the discharge current, i.e. (I
a
, V
01
), (I
b
, V
02
), (I
a
, V
03
), (I
b
, V04),—, by using the method of least squares, coefficients a
1
and b
1
in an equation V=all +b
1
, i.e. a linear relationship between the voltage and current of the battery are obtained, wherein the equation V=a
1
I+b
1
is placed as the voltage-current characteristic of the battery corresponding to the capacity during the above sampling.
Then, the similar discharge to the discharge described above is continuously carried out by using an impulse current, in which currents I
a
and I
a
are periodically mutually appear, and then the predetermined number of the sets of the terminal voltage having a reverse phase with respect to the discharge current and the discharge current, i.e. (I
a
, V
11
), (I
b
, V
12
), (I
a
, V
13
), (I
b
, V
14
),—are continuously sampled. Then, from thus sampled sets of the terminal voltage and the discharge current, by using the method of least squares, coefficients a
2
and b
2
in an equation V=a
2
I +b
2
, i.e. a linear relationship between the voltage and current of the battery are obtained, wherein the equation V=a
2
I+b
2
is placed as the voltage-current characteristic of the battery corresponding to the capacity during the above sampling.
Thereafter, similarly, coefficients an and bn in an equation V=a
2
I +b
2
, i.e. a linear relationship between the voltage and current of the battery are obtained, wherein the equation V=a
2
I+b
2
is placed as the voltage-current characteristic of the battery corresponding to each mutually different capacity which gradually decreases, thereby the voltage-current characteristics of the battery corresponding to the respective capacities ranging from 100% to 0%.
In
FIG. 12
, there is schematically shown a relation between the sampled sets of the terminal voltage and the discharge current. i.e. (I
a
V
n1
), (I
b
, V
a2
), (I
a
, V
n3
), (I
b
, V
n4
),—and the linear voltage-current eq

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