Electricity: battery or capacitor charging or discharging – Battery or cell discharging – Regulated discharging
Reexamination Certificate
2000-02-04
2001-04-03
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
Regulated discharging
Reexamination Certificate
active
06211652
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a discharge protection circuit for battery-operated devices such as portable electric lights, electric tools, and the like. More particularly, the invention relates to a discharge protection circuit of relatively simple design that protects re-chargeable batteries from damage caused by overdischarging.
Rechargeable batteries are used to power a wide variety of battery-operated or “cordless” electric devices. Ideally, a rechargeable battery could be charged and discharged an infinite number of times. In practice, batteries have a limited useful life, which can be further shortened if the batteries are damaged. Overdischarging is a significant cause of damage to rechargeable batteries. Typically, overdischarging occurs as a result of the electric device being inadvertently left on. When this happens, the battery (or batteries) in the device continues to discharge until the voltage of the battery drops to a zero or near zero value. Allowing the voltage to drop to such a low value typically causes cell reversal and damage to the battery. Often, the damage is so severe that the battery must be replaced. At a minimum, the battery's ability to fully recharge is affected.
Devices that prevent overdischarge are available. Many available devices attempt to solve the problem of overdischarge by monitoring the voltage level of the battery and disconnecting a battery from its load once the battery has discharged to a predetermined level. One difficulty with this type of design is that a discharged battery's open circuit voltage returns to a level that is nearly the same as a fully charged battery (“open circuit recovery”). Thus, in cordless devices equipped with voltage-monitoring protection devices, a battery will continue to discharge after open circuit recovery occurs, further depleting the battery. Using voltage hysteresis in a voltage monitoring circuit does not solve this problem.
Some protection devices provide a latching function to prevent discharge of an already depleted battery. However, most methods and components used to provide a latching function are complex and costly. Expensive protection devices are, in general, not suitable for cordless devices that sell at relatively low prices. Yet, if inexpensive and simple protection devices were available, protection devices could be fitted in many inexpensive cordless devices and significant cost savings could be realized for nearly all cordless devices.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a discharge protection circuit that is capable of preventing a battery from discharging to an undesirably low level. The discharge protection circuit is relatively simple and may be constructed with relatively inexpensive components. The circuit is designed to be directly integrated into battery-operated devices having a battery, a power switch that controls the battery, and a load connected to the power switch. The protection circuit includes a thyristor or similar current-controlled, latched switch that has a control gate or input. The thyristor is coupled in series with the load. A capacitor or similar electric energy storage device is coupled to the load and to the control input of the thyristor. A resistor is coupled to the load and to the control input of the thyristor, in parallel to the capacitor.
When the power switch is turned on, it delivers a current pulse to the capacitor. In response to the pulse, the capacitor shorts and provides a trigger pulse to the control input of the thyristor. The trigger pulse causes the thyristor to become conductive, which allows current to travel from the battery through the load to ground. The thyristor remains conductive so long as the current through the load remains at or above the holding current of the thyristor. If the current through the load drops below the holding current (as a result of the battery discharging), the thyristor turns off and remains off even if the battery experiences an open circuit recovery.
As current flows through the load, the capacitor charges. When the load current is shut off, such as when the power switch is shut off, the capacitor discharges through the resistor allowing the circuit to cycle as fast as the power switch can be cycled.
In addition to using a thyristor, the invention may be implemented with alternative current-controlled, latched switches such as SCRs or SIDAC™ devices.
As is apparent from the above, it is an advantage of the present invention to provide a simplified, current-dependent discharge protection circuit that does not employ microprocessor or other expensive circuit components. Other features and advantages of the present invention will become apparent by consideration of the detailed description and accompanying drawings.
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Michael & Best & Friedrich LLP
Milwaukee Electric Tool Corporation
Tso Edward H.
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