Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2000-11-24
2001-09-04
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S149000, C324S426000
Reexamination Certificate
active
06285163
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a means for estimating the state of charge of a battery, especially by using a battery model to minimize an estimation error, and an improved method for estimating the degraded state of battery (battery condition).
RELATED ART
A method for estimating state of charge (SOC) of a battery by adding an integrated value of charging/discharging current to an initial SOC of the battery has conventionally been known. With this method, however, it is difficult to accurately estimate the SOC of a battery because of factors such as the error inherent in integration of the charging/discharging current values, the accumulation of such error, change in the initial SOC due to self-discharge when the battery is not used, and so on.
To improve accuracy of estimation, a method for estimating the SOC also from a voltage of a battery and correcting the estimation result of the SOC obtained by integrating the charging/discharging current values has also been studied. For example, Japanese Patent Laid-Open Publication Hei 9-96665 discloses such an improved estimation method.
However, even by using the method for estimating the SOC from the battery voltage and correcting the integration error of the charging/discharging current values, it remains difficult to ensure a high estimation accuracy because there is no simple and accurate method for estimating the SOC from the voltage of the battery.
FIG. 6
shows a relationship between current and voltage of a battery having an SOC of 68%. As indicated in
FIG. 6
, the current-voltage relationship of the battery is not linear and has a large hysteresis. If the SOC is estimated from this current-voltage relationship, a large error may result depending on where in the changes in current and voltages of the battery the SOC is judged. In
FIG. 6
, although the real SOC is 68%, the SOC is judged to be 80% during increase of the charging current, and the SOC is judged to be 20% during increase of the discharging current.
As such, a significant estimation error is produced if the SOC is estimated based on the battery voltage, because, even if the SOC is unchanged, the voltage of the battery changes widely reflecting the charging/discharging state of the battery immediately before the voltage is measured. Thus, accurate estimations of the SOC could not be realized by any conventional method. The estimation error of the SOC is particularly large in a hybrid vehicle where the battery repeats charging and discharging in short cycles.
In the case of a battery used in an electric vehicle or the like, the degraded level of the battery is estimated in order to determine the replacement timing of the battery or to predict occurrence of troubles.
The state of decay of a battery can be measured out by determining the internal resistance of the battery. The internal resistance may be determined, for example, from the voltage of the battery after discharging a predetermined amount of discharging current for a certain period of time from the battery in a predetermined state of charge (SOC). In electric vehicles, it is also possible to determine the internal resistance R from several current/voltage values, while the vehicle is driven, by
Vb=−R·Ib+V
oc (1)
where Vb is a voltage value, Ib is a current value, and Voc is an open voltage.
In the above method where a predetermined amount of discharging current is discharged for a certain period of time from the battery having the predetermined SOC, measurement is done offline. Therefore, if the battery is used, for example, in an electric vehicle, it is impossible to estimate the internal resistance of the battery while the vehicle is driven.
On the other hand, by the method for determining the internal resistance of the battery using the above equation (1), it is possible to determine the internal resistance of the battery while the electric vehicle is driven. However, the influence of polarization of the battery is not considered in this method, so that a significant estimation error may be present in the obtained internal resistance. More specifically, the charging/discharging current of the battery is correlated to the voltage of the battery as shown in
FIG. 10
, in which the discharging state is indicated by positive current values and the charging state is indicated by negative current values. As shown in
FIG. 10
, the current/voltage characteristic of the battery includes hysteresis, so that a gradient between voltage and current, i.e., the internal resistance (−R), is changed depending on the measurement timing. This happens because the polarization is not considered, as described above. Thus, this conventional method can not be used to simply and accurately estimate the internal resistance of the battery.
In view of the above, an object of the present invention, therefore, is to provide a means for estimating the charged state of a battery capable of accurately estimating an SOC even though the battery repeats charging/discharging in short cycles.
Another object of the present invention is to provide a method for estimating the degraded state of battery capable of accurately measuring variations of internal resistance of the battery while the battery is used and of correctly ascertaining the degraded state of battery.
DISCLOSURE OF THE INVENTION
To achieve the above objects, there is provided in the present invention a means for estimating the charged state of a battery, in which a battery model is provided for determining a pseudo-SOC (state of charge) as a temporary value representative of the SOC of the battery, and voltage of the battery is estimated by considering the pseudo-SOC and a change in the state of the battery, whereby a real SOC is estimated by correcting the pseudo-SOC so that the estimated battery voltage is equal to an actually measured battery voltage.
In the means for estimating the charged state of the battery, the battery model includes a pseudo-SOC estimating means for determining the pseudo-SOC from charging/discharging current of the battery; an electromotive force estimating means for estimating the voltage of the battery based on the pseudo-SOC output from the pseudo-SOC estimating means; a voltage change estimating means for estimating a change in the voltage of the battery caused by internal resistance; and a dynamic voltage change estimating means for estimating a change in the voltage of the battery based on a change in charging/discharging current of the battery, whereby the battery voltage is estimated from the sum of output values of the electromotive force estimating means, the voltage change estimating means, and the dynamic voltage change estimating means.
In the means for estimating the charged state of the battery, the correction of the pseudo-SOC consists of a component proportional to a difference between the estimated battery voltage and the actually measured battery voltage, and a component proportional to an integrated value of the difference.
In the means for estimating the charged state of the battery, the pseudo-SOC estimating means, the electromotive force estimating means, the voltage change estimating means, and the dynamic voltage change estimating means carry out correction during respective estimating operations in accordance with a temperature of the battery.
In the means for estimating the charged state of the battery, the pseudo-SOC estimating means, the electromotive force estimating means, the voltage change estimating means, and the dynamic voltage change estimating means carry out correction during respective estimating operations in accordance with the estimated SOC of the battery.
In the means for estimating the charged state of the battery, the dynamic voltage change estimating means is formed by a neural network having a feedback path.
In another aspect of the present invention, there is provided a method for estimating the degraded state of battery, including the steps of measuring charging/discharging current and a voltage of the battery; determining a pseudo-SOC (state of
Kikuchi Yoshiaki
Kuroda Yukio
Watanabe Nobuo
Oliff & Berridg,e PLC
Toyota Jidosha & Kabushiki Kaisha
Tso Edward H.
LandOfFree
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