Electricity: battery or capacitor charging or discharging – Battery or cell charging – With detection of current or voltage amplitude
Reexamination Certificate
2001-03-05
2002-11-05
Toatley, Jr., Gregory J. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
With detection of current or voltage amplitude
C320S134000
Reexamination Certificate
active
06476585
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the field of devices and methods for charging lithium-ion cells (or batteries) and specifically to a charging circuit including a power transformer in which the loading curve of the power transformer is used to limit the current flow to the lithium-ion cell (or battery) and a method for charging lithium-ion cells (or batteries) in which the loading curve of a power transformer is used to limit the current flow to the lithium-ion cell (or battery).
BACKGROUND OF THE INVENTION
Lithium-ion cells are used in battery packs where high energy density and low weight are required. However, lithium-ion cells can be dangerous if operated outside of their rated specifications. Typically, such batteries are used in controlled environments and are accompanied by suitable protective devices to prevent such problems as short circuits, unduly high temperatures and over-discharge. A number of such protective devices are typically installed in the battery pack. It is standard industry practice that lithium-ion cells are equipped with in-pack circuitry that provides the necessary protection for the cell in use. Although the in-pack circuitry will provide over-all protection, suitable cell charging circuitry is required to provide repeated charging of the cell while satisfying applicable charging and operational constraints that vary somewhat from one cell type to another, as the manufacturer may have specified for any given design.
Particularly, lithium-ion cells carry a risk of generating excess gas due to overcharge or overdischarge—this may cause the safety vent of the battery pack Lo open and release electrolyte into the atmosphere. If this release of electrolyte is continued, the cells can lose sufficient electrolyte that they are disabled. Further, overcharge or overdischarge may generate excess heat, causing a severe rise in temperature that can reduce the ability of the cell to retain energy and reduce the number of charging cycles the cell can undergo before it must be replaced. More seriously, overcharging or overdischarging may occur to such an extent that the lithium metal is isolated from the other elements and may become plated onto one of the electrodes. Lithium metal is explosive in water and will, in varying degrees, react with the moisture in the atmosphere. Lithium-containing batteries have been known to catch fire, although more recent safety designs have reduced the chances of this occurrence. The avoidance of overcharge voltage and overcharge current during charging of a lithium-ion cell is therefore an important objective in the use of lithium-ion cells, has been achieved by a number of known regulator circuits, and is also a principal objective of the present invention.
It is known that the attained charge capacity of a lithium-ion cell is significantly reduced if the charging voltage is less than the manufacturer's recommended maximum charging voltage (say 4.1 volts). With a drop of charging voltage of only 0.05V (approximately 1%), a loss of up to 5% in charge capacity occurs. However, if the charging voltage reaches only 4.0 volts (a drop of 0.1V or approximately 2%) then a loss of charge capacity of up to 12% occurs. On the other hand, as pointed out previously, if one exceeds the manufacturer's recommended maximum charging voltage, the life cycle of the cell is decreased, or worse, catastrophic breakdown of the cell can occur. Therefore one is compelled by these combined constraints to charge the lithium-ion cell at a voltage (at least at the end of the charging cycle) that is as close as reasonably possible to the maximum charging voltage without exceeding it.
Previous battery charging circuits for lithium-ion cells or batteries are known that include suitable regulator devices to maintain charging voltage and current within acceptable constraints. The “charge inhibition voltage” refers to the value that the cell manufacturer has set as the upper limit of operating/charging voltage of the cell. If the voltage exceeds this value, lithium metal may become plated to an electrode, with potentially dire consequences as discussed above. The “maximum charging voltage” is also established by the manufacturer at a lower value than the charge inhibition voltage; if for example the charge inhibition voltage is 4.35 volts for a representative cell, the maximum charging voltage is typically set at about 4.1 or 4.2 volts. Lithium-ion cell manufacturers have found that operation above the maximum charging voltage tends to reduce severely the recharging life cycle of the battery. Accordingly, in order to ensure that charging voltage is no greater than the set maximum charging voltage for the cell, controlled lithium-ion cell charging circuits typically provide a maximum output charge voltage that is no more than the maximum charging voltage.
In a typical charging circuit, an alternating current source operating at line voltage (typically 110-120 volts in North America) is applied to the primary winding of a transformer whose secondary winding applies a relatively low AC voltage to a bridge rectifier. The output of the bridge rectifier is applied across a smoothing capacitor to the load (the load in the charging circuit is the lithium-ion cell or battery to be charged). If no circuit elements were present other than the foregoing, the output voltage delivered to the lithium-ion cell would be at risk of exceeding the maximum charging voltage and ultimately might exceed the charge inhibition voltage of the lithium-ion cell. Accordingly, interposed between the bridge circuit and the lithium-ion cell or battery is a regulator circuit for limiting the voltage and current applied to the lithium-ion cell or battery during the charging operation.
A general purpose battery charger is described in U.S. Pat. No. 3,736,490 (Fallon et al.). This patent describes a battery charger incorporating a high leakage transformer and multiple rectifiers for regulating the charge current and the charge voltage applied to a battery. The high leakage transformer is used to provide impedance isolation between the input and the output circuit of the transformer and thus to protect the semiconductor components from line transients. The transformer is selected for maintaining a trickle charge current to the battery after a controlled rectifier providing supplemental current to the battery has been cut-off.
Another charging device defining the general state of the art is described in the abstract of Japanese Patent publication no 07296854 (Mitsui). The abstract describes a device for charging a battery that includes a constant current generator for charging the battery at a constant current at the initial stage of charging, and a constant voltage generator for performing constant voltage charging after a predetermined charging voltage has been reached.
Two types of regulator circuit are conventionally used, both of which are constant current/constant voltage regulator circuits, viz a linear regulator circuit, and a switching regulator circuit.
A switching regulator circuit includes a specially-designed charge control integrated circuit (IC) device for use with the other circuit elements. Such IC device is connected within the switching regulator circuit in constant-current mode. With the regulator operating in constant-current mode, charging continues at a constant current until the voltage across the lithium-ion cell or battery reaches the pre-set maximum charging voltage. The circuit then limits the output charging voltage to the maximum charging voltage, using a pulse-width modulation technique. According to this technique, the length of time that charge current is applied to the lithium-ion cell load during each AC. cycle is progressively and gradually decreased as charging proceeds.
The commercially available Benchmarq™ model bq2054 IC device and the 4C™ Technologies 4C-101656Li device are representative examples of charge control IC elements for use with a switching regulator circuit of the type described above.
As an alternative to the switching re
Barrigar Robert H.
Toatley , Jr. Gregory J.
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