Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2003-04-28
2004-12-28
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
Reexamination Certificate
active
06836095
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to rechargeable batteries, and particularly relates to battery charging methods and apparatus that offer improved charge current sensing accuracy.
Rechargeable batteries appear in a growing range of electronic devices. The use of rechargeable batteries is particularly common in portable electronics, such as cell phones, Portable Digital Assistants (PDAs), pocket and notebook computers, Global Positioning System (GPS) receivers, etc. No one rechargeable battery type finds universal usage across this diverse range of devices, as each battery type offers its own set of tradeoffs regarding performance, size and cost.
For example, types of rechargeable batteries include, but are not limited to, lead-acid cells, nickelcadmium cells, nickel/metal hydride cells, sodium/sulfur cells, nickel/sodium cells, lithium ion cells, lithium polymer, manganese-titanium (lithium) cells, nickel zinc cells, and iron nickel cells. Each of these battery chemistries offers its own mix of advantages and disadvantages regarding size, energy density (volumetric or gravimetric), cost, cell voltage, cell resistance, safety, toxicity, etc.
Despite such differences, some charging algorithms find broad applicability across a wide range of battery chemistries. As an example, the constant-current/constant-voltage (CC/CV) charging algorithm is adaptable to many different types of battery chemistries and, therefore, finds wide usage in a variety of battery charging products. With the CC/CV charging algorithm, a discharged battery is charged at a constant current until its cell voltage rises to a defined threshold voltage, e.g., the battery's “float voltage,” at which point the charging control is switched to constant/voltage to charge the remaining capacity of the battery without exceeding the voltage limit of the battery. Thus, the CC/CV charging algorithm initially relies on current feedback charging control and then switches over to voltage-feedback control once the battery-under-charge reaches its float voltage.
As a matter of convenience, the charging current during the CC phase of recharge should be as high as possible within recommended limits because higher charging currents equate to lower recharge times. Battery manufacturers often rate battery capacity in terms of a given battery's “C rating,” which is a scaling unit for the battery's charge and discharge currents. Charging or discharging the battery at rates beyond the “C” rating exceeds the safe rating of the battery. For example, a charge current of 1000 mAh (1 C) will charge a 1000 mAh battery in about an hour. Thus, charging current during the CC phase of recharging may be set according to the manufacturer's recommended C rate limit.
Proper C rate based charging requires relatively accurate current sensing to ensure that the CC phase of charging actually is regulated to the recommended C rating of the battery. In the above example, the 1 C charging current is 1 Amp and thus the current sensing used to regulate the charging current must sense current in the 1 A range with relatively good accuracy.
Most simple and economical current sensing techniques rely on Ohm's Law (V=IR), and thus measure current by sensing a current-induced voltage drop across a resistor or other series impedance element. For efficiency and low voltage dropout, such sense resistors typically are sized for a particular current range of interest. For example, a 100 m&OHgr; sense resistor provides a very usable 100 mV sense voltage at 1 A of charging current. Obviously, as the charging current falls, so too does the sense voltage, and therein lies one of the many challenges faced by designers of battery charging systems. That is, the sense voltage decreases with decreasing charge current and circuit error sources, such as sense amplifier voltage offsets, become increasingly significant obstacles to accurate charge current measurement.
Further compounding these design challenges, not all charging environments offer the ability to charge at a battery's recommended C rate. For example, many battery-powered devices interface to Personal Computers (PCs) and the like via Universal Serial Bus (USB) connections. Portable music players, such as those based on the popular MP3 digital audio format, are just one example of such devices. Regardless, the USB standard defines low-power devices as those requiring less than 100 mA, and high power devices as those requiring up to 500 mA. Some USB ports support both low power and high power, and thus offer attached devices the ability to draw charging currents up to the high power limit of 500 mA. However, some USB ports support only low power devices and thus limit charging current to 100 mA. Ideally, a rechargeable USB device would operate with accurate charge current sensing regardless of the type of USB port to which it is attached.
Further charging scenario variations create additional challenges to reliable and accurate current sensing. For example, some types of battery technologies are incompatible with so-called “trickle” charges, i.e., continuous low current into the battery after it reaches its float voltage. Because of the inability of conventional charging systems to accurately sense very low levels of battery current, it is a common practice to include a transistor switch or other isolation device to “disconnect” the battery from the charging circuit after reaching an end-of-charge condition. The addition of the extra switching element adds undesirable expense and size to the charging circuit.
For these and other reasons, an ideal battery charging circuit would offer highly accurate current sensing over a wide range of charging currents. Accurate current sensing over a wide dynamic range would thus allow reliable and safe charging in various modes (e.g., low power and high power), would permit accurate end-of-charge current sensing, and would permit regulation down to an effectively “zero” charge current. That latter capability would eliminate the requirement for battery isolation switches as the charging circuit itself could regulate current into the battery essentially down to zero.
SUMMARY OF THE INVENTION
The present invention comprises a system and method to charge batteries based on accurately sensing battery charging currents over a wide dynamic range. In an exemplary embodiment, a battery charging circuit includes a sense circuit that includes a time-averaging amplifier circuit to accurately sense the charging current of a battery under charge. An exemplary time-averaging amplifier circuit comprises a polarity-switched amplifier that periodically switches amplifier signal polarities to null amplifier-offset errors from the sensed charging current.
For example, in at least one embodiment, the sense circuit generates the sense signal as a voltage that is proportional to the charging current, wherein the polarity-switched amplifier is used as a differential sensing amplifier that controls the sense signal. An adjustable element, such as a user-set resistor, may be used to provide a desired scaling between the sense signal and the battery charging current magnitude. In one polarity configuration, the polarity-switched amplifier's offset errors add to the sense signal and in the opposite polarity configuration those same offset errors subtract from the sense signal. Thus, by periodically switching between these polarity configurations, the amplifier's offset errors effectively are averaged out of the sense signal.
In at least one embodiment, the polarity-switched amplifier includes a differential amplifier having switched amplifier input and output connections. In an exemplary embodiment, one input switch selectively couples a current sensor, such as a series resistor disposed in the charging current path, to the non-inverting and inverting inputs of the amplifier, and another input switch selectively couples a sense feedback signal to the inverting and the non-inverting inputs of the amplifier. Similarly, an output sw
Coats & Bennett P.L.L.C.
Semtech Corporation
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