Electricity: battery or capacitor charging or discharging – Battery or cell charging – With thermal condition detection
Reexamination Certificate
2002-07-18
2004-03-16
Berhane, Adolf D. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell charging
With thermal condition detection
C320S137000
Reexamination Certificate
active
06707273
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a silicon controlled rectifier (SCR) nickel metal hydride (NiMh) battery charging circuit. More particularly, the present invention relates to a SCR battery charging circuit particularly adapted to charge the kind of battery, such as the NiMh type battery, which has a critical temperature limit and a need for a certain amount of self-generated heat to reach a full capacity of recharge. The invention is further directed to a battery charging circuit which generates for a given range of ambient temperature a profile of SCR set voltage, i.e. the voltage at which the SCR becomes non-conducting, dependent on both cell temperature, under the influence of the prevailing ambient temperature, and cell voltage in a manner designed to accommodate to the aforesaid limitations.
BACKGROUND OF THE INVENTION
SCR-type charging circuits have been in manufacture for years. In a typical circuit the voltage potential at the SCR's cathode must be less than the voltage potential at the SCR's gate for the SCR to conduct, or turn to the “ON” state. The gate potential can be set at a desired level for a specific type of battery and the charger will return energy to the battery until that set potential is reached, at which point the SCR will automatically turn “OFF”. However, in the typical SCR type charging circuit the set potential is not made to vary as in the present invention dependent on changes in both the cell temperature and cell voltage.
Here it should be noted that batteries of the kind being referred to are made up of one or more “cells” which are spoken of as comprising a “battery pack”. Therefore, the terms “cell”, “cells”, “battery” and “battery pack” are sometimes used interchangeably.
The SCR circuit has worked well for charging SLA (sealed lead acid) and similar types of battery products which do not require critical cell temperature control under charge. More recently, new battery systems and in particular, nickel metal hydride (NiMh) batteries have become popular since NiMh batteries have an advantage in that they do not contain cadmium, as do nickel cadmium (NiCad) batteries, cadmium being a non-desired pollutant. Furthermore, the NiMh cells which make up a NiMh battery have an energy density about 80% greater than that of the cells which make up a NiCad battery thus allowing for longer equipment run time, or, smaller and lighter equipment with the same run time.
There are special concerns for NiMh batteries under charge. As previously mentioned, NiMh batteries have a critical upper temperature limit. In addition, they require a certain amount of self-generated heat to reach a full capacity of recharge. These two temperature conditions are relatively close together, requiring a charging system, which can allow one to be met, and, prevent the other from being reached. Attempts have been made to meet the NiMh battery charging control requirements. However, such attempts have led to very costly control means. The high cost of the control means has limited the market for both NiMh batteries and NiMh battery rechargers. Uncontrolled and relatively inexpensive charging circuits, like those presently available for recharging NiCad type batteries, can be used for recharging NiMh batteries, but such NiCad chargers if used for recharging a NiMh battery risk shortening the life of the NiMh battery because of excessive heat.
As background for later description reference is next made to
FIG. 1A
which is a schematic representation of a known SCR-type battery charger. The voltage potential at the cathode “C” of the SCR shown in
FIG. 1A
must be less than the voltage potential at the gate “G” for the SCR to conduct, or turn to the “ON” state. The gate potential can be set at a desired level for a specific type of battery and the charger will return energy to the battery until that set potential is reached, at which point the SCR will automatically turn “OFF.” In the circuit of
FIG. 1A
, Zener Diode (ZD), resistors R
1
, R
2
, and potentiometer P are used to provide a controlled and consistent voltage to the gate G of the SCR over a wide range of AC input voltage to the transformer T. However, the
FIG. 1A
circuit lacks means for cell temperature control.
Two types of rectification are shown in schematic
FIGS. 1A and 1B
.
FIG. 1A
shows a Full Wave (FW) center tap rectifier, and
FIG. 1B
illustrates an alternative FW bridge rectifier either of which can be used without concern to the rest of the circuit. While the portion of the circuit connected to the rectifiers is not shown in
FIG. 1B
, it should be understood that such portion is similar to that of FIG.
1
A. As previously stated, either type of rectification can be used with either of the circuits of the invention, but for simplification, only the FW center tap rectifier will be discussed by way of example throughout the rest of the description. The boxes BX, with an “X” therein on either side of the SCR as shown in
FIG. 1A
, schematically represent various accessory items that can be used in the circuit, but do not affect its operation. The accessories may include, by way of example, accessories such as a state of charge display, overload protection circuits, and impedance control.
Here it should be recognized that circuits of the kind illustrated in
FIGS. 1A and 1B
will not charge NiMh batteries with proper results. Use of such circuits for recharging NiMh batteries will cause the cells to overheat and experience a reduced cell life. On the other hand, NiCad batteries are sometimes recharged by use of a
FIG. 1A
or
1
B circuit.
As further background, reference is next made to
FIG. 2
which is a schematic diagram of another known type of battery charging circuit that helps to overcome the previously mentioned excessive heating problem particularly when charging NiMh batteries. The circuit illustrated in
FIG. 2
is similar to that of
FIG. 1A
, but includes the addition of resistor R
3
and negative temperature coefficient (NTC) thermistor which operates both to control the end of voltage, and to provide a means by which to utilize cell temperature as a means for terminating charge. Thus, the circuit of
FIG. 2
, unlike the circuit of
FIG. 1A
, does provide a form of cell temperature control under charge. Those skilled in the art will understand that when making reference to a circuit such as shown in
FIG. 2
, the phrase “end of voltage” is conventionally used interchangeably with the term “set voltage” and that the “gate” voltage is not exactly the true “set” voltage but for practical purposes is treated as being equal to the true set voltage. The NTC thermistor has a variable resistance that changes in value to the inverse of the temperature being sensed. In other words, its resistance decreases with increasing temperature and visa versa. In order to function properly in a battery charging circuit, the NTC device is bonded to a cell in the battery pack to accurately sense battery temperature.
With a proper set of charge circuit components, it is recognized that the battery pack shown in
FIG. 2
could achieve a desired rise in temperature to attain an approximate but less than full charge particularly of a NiMh battery, and the gate voltage potential could be set such that the charging circuit will cut off without reaching an unacceptable battery temperature when the battery is operating in a given ambient temperature. Of particular significance however is the fact that the
FIG. 2
type circuit only allows an approximate but less than full capacity to be returned to a NiMh battery. That is, the charging circuit necessarily cuts off before the battery is fully charged. Furthermore, the time for recharge, with a circuit of the
FIG. 2
type, can vary from about 1.5 hours up to about 12 hours depending on the charge current delivered by the recharger. The remaining small percentage, say for example 5%, of returned capacity is not obtainable in a well defined time period because of the random nature of the charge pulses in the nearly “OFF” state. Here it als
Berhane Adolf D.
Electronic Design & Sales, Inc.
Luk Lawrence
Olive & Olive P.A.
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