WO1997032383A1

WO1997032383A1 - Automatic deep discharge circuit for rechargeable batteries

Info

Publication number: WO1997032383A1
Application number: PCT/US1997/003802
Authority: WO
Inventors: Brad H. Sherman
Original assignee: Meridian Wireless Technologies, Inc.
Priority date: 1996-02-28
Filing date: 1997-02-26
Publication date: 1997-09-04
Also published as: AU2204597A

Abstract

An automatic discharge circuit (20) is located inside a cellular telephone to prevent memory effects from occurring in a rechargeable battery (14). When activated, the discharge circuit (20) is coupled across the rechargeable battery (14) deeply discharging the battery (14) to a sufficiently low level to prevent memory effects. A control circuit (16) located in the cellular telephone activates the discharge circuit (20) when the battery output power level is below a minimal power level necessary for conducting normal operations in the electrical device. The deep discharge takes place after the battery charge has already fallen below a minimum power level required to effectively conduct a cellular telephone call. However, the battery (14) still has sufficient charge to power logic circuitry in the telephone. Thus, the discharge circuitry can automatically discharge the battery (14) without effecting normal telephone operations.

Description

AUTOMATIC DEEP DISCHARGE CIRCUIT FOR RECHARGEABLE BATTERIES

TECHNICAL FIELD This invention relates generally to rechargeable batteries used in electrical devices and more particularly to a deep discharge circuit that auto¬ matically discharges batteries in a cellular telephone to prevent memory effects.

BACKGROUND ART Rechargeable batteries, such as Nickel Cadmium (NiCd) batteries, have a recognized problem of battery memory which lowers cell capacity. Battery memory is caused from repetitive charge-discharge cycles which fail to fully discharge the battery, from continuous trickle charging for long periods of time and from operations at elevated temperatures. A battery that develops memory effects is unable to deliver its full rated capacity. Battery memory effects may be disabling to the electrical device powered by the battery and may result in a potentially dangerous situation to the user of the equipment.

Battery capacity is recovered by fully discharging and charging the battery several times, such as disclosed in U.S. Patent No. 4,302,714 to Yefsky entitled: RECHARGEABLE BATTERY CHARGER SYSTEM FOR CHARGING TESTING, REJUVENATION AND PREVENTATIVE MAINTENANCE. The battery charger discussed in Yefsky uses a deep discharge process for removing the memory effects from the battery.

Battery chargers, such as described in Yefsky, are designed to recondition a battery with memory effects, but do not prevent the memory effects from occurring. A battery must be removed from the electrical device which the battery is currently operating and then inserted into the battery charger. The battery charger then performs multiple charge and discharge cycles on the battery until the memory effects are erased.

The charge and discharge process used during the battery reconditioning process are time-consuming. For example, as explained in Yefsky, a normal discharge cycle for a battery charger takes around an hour. During the reconditioning period, the electrical device powered by the battery is not operational. The additional recon¬ ditioning circuitry used in the charger system makes the charger system more expensive to manufacture than other battery chargers that simply charge the battery to a predefined power level.

Many standard battery chargers do not include circuitry for eliminating memory effects. Batteries with memory effects are often placed in the standard battery charger. Regardless of how long the battery is placed in the battery charger, the battery will not reach a full charge capacity.

Accordingly, a need remains for preventing memory effects from occurring in rechargeable batteries.

DISCLOSURE OF THE INVENTION An automatic discharge circuit is located inside an electrical device operated by a rechargeable battery. The discharge circuit discharges the battery to a deep discharged condition sufficient to prevent memory effects. A control circuit located inside the electrical device activates the discharge circuit when the battery output power level is below the minimal power level necessary for operating the electrical device. The battery is discharged after the battery charge falls below the power level necessary to effectively operate the electrical device. Thus, the deep discharge process begins after the device is deactivated and does not affect normal operation.

In one embodiment, the discharge circuit comprises a discharge switch and a load resistor which are selectively coupled across the electrical device battery. The control circuit comprises an analog-to- digital converter that converts the battery voltage into a digital voltage signal. A processor is coupled to the analog-to-digital converter and compares the digital voltage signal to a predetermined minimal power level. When the battery voltage drops below the predetermined minimum power level, the discharge circuit is enabled, coupling the load resistor across the battery for the deep battery discharge.

A system switch is coupled between the battery and application circuitry in the electrical device. The system switch disconnects the application circuitry from the battery when the battery voltage drops below the minimal power level. A light-emitting diode is activated by the control circuit while the discharge circuit is conducting the deep discharge.

In one embodiment of the invention, the automatic discharge circuit is used in a cellular tele- phone. The invention uses an existing battery monitoring circuit in the cellular telephone to control the discharge circuit. The monitoring circuit determines when the battery charge has dropped below a minimum output level necessary for operating transmitter and receiver circuitry in the cellular telephone. When the battery charge drops below the minimum output level, the transmitter and receiver circuitry is automatically disconnected from the battery and the deep discharge circuitry is automatically enabled. The automatic discharge circuitry prevents memory effects by ensuring that the battery is placed in a deep discharge stage anytime the battery is discharged enough during normal use to prevent further device opera¬ tion. The discharge circuit continues to operate after the normal functional operation of the device has been discontinued. Thus, the time and cost of reconditioning batteries with memory effects in a battery charger is reduced.

The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment of the invention which proceeds with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram of the automatic discharge circuitry according to the invention.

FIG. 2 is a detailed diagram of the automatic discharge circuitry shown in FIG. 1.

FIG. 3 is a flow diagram showing one method for deeply discharging a battery using the circuitry shown in FIG. 2.

BEST MODES FOR CARRYING OUT THE INVENTION FIG. 1 is a diagram of an automatic deep discharge system 12 according to the invention. The deep discharge system includes a rechargeable battery 14 coupled to a control circuit 16. The control circuit 16 is coupled to transmitter and receiver circuitry 18 and a deep discharge circuit 20. The entire deep discharge system 12 is located inside a cellular telephone.

The battery in one embodiment is Nickel Cadmium (NiCd) and the cellular telephone is a dual-mode tele¬ phone transmitting telephone calls in either an analog or digital mode. The battery discharge system, while described in the context of a cellular telephone, can be used to prevent memory effects in any electronic device powered by a rechargeable battery.

FIG. 2 is a detailed diagram of the battery discharge system 12 shown in FIG. 1. The battery 14 is coupled through a switch 26 to an analog-to-digital converter (ADC) 36. A processor 38 is coupled between ADC 36 and a memory 40. The switch 26 is typically a metal oxide semiconductor field effect transistor (MOSFET) but represents any switch used for connecting and disconnecting battery 14. A filter/voltage divider circuit 34 is coupled between switch 26 and ADC 36. The filter/voltage divider circuit 34 comprises resistors 28 and 30 and capacitor 32.

Voltage regulators 50 are coupled between battery 14 and a receiver circuit 54 and a transmitter circuit 52. The receiver and transmitter circuits are each connected to an antenna 48 for wireless transmission of cellular telephone calls. The voltage regulators 50 provide individual reference voltages to transmitter circuit 52 and receiver circuit 54. The voltage regula¬ tors 50, transmitter circuit 52, and receiver circuit 54 are all defined as telephone circuitry or, alternatively, as application or functional circuitry of the cellular telephone. A control and data bus 44 is coupled between the telephone circuitry 18, ADC 36, a discharge circuit 20 and processor 38. The discharge circuit comprises a silicon controlled rectifier (SCR) 25 connected in series with a load resistor 22. The discharge circuit 20 is operably coupled across battery 14 when line 42 in bus 44 is acti¬ vated. A SCR 15 is connected between battery 14 and the telephone circuitry 18 and is activated via line 43 in bus 44. A light-emitting diode (LED) 46 is coupled to line 42 in bus 44 and serves as an annunciator for visually identifying that the cellular telephone is in a deep discharge state. The ADC 36 constantly monitors the voltage level output from battery 14 and generates a digital voltage signal to processor 38. The processor 38 compares the digital voltage signal with a minimum power level value previously stored in memory 40. The minimum power level value represents the minimum charge level for battery 14 required to operate telephone circuitry 18. If the output charge of battery 14 drops below the minimum power level, processor 38 activates line 42 coupling load resistor 22 across battery 22 and acti¬ vating LED 46. At the same time line 42 is activated, line 43 is deactivated, thus disconnecting battery 24 from telephone circuitry 18.

During deep discharge, ADC 36 continues to monitor battery 14. Processor 38 compares the measured output from battery 14 to a second prestored power level stored in memory 40. The second prestored power level represents the output level at which battery 14 is discharged sufficiently to prevent significant memory effects. When the battery voltage drops below the second power level, line 42 is deasserted, thus disconnecting load resistor 22 from battery 14. Minimum battery power levels required for operating the transmitter and receiver circuitry 18 is in the range of 5.7 to 5.9 Volts. Other logic circuitry in the telephone is not associated with transmitting and receiving telephone calls and can operate down to around 5 Volts. Thus, logic such as processor 38 can continue to operate and control the automatic discharge operations for the battery even after the battery charge is no longer sufficient to operate the transmitter and receiver circuitry 18. FIG. 3 is a flow diagram showing a method for deeply discharging a battery using the circuitry shown in FIG. 2. Step 70 measures the battery voltage. Decision step 72 compares the measured battery voltage with the minimum power level. If the measured battery voltage is greater than the minimum power level, step 72 jumps back to step 70 and continues to monitor the battery voltage. As mentioned above, the minimum power level is defined as the minimum output voltage from battery 14 required for operating transmitter and receiver circuitry 18 (FIG. 1) . However, the minimum power level can be any preselected minimum charge level that initiates a deep discharge. If the battery voltage drops below the minimum power level, the cellular telephone transmitter and receiver circuitry is automatically disconnected from the battery in step 74. Since the battery can no longer effectively operate the cellular telephone, for deep discharge operations do not disrupt normal telephone operations.

Step 76 activates the discharge circuitry for the deep discharge. During deep discharge, the battery voltage is measured in decision step 80. As long as the battery voltage remains above a deep discharge power level, the discharge circuitry remains activated and step 78 continues to monitor the battery voltage. When the battery voltage drops below the second deep discharge power level, the discharge circuit is disconnected from the battery. Since the battery is now in a deep dis¬ charged condition, any subsequent recharging of the battery will be performed without memory effects.

Some electrical devices include a LED which is activated when the battery charge drops below a given low power level. The same LED or a second LED can be acti¬ vated to indicate that the deep discharge is taking place.

If the measured battery voltage (M) falls below the deep discharge power level, step 82 turns the tele¬ phone completely off. In turn, the LED is deactivated indicating that the deep discharge has ended.

Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such prin¬ ciples. I claim all modifications and variation coming within the spirit and scope of the following claims.

Claims

WE CLAIM :

1. An automatic discharge system for a cellular telephone, comprising: a rechargeable battery for operating the cellular telephone; a cellular telephone circuit for both transmitting and receiving telephone calls; a discharge circuit coupled to the battery, the discharge circuit when activated deeply discharging the battery; and a control circuit enabling one of said cellular telephone circuit and said discharge circuit according to a predefined minimum power level in the cellular tele- phone necessary for operating the cellular telephone circuit.

2. A discharge system according to claim 1 wherein the discharge circuit comprises a switch and a load resistor selectively coupled across the battery.

3. A discharge system according to claim 1 wherein the control circuit comprises an analog-to- digital converter for transforming the battery voltage into a digital voltage signal and a processor coupled to the analog-to-digital converter for comparing the digital voltage signal to a predefined minimum power level.

4. A discharge system according to claim 3 wherein the control circuit includes a memory coupled to the processor for storing the predefined minimum power level.

5. A discharge system according to claim 1 wherein the control circuit includes a switch coupled between the battery and the cellular telephone circuit for selectively disconnecting the cellular telephone circuit from the battery when a battery output charge drops below the predefined minimum power level.

6. A discharge system according to claim 1 including an annunciation device activated at the same time as the discharge circuit.

7. A method for automatically discharging a rechargeable battery in a cellular telephone, comprising: determining a minimum power level for the battery to operate the cellular telephone; measuring an output power level for the battery while the battery is operating the cellular telephone; comparing the measured output power level with the minimum power level; and automatically deep discharging the rechargeable battery and deactivating the cellular telephone when the output power level is below the minimum power level.

8. A method according to claim 7 including storing the minimum power level in a cellular telephone memory.

9. A method according to claim 8 wherein the step of measuring the output power level for the rechargeable battery comprises converting an analog battery voltage into a digital battery voltage and the step of comparing comprises comparing the digital battery voltage to the stored minimum power level.

10. A method according to claim 7 wherein the step of deeply discharging the rechargeable battery comprises selectively coupling a load resistor across the battery until the battery voltage drops below a second deep discharge power level.

11. A method according to claim 7 wherein the step of deactivating the cellular telephone comprises disconnecting transmitter and receiver circuitry in the cellular telephone from the battery.

12. A method according to claim 7 including annunciating a signal when the battery is being deeply discharged.

13. A method according to claim 7 wherein the cellular telephone transmits and receives telephone calls in one of an analog mode and a digital mode.

14. An automatic discharge system, comprising: an electrical device having application circuitry, the application circuitry having a minimum power level required for operation; a rechargeable battery generating output power for operating the electrical device; a discharge circuit located inside the electrical device and coupled to the rechargeable battery, the discharge circuit when activated deeply discharging the battery; and a control circuit located inside the electrical device for activating the discharge circuit when the battery output power is below the minimum power level necessary for operating the application circuitry.

15. A discharge circuit according to claim 14 wherein the discharge circuit comprises a switch and a load resistor coupled across the rechargeable battery.

16. A discharge circuit according to claim 15 wherein the control circuit comprises an analog-to- digital converter for generating a digital voltage signal and a processor coupled to the analog-to-digital

SϋBSTTTiπtSHEEl (Rl)LE26) converter for comparing the digital voltage signal to the minimum power level.

17. A discharge circuit according to claim 16 wherein the control circuit includes a memory coupled to the processor for storing the minimum power level.

18. A discharge circuit according to claim 17 wherein the control circuit includes a switch coupled between the battery and the application circuitry in the electrical device, the switch selectively disconnecting the application circuitry from the battery when the battery output power drops below the minimum power level.

19. A discharge circuit according to claim 18 including a light-emitting diode activated by the control circuit when the discharge circuit is activated.