Electricity: battery or capacitor charging or discharging – Sequential charging or discharging of batteries or cells
Reexamination Certificate
2000-11-29
2001-12-04
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Sequential charging or discharging of batteries or cells
C320S160000
Reexamination Certificate
active
06326769
ABSTRACT:
BACKGROUND
The present invention pertains to a method and system for charging a battery which reduces the heat dissipation in the charging circuitry. More particularly, the present invention relates to methods and apparatus for reducing heat dissipation in one or more transistors that which controls the charge current of a battery cell by altering the charging cycle to provide a reduced current after the constant current stage and before constant voltage stage.
Due to the miniaturization of electronic circuits in consumer electronics, there are large numbers of devices presently in use which rely upon batteries for their power. Such battery operated consumer electronics devices include mobile phones, laptop computers, video cameras, and like electrical devices. Since the majority of these devices use rechargeable batteries, there is a large demand for rechargeable batteries and need for improving the charging characteristics thereof.
Rechargeable batteries must periodically be connected to an external charger or supply of power to be recharged. One consequence of recharging a battery is the heat produced during the charging operation. The heat due to recharging is generally undesirable for a number of reasons. For example, the heat may damage the battery or reduce its useful life, or even cause problems in the circuitry of the device itself. In extreme cases, the heat due to recharging may be a hazard to the user, or result in unsafe conditions.
There are several types of rechargeable battery cells which are in common use. Among the most commonly used rechargeable batteries are nickel-cadmium (Ni-Cd), sealed lead acid (SLA), nickel-metal-hydride (NiMH), lithium-ion (Li-ion) and lithium-polymer (Li-polymer). Each type of rechargeable battery is characterized by properties resulting from its battery chemistry and design.
Ni-Cd batteries are known to be inexpensive, but are subject to the memory effect, that is, voltage depression. The memory effect reduces the capacity of a Ni-Cd cell if the battery is not fully discharged before re-charging it. Ni-MH batteries tend to be more expensive than Ni-Cd, but have a higher charge capacity per unit of weight than Ni-Cd batteries. Furthermore, Ni-MH batteries are not subject to the pronounced memory effect of Ni-Cd batteries.
Li-ion batteries are advantageous over the aforementioned nickel-based batteries in that Li-ion batteries have a higher energy density per unit of mass. Li-ion batteries are also not subject to the memory effect that exists in other types of nickel-based battery cells, particularly Ni-Cd cells. An advantage of Li-polymer batteries is that they may be designed to be very thin, and even exhibit some flexibility instead of being structurally rigid. However, Li-polymer batteries are fairly high cost, relative to non-lithium battery technologies.
Another battery technology worth noting is that of sealed lead-acid (SLA) batteries. SLA batteries are based on well known lead-acid battery technology. SLA batteries are relatively low cost, but tend to be relatively heavy and cumbersome as compared to other types of batteries.
FIG. 1
is a conventional battery charging system
100
. The charging system
100
typically includes a power supply
110
and a battery charger assembly
120
in connection with the battery
130
. The charging system
100
may be configured so as to have the negative terminal of the battery
130
and the power supply
110
both connected to a ground potential, as shown in FIG.
1
. Alternatively, the battery charger assembly
120
may be provided with connections to separately connect the negative terminal of the battery
130
to a negative terminal of the power supply
110
(not shown), with no need for use of a ground return.
The battery charger assembly
120
receives current and voltage inputs from the power supply
110
, and in turn, provides current and voltage to the battery
130
in accordance with a conventional charging scheme. The constant current-constant voltage (CC-CV) charging operation is typically the conventional charging scheme which is used to recharge batteries.
FIG. 2
depicts the typical current and voltage profile of a CC-CV charging operation. Rechargeable battery cells are often charged using such a CC-CV two-stage charging process in which the charger first provides a constant current, I
CC
, which has an associated voltage V
CC
. The charging process is then completed during a CV stage by providing a constant voltage, V
CV
, which has an associated current C
CV
.
During the CC stage of a conventional CC-CV charging process, the constant current I
CC
is applied to the battery until the cell approaches its rated voltage, V
MAX
, sometimes referred to as the maximum voltage. The cell voltage steadily increases during the CC stage until the fully-charged cell voltage is reached. During the CV stage of a conventional CC-CV charging process, a constant voltage equal to the fully-charged cell voltage is applied to the battery until the battery is fully charged. A battery is characterized by a rated voltage, which is often defined in the specifications provided by the manufacturer of the battery. The rated voltage, which may also be referred to as the specified charging voltage or the rated charge voltage, is the maximum recommended voltage for charging the battery. The rated voltage depends upon the battery chemistry and other design parameters of the battery. Typically, the point at which the charging process transitions from the CC stage to the CV stage occurs when the charging voltage during the CC stage reaches the rated voltage of the battery.
SUMMARY
One drawback of conventional battery charging operations is the heat generated during charging. In particular, the present inventor recognized that much of the generated heat occurs during the first portion of the constant voltage charging period of the charging cycle. The generated heat results from a voltage drop in the charge current-switch transistor of the battery charger. The heat dissipation in the charge current-switch transistor may detrimentally affect the transistor itself, the charging circuitry, or even the device in which the battery is used. Accordingly, the present invention overcomes drawbacks of conventional charging systems by reducing the heat generated during this juncture of the charging process. Hence, through use of the present invention, the transistor heat dissipation may be reduced which enables the use of smaller less expensive charge-current transistors. This is advantageous in mobile communications devices or other portable electronics assemblies in which miniaturization and weight reduction is important.
In accordance with an exemplary method of the present invention for charging a battery, following a first stage of charging the battery in which a constant current is provided to the battery, a reduced amount of current is provided to the battery during a second stage of charging the battery, said reduced amount of current being less than said constant current. Then, during a third charging stage, a constant voltage is applied to the battery to complete the charging operation.
A battery charging apparatus in accordance with an exemplary embodiment of the present invention has battery charging circuitry and a controller. Following a first constant current stage of charging the battery, the controller causes the battery charging circuitry to provide a reduced amount of current to the battery during a second stage of charging the battery. Then, following the second stage, a constant voltage is applied to the battery in a third stage of charging the battery.
It should be emphasized that the terms “comprises” and “comprising,” when used in this specification, are taken to specify the presence of stated features, integers, steps or components; but the use of these terms does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
REFERENCES:
patent: 5952815 (1999-09-01), Rouillard et al.
patent: 5998966 (1999-12-01), Gaza
patent: 6087810 (2000
Burns Doane Swecker & Mathis L.L.P.
Telefonaktiebolaget LM Ericsson (publ)
Tso Edward H.
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