Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
1999-06-11
2001-07-24
Wong, Peter S. (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
C324S433000, C320S136000
Reexamination Certificate
active
06265877
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of detecting remaining capacity for secondary batteries wherein nickel hydroxide is used as a positive electrode active material, such as a nickel metal-hydride secondary battery and a nickel cadmium secondary battery.
In appliances in which secondary batteries are used as a power source, it is necessary to detect the remaining capacity of the secondary batteries in order to prevent overdischarge and overcharge, so that the appliances are used within a suitable range of the remaining capacity of the batteries.
The remaining capacity of a secondary battery can be indexed by the state of charge (SOC) which indicates the rate of accumulated electrical quantities in relation to battery capacity. This SOC can be determined from voltage because charge/discharge characteristic curves corresponding to charge/discharge electric current and temperature can be determined through experimentation. That is, by detecting voltage at a certain electric current value and temperature, and by utilizing the charge/discharge characteristic curves corresponding to the above electric current value and temperature, the SOC of the battery can be obtained.
Accordingly, it is possible to detect, by measuring voltage of a secondary battery, whether the SOC after a final discharge (i.e., the last discharge before the battery is recharged) has reached a permissible lower limit (an SOC of 20%, for example). A determination is then made in accordance with the results whether further discharge will induce an overdischarged state, whereupon discharge can be stopped and the battery charged.
The determination voltage (also referred to as end of discharge voltage) for determining whether the SOC has reached a permissible lower limit as described above has usually been determined in the prior art as follows. The determination voltage was determined based on a discharge characteristic curve (complete discharge characteristic curve) indicative of the relationship between the SOC and voltage that is obtained at the time of complete discharge (first discharge). By way of example, in the case of the discharge characteristic curve A shown by solid lines in
FIG. 1
, the voltage value 1.237V that corresponds to the SOC of 20% was defined to be the determination voltage for determining whether the SOC has reached the permissible lower limit.
However, in the case of nickel metal-hydride or nickel-cadmium secondary batteries using nickel hydroxide as a positive electrode material, as the charge and incomplete discharge cycles are repeated, a memory effect is observed wherein, as shown in the discharge characteristic curves in
FIGS. 2 and 3
, voltage corresponding to the SOC decreases in accordance with the number of charge/discharge cycles (represented by [
1
]~[
10
]), and drops considerably, particularly at the final discharge. Thus, if the determination voltage is defined based on the complete discharge characteristic curve as above, there is a problem that it may be erroneously determined that the SOC has reached the permissible lower limit (for example, an SOC of 20%) although there actually remains a sufficient amount of remaining capacity (an SOC of 30%, for example) in the case where charge and discharge are repeated a large number of times as shown in FIG.
3
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method of detecting the determination or end of discharge voltage of secondary batteries using a nickel-hydroxide positive electrode, by which a correct determination of whether the SOC has reached a permissible lower limit even after the final discharge is possible by measuring the voltage of the batteries.
In order to achieve the above object, the method of detecting the determination voltage based on the SOC of a secondary battery which uses a nickel-hydroxide positive electrode according to a first aspect of the present invention includes the steps of setting a permissible lower limit of the SOC designed, for example, to prevent overdischarge of the battery, and determining the voltage of the battery when the SOC reaches the lower limit after multiple discharges, the voltage for discharges after the first discharge gradually decreases from the voltage after the first discharge due to a memory effect. Thereafter, the voltage for determination, i.e., the voltage at which discharge should be stopped and charging commenced, is determined in consideration of the voltages at which the SOC is the set lower limit after the multiple discharges whereby the determination voltage is lower than the voltage the battery reaches after the first discharge for the lower limit of the SOC, and a determination is made whether the SOC has reached the lower limit based on this voltage for determination, which entails a comparison of the actual voltage to the determination voltage.
According to the above method, the determination voltage is defined lower than the voltage when the SOC reaches the permissible lower limit after the first discharge in consideration of a memory effect observed in secondary batteries using a nickel-hydroxide positive electrode. As a result, even in the case of repeating charge and discharge cycles a large number of times, the actual SOC when it is determined to have reached the permissible lower limit can be made closer to the lower limit of the SOC defined as the determination reference.
It is preferable to set the voltage for determination 0.01~0.10V/cell lower than the voltage when the SOC reaches the permissible lower limit at the first discharge.
Also, to achieve the above object, the method of detecting the determination voltage based on the SOC of a secondary battery which uses a nickel-hydroxide positive electrode according to a second aspect of the present invention includes the steps of setting a permissible lower limit of the SOC designed, for example, to prevent overdischarge of the battery, and determining the voltage of the battery when the SOC reaches the lower limit after multiple discharges, the voltage for discharges after the first discharge gradually decreases from the voltage after the first discharge due to a memory effect. Several determination voltages are determined from the voltages at which the SOC is the set lower limit after the discharges and thus the determination voltage of the battery is variable in accordance with the number of times of charge and discharge of the battery.
According to the above method, the voltage for determination is defined in accordance with the number of charge and discharge cycles repeated in consideration of a memory effect observed in secondary batteries using a nickel-hydroxide positive electrode. As a result, at any time of discharge, the actual SOC when it is determined to have reached the permissible lower limit can be made remarkably closer to the lower limit of the SOC defined as the determination reference.
REFERENCES:
patent: 5691624 (1997-11-01), Im et al.
patent: 5804894 (1998-09-01), Leeson et al.
patent: 6023151 (2000-02-01), Hudson et al.
patent: 8-78065 (1996-03-01), None
patent: 8-185890 (1996-07-01), None
patent: 8-336202 (1996-12-01), None
Kimura Tadao
Murakami Yusai
Nakanishi Toshiaki
Yonemoto Masaru
Jordan and Hamburg LLP
Matsushita Electric - Industrial Co., Ltd.
Toatley , Jr. Gregory J.
Wong Peter S.
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