Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2001-08-29
2002-06-04
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
Reexamination Certificate
active
06400123
ABSTRACT:
TECHNICAL FIELD
The invention relates to battery powered electronic devices. In particular, the invention relates to battery fuel gauging for indicating an amount of charge remaining in a battery.
BACKGROUND OF THE INVENTION
Electronic devices that derive some or all of their operating power from a battery are popular, widely available, and in widespread use. Many of these so-called battery powered electronic devices would be much less marketable without the availability of reliable battery power. In fact, many popular portable electronic devices, such as notebook and laptop computers, hand-held computers and personal digital assistants (PDAs), digital cameras, and cellular telephones, would be of little or no use without a reliable and dependable battery power supply.
Many battery powered electronic devices provide a battery ‘fuel gauge’ to keep the user informed regarding the reliability of the battery power. A battery fuel gauge is an indicator that shows remaining energy capacity or charge level of the battery. The fuel gauge is intended to keep a user of the device informed of a current or remaining battery charge level and, by extension, a probable remaining operating time of the electronic device. In addition, the fuel gauge, or more precisely, data collected by the electronic device and used to generate the fuel gauge, is often used to determine whether or not a predetermined cut-off point in a discharge profile of the battery has been reached. The cut-off point is a point in the battery discharge profile beyond which adequate power may not be available to insure proper device operation. By detecting if and when the cut-off point has been reached, the device can, among other things, initiate a ‘soft shut-down’. Such a device initiated soft-shut down can help to prevent or at least mitigate various inopportune or inconvenient consequences of an unexpected loss of adequate operational power at or near an ‘end of discharge’ of the battery.
While having an accurate battery fuel gauge is useful for and enhances the reliability of battery power supplies for electronic devices, generally implementing such a fuel gauge is not a simple, straightforward task. Simply put, there is no direct means of determining or measuring the current charge level of a battery. Therefore, battery fuel gauging generally employs an indirect approach. Instead of directly measuring charge level, battery fuel gauges generally attempt to predict or infer the charge level from measurements of the dynamic electrical behavior or characteristics of the battery. In most cases, fuel gauging is based on monitoring battery characteristics, such as battery voltage or battery current, as a function of time. The measured data from monitoring one or more of these battery characteristics are then translated into a fuel gauge reading or result using a fuel gauge algorithm.
Unfortunately, the problem of battery fuel gauging is made even more difficult in devices capable of accepting and utilizing batteries of any one of different battery chemistries. In simple terms, a battery is a device that converts chemical energy into electrical energy or electricity. The ‘chemistry’ of the battery refers to the specific combination of electrolytes and electrode materials used in the battery to create and sustain chemical reactions within the battery that produce electricity. A variety of different battery chemistries are currently commercially available including alkaline, high-energy alkaline, nickel-metal hydride (NiMH), nickel-cadmium (NiCd), and photo lithium or lithiumn-iron sulfide (Li—FeS
2
). Moreover, all of these chemistries are available in a variety of common battery sizes or form factors, including, but not limited to, an ‘AA’ size.
The chemistry of a battery is relevant to fuel gauging because of a direct relationship between the chemical reactions and the electrical characteristics of the battery. Essentially all measurable electrical characteristics of a given battery, including but not limited to, open-circuit voltage, loaded voltage, charge capacity, peak current and even re-chargeability are a direct result of the specific chemical reactions taking place within the battery. The unique qualities of a battery's chemical reaction, such as reaction rate, reaction path, and reactants involved, are sometimes referred to collectively as the ‘reaction kinetics’ or simply ‘kinetics’ of the battery. The reaction kinetics of the battery dictate the electrical characteristics of the battery. Thus, any of the electrical characteristics of the battery that might be usefully monitored for fuel gauging will be directly affected by or depend on the battery chemistry.
For example, the open-circuit voltages at full charge, mid charge and end of discharge can and do differ from one chemistry to another. In addition, peak current levels and internal resistance levels differ among the various chemistries, leading to different measured voltages when the batteries are placed under a load. Thus, a fuel gauging approach designed or ‘calibrated’ for one chemistry may not be particularly accurate or effective for another battery chemistry even in the same form factor.
Despite these problems, most battery powered electronic devices employ one of two methodologies in conjunction with monitoring batteries and providing fuel gauging: current monitoring or voltage slope monitoring. Current monitoring, sometimes called power monitoring, determines the energy capacity remaining in a battery by monitoring the power or current passing into and out of the battery. Current monitoring requires knowledge of the approximate amount of energy that can be drained from the battery before it is fully discharged. As such, the use of power/current monitoring is generally restricted to electronic devices that utilize a battery having known characteristics such as an application-specific battery pack. An application-specific battery pack is typically manufactured and/or distributed under the control of a manufacturer of the electronic device. Therefore, the manufacturer can control the battery pack specifications and thus effectively has some considerable control over the accuracy of the battery monitoring and fuel gauging system. Essentially, the fuel gauge can be calibrated based on a priori knowledge of the application-specific battery pack performance characteristics and chemistry.
Because a priori knowledge of battery characteristics is not available to devices that accept multiple battery chemistries (i.e., devices that do not use an application-specific battery pack), generally the current monitoring approach is not used for fuel gauging in these devices. Instead, these devices generally employ the second methodology, voltage slope monitoring. Voltage slope monitoring monitors a change in battery voltage over a change in time (dv/dt) during battery discharge. The change in voltage with respect to time (dv/dt) is referred to as the voltage slope of the battery. If the voltage slope characteristics are known for a given battery chemistry, a reasonably accurate prediction can generally be made regarding charge level based on a measured voltage at various points during the discharge cycle of the battery. Therefore, a periodic measurement of the battery voltage can be used to monitor the battery and provide battery fuel gauging for the electronic device.
In devices designed to work with any one of multiple battery chemistries, the battery chemistry may not be known a priori. In such multi-chemistry situations, the battery fuel gauge is generally designed to accommodate a lowest common denominator among the potential battery chemistries. In most cases, the fuel gauge is simply calibrated for the battery chemistry most likely to be commonly used in the device. As a result, while the fuel gauge may be relatively accurate for the calibrated battery chemistry, the fuel gauge may be grossly inaccurate for other chemistries. In other words, the accuracy of such a fuel gauge directly depends on the battery chemistry being used at a given moment.
Consider, fo
Bean Heather N.
Whitman Christopher
Hewlett--Packard Company
Tso Edward H.
LandOfFree
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