Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2000-07-21
2001-11-20
Toatley, Jr., Gregory J. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S157000, C324S433000
Reexamination Certificate
active
06320354
ABSTRACT:
TECHNICAL FIELD
The present invention relates generally to techniques concerning the use of rechargeable batteries and, in particular, to a method and apparatus for charging such batteries.
BACKGROUND OF THE INVENTION
Rechargeable batteries and their use are well known in the art. Generally, a rechargeable battery is any device capable of storing a charge that is gradually depleted over time as the battery is used to drive a load. Such devices include, but are not limited to, batteries constructed from lithium material, nickel-cadmium (Ni—Cd) material or nickel metal hydride (NiMHO
2
) material. When the charge is at least partially (but preferably, fully) depleted, the rechargeable battery may be coupled to a charging circuit in order to recharge the battery. (As used in the context of the present invention, the terms “charging”, “recharging” and their variants are used interchangeably.) A system for recharging a battery is illustrated in FIG.
1
. As shown, the system comprises a charging circuit
102
electrically coupled to a battery
104
to be recharged and to an external power source
106
. Operation of the charging circuit
102
usually depends on the type of battery being charged and the type of charging to be performed (i.e., rapid versus normal recharging). In general, however, the charging circuit
102
operates to provide, in a controlled manner, power from the external power source
106
to terminals of the battery
104
. The flow of current at various voltage levels through the charging circuit
102
into the battery
104
causes a charge to be increasingly stored within the battery
104
. Typically, the battery
104
is considered recharged when it is capable of maintaining a predetermined current level at a predetermined voltage level, i.e., constant power, without the aid of the external power source
106
.
A lithium-type rechargeable battery typically exhibits favorable recharging properties in that it can be recharged through the application of a fixed voltage at the battery's terminals. A typical charging profile for lithium-type batteries is illustrated in FIG.
2
. During the recharge process of lithium-type batteries, the battery voltage is allowed to increase up to a fixed threshold, and then the current from the external power supply must be gradually decreased in order to maintain this threshold. If the external power supply has a limited output voltage, then the charge current into the battery will naturally taper to a low value as illustrated in FIG.
2
. The peak current during the initial charging period (when the battery is low) will be limited to the maximum output of the power source, and the power source output voltage will be pulled down to the battery voltage. The battery is fully recharged when the charge current into the battery decreases substantially to zero or a known low level.
However, in order to determine when the battery has reached such a state, the charging circuit typically comprises relatively complex circuitry for measuring the state of the battery and for controlling the voltage and currents applied to the battery's terminals. For example, conventional methods of battery metering require that the charging circuit measure voltage of the battery, and also measure and/or control current flow into the cell during the charge process. Coulomb-counting methods of battery metering require that the current flow out of the battery during discharge must also be measured. A typical prior art charging circuit
304
is illustrated in
FIG. 3. A
variable-output power supply
302
is electrically coupled to the charging circuit
304
, which in turn is electrically coupled to a battery
306
being recharged. The variable output of the power supply
302
is determined by the battery voltage (via feedback buffer
318
) during the recharge process. As the battery increases in voltage, the power supply
302
output increases proportionally in order to provide sufficient “headroom” for the desired current to flow through a sense resistor
308
and a gating device
316
. Headroom addresses the fact that the additional circuitry illustrated in
FIG. 3
will cause various voltage drops. In order to maintain a given voltage and current level at the battery
306
, a margin accounting for these voltage drops, i.e., headroom, must be provided. Current sensing is provided by a sense resistor
308
monitored by a current sense amplifier
310
. A control amplifier
312
compares the current measurement provided by the current sense amplifier
310
with a threshold
314
provided by a controller (not shown). The output of the control amplifier
312
is used to control the gating device
316
such that current applied to the battery
306
is properly controlled. Because the controller (typically, a microprocessor) actively controls the charge current through the control amplifier
312
, the battery's current charge level can be determined by the amount of taper current into the battery required to maintain the constant voltage.
The additional circuitry required to measure and control current and voltage levels represent an additional cost to charging circuits. In turn, the increased cost of such charging circuits increases the cost of various devices that incorporate them. For example, wireless communications units, such as handheld radiotelephones and the like, typically incorporate such charging circuits. In this manner, a user of a wireless communication unit can simply plug their unit into an external power source and thereby recharge the batteries (either external or internal to the unit) used to power the unit. Previous realizations of portable product designs have eliminated current sensing and/or control circuitry from the device, however these types of systems have lost the capability to accurately determine the battery level, particularly during charging as demonstrated, for example, in applications of the LM3420 series of lithium-ion battery charge controller manufactured by National Semiconductor Corporation. The ability to determine battery level during recharging is a convenient feature that many users of portable devices find useful. Thus, it would be advantageous to provide a technique that solves the problem of determining a battery's level during charging without the requirement for precise current monitoring or control.
SUMMARY OF THE INVENTION
The present invention provides a technique for battery charging that allows a charge state of a rechargeable battery to be determined without the use of complex circuitry for current monitoring or control. In particular, a battery charging circuit is provided comprising a recharge input and a recharge output with a switching element electrically coupled in series between the recharge input and output. A recharge controller controls a conducting state of the switching element. When in a conducting state, recharge current can flow from the recharge input to the recharge output, thereby allowing a rechargeable battery coupled to the recharge output to be recharged. During periodically occurring intervals, the recharge controller switches the switching element to a non-conducting state such that the recharge current no longer flows to the recharge output. During the intervals, a battery voltage input is sampled to determine a charge state of the battery. Depending on the sampled charge state, the switching element is either returned to a conducting state such that further recharging of the battery may occur, or it is left in a non-conducting state when the battery has been sufficiently recharged. Furthermore, the sampled charge state information may be used to drive an indication of the battery's charge state. In this manner, the need for complex current monitoring and control is eliminated while still allowing information regarding the charge state or level of the battery to be determined. These and other advantages and features of the subject invention will become apparent from the detailed description of the invention that follows.
REFERENCES:
patent: 5032825 (1991-07-01)
Oglesbee John W.
Sengupta Upal
Mancini Brian M.
Motorola Inc.
Toatley , Jr. Gregory J.
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