Electricity: battery or capacitor charging or discharging – Battery or cell charging – Pulsed
Reexamination Certificate
2000-03-31
2002-04-02
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
Pulsed
Reexamination Certificate
active
06366056
ABSTRACT:
TECHNICAL FIELD
The present invention is a fast charging process for lithium-based batteries.
BACKGROUND OF THE INVENTION
The number and variety of portable devices has been constantly increasing, from small applications such as cellular phones, personal digital assistants (PDAs) and portable computers, to applications as large as electric vehicles and lift trucks. As a result, the focus of improving the performance of these devices has been concentrated in two areas: better batteries, and more energy efficient products. More energy efficient products means computer chips and other components which consume less energy, thereby reducing the frequency of recharging.
The area of better batteries involves two aspects: the battery itself, and the method of charging the battery. With respect to the battery itself, better batteries means smaller and lighter batteries which can store more energy, with a greater energy density, more effectively, for longer periods, under varying conditions of operation, and with more flexibility in packaging. Although battery manufacturers have been successful in developing new batteries that exhibit one or more of these characteristics, those batteries frequently have a shorter cycle-life and use increasingly unstable and less understood elements to achieve those characteristics.
There has also been a strong focus on faster and more efficient methods of charging the battery. However, there are significant differences in the electrochemical natures of different battery types so different types of battery chargers, and different methods of charging, are necessary to address these different battery types and their associated different charging requirements and limitations. However, even with continued advances with respect to more energy efficient devices and better batteries, the users of battery-operated devices continue to experience problems.
Most prior art battery charging methods have focused on charging methods for lead acid, nickel cadmium, and nickel metal hydride batteries. In practice, the majority of chargers in use today for these battery chemistries feature traditional constant current and trickle charging techniques. Others chargers use fast charging techniques which are often nothing more than a higher level of constant current. Some fast charging methods are accompanied by a basic charge termination method, and some use a current cutback as the battery nears the end of charge cycle or approaches a predetermined charge level. Other chargers utilize pulse charging, which consists of positive current pulses which are separated by rest periods, discharge pulses, or both. However, these charging methods often result in long charging times and/or premature battery degradation, and therefore a lack of availability and/or reliability in those battery-operated devices.
Some prior art battery chargers, especially those for use with NiCd and NiMH batteries, measure the temperature of the battery, such as by sensing the resistance of a thermistor which is located inside the battery. If the battery temperature is not within predetermined parameters, a fault condition exists and the charger does not initiate, or stops, the charging process. If there is no fault condition, a rapid charge sequence may be initiated. The temperature and voltage of the battery is monitored. In addition, changes in battery voltage can be monitored. When the slope of the battery-charging curve becomes negative, or the battery temperature reaches a predetermined value, the battery is fully charged, so fast charging is terminated, and a trickle charge process is started.
However, this type of fast charging process does not work well with lithium-based batteries, such as lithium-ion and lithium-polymer batteries, and problems occur when the above fast-charging techniques are used with lithium-based batteries. Continuous high current, i.e., greater than the
1
C rate where C is the capacity of the battery, causes metallic lithium to plate or be deposited onto the electrode. This permanently reduces the capacity of the cell. Another problem is the decomposition of the electrolyte. Another problem caused by conventional fast charging methods is overheating of the battery, which causes the battery's useful life to be shortened. Still another problem is explosive, destructive failure of the battery.
Lithium-ion cells have unique characteristics which make rapid charging difficult: lithium ion cells cannot tolerate the application of a high amplitude direct current. Moreover, lithium ion cells have demonstrated a propensity to explosively fail upon the application of excessive charging voltages. Thus, for safety reasons, all manufacturers impose a voltage limit of approximately 4.2 volts. Further, continuous high current (i.e., greater than the cells
1
C rate) causes metallic lithium to plate onto the electrode rather than being adsorbed into the electrode. This can permanently reduce the capacity of the cell.
Thus, for charging lithium-based batteries, a constant current/constant voltage (CC/CV) technique is the most common method. In conjunction with the CC/CV method, many chargers will complete the charge process with a trickle charge stage. If the battery temperature reaches a predetermined value, or the battery voltage reaches some predetermined value, then primary charging is terminated in order to prevent overheating of the battery. The charger is then placed in a trickle-charge mode where the battery is charged at the rate of approximately C/10 to C/20. For example, if the battery has capacity of 1000 mAh, at a C/10 charge rate the charger would charge the battery using a current of 100 mAh. These prior art methods typically require 3 to 10 hours to fully charge a lithium-based battery and still tend to heat the batteries, which causes the battery's useful life to be shortened.
The long charging time and short battery life result when the concentration gradient increased and the diffusion rate or intercalation of lithium ion into the carbon or graphite electrode decreased and the battery approached a steady state condition wherein the battery would not accept the charging current. When this condition was reached the low amplitude of the charging current resulted in a charging time of hours to complete the charge. When this condition occurred, the charging voltage was often increased so as to force a higher charging current into the battery in an attempt to reduce the charging time. This results in dissolution of the electrolyte, the plating of metallic lithium, and a consequent shortening of the battery life. Thus, in addition to a still too long charging time of 3 to 4 hours, the actual lifetime of those batteries was reduced, generally to about 300 cycles.
Thus, there is a need for a charging technique which provides for safe, fast recharging of lithium-based batteries without causing degradation of the batteries.
SUMMARY OF INVENTION
The present invention provides a method and an apparatus which safely and rapidly charges lithium-based batteries while reducing the negative side effects which result in premature battery capacity degradation or destructive failure.
In accordance with the preferred embodiment of the present invention, several stages are implemented to effect fast and efficient charging of the battery, full charging of the battery, and terminating the charging of the battery. These stages include a charging stage, two removal stages, and a measurement stage. The charging stage comprises one or more charge pulses separated by rest periods. One removal stage comprises a plurality of alternating charge pulses and discharge pulses, separated by rest periods. Another removal stage comprises one or more large magnitude discharge pulses followed by rest periods. These three stages are applied to the battery in a sequence that rapidly charges the battery, efficiently mixes the electrolyte within the battery, provides voltage and impedance measurements to determine the condition of the battery, and restores a battery's capacity. The measurement stage c
Cope Richard C.
Kusharskaya Galina K.
Podrazhansky Yury M.
Enrev Corporation
Troutman Sanders LLP
Tso Edward H.
Warner Charles L.
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