Final discharge of a cell activated by a circuit that senses when a charging fault has occurred

• US 6,531,846 B1
• Filed: 05/03/2001
• Issued: 03/11/2003
• Est. Priority Date: 05/03/2001
• Status: Expired due to Term

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Smith, Gregory J.
• Primary Examiner(s): Tso, Edward H.
• Assistant Examiner(s): LUK, LAWRENCE W
• Application Number:
  US09/848,744
• Time in Patent Office: 677 Days
• Field of Search: 320/134, 320/135, 320/136, 320/132
• US Class Current: 320/134
• CPC Class Codes: H02J 7/00302 Overcharge protection
• CPC Class Codes: H02J 7/00309 Overheat or overtemperature protection

Abstract

A method and apparatus is provided for discharging an improperly charged cell. A circuit detects when an unsafe event occurs that disables the rechargeable battery. When such an event occurs, a discharge circuit discharges the cell to a level making it safer to dispose, thereby lowering the risk of explosion. A thermal circuit may also be provided that senses the temperature of the cell and discharges the cell based on the sensed temperature.

Claims

1. An apparatus for discharging a cell, comprising:

a charge input circuit arranged to provide a charging signal to a protection circuit;

the protection circuit arranged to provide the charging signal to the cell during normal operation and arranged to provide a notification signal to a discharge circuit when a fault condition exists for the cell; and

the discharge circuit arranged to receive the notification signal, and when the notification signal is received arranged to discharge the cell to a safe level such that the cell may be discarded safely.

2. The apparatus of claim 1, wherein the protection circuit, further comprises, an improper charge circuit arranged to detect when a current exceeds a predetermined level.

3. The apparatus of claim 1, wherein the protection circuit, further comprises, an improper charge circuit that detects when a voltage exceeds a predetermined potential.

4. The apparatus of claim 3, wherein the improper charge circuit, further comprises a current detector circuit arranged to detecting when a current exceeds a predetermined level.

5. The apparatus of claim 1, wherein the protection circuit, comprises a sense amplifier circuit arranged to sense when the fault condition exists and arranged to provide the notification signal when the fault condition exists.

6. The apparatus of claim 5, wherein the discharge circuit, comprises:

a resistor circuit; and

a transistor circuit coupled to the resistor circuit and arranged to be active during the fault condition.

7. The apparatus of claim 1, wherein the protection circuit, further comprises, a thermal protection circuit that determines when a predetermined temperature is exceeded.

8. An apparatus for discharging a cell to a safe level, comprising:

a sense amplifier having a first input coupled to a first node and a second input coupled to a second node, and an output coupled to the gate of a transistor circuit;

a fuse circuit coupled between the first node and the second node, the fuse circuit arranged to open when a predetermined event occurs;

a switch circuit coupled between the first node and a fourth node, the switch circuit arranged to short when an illegal charging condition occurs;

the cell coupled between the first node and the fourth node; and

a discharge circuit including a resistor circuit coupled between the first node and the drain of the transistor circuit, the source of the transistor circuit coupled to the fourth node;

the transistor circuit arranged to drain the cell to a safe level based on the output of the sense amplifier.

9. The apparatus of claim 8, further comprising:

a thermal protection circuit arranged to provide a thermal signal based on the temperature of the cell; and

a detection circuit arranged to receive the thermal signal and the output of the sense amplifier;

the detection circuit arranged to activate the discharge circuit based on the thermal protection circuit and the state of the fuse circuit.

10. A method for discharging a cell within a battery pack, comprising:

determining when a fault condition exists;

disabling the cell from being charged when the fault condition exists;

discharging the cell to a safe level when the fault condition exists;

wherein the safe level relates to a charge level of the cell such that the cell may be discarded safely.

11. The method of claim 10, wherein determining when the fault condition exists, further comprises detecting when a charging current is above a predetermined threshold.

12. The method of claim 11, wherein the predetermined threshold is based on the charging characteristics of the cell being charged.

13. The method of claim 10, wherein determining when the fault condition exists, further comprises detecting when a predetermined temperature is exceeded within the battery pack.

14. The method of claim 13, wherein the predetermined temperature is based on the temperature characteristics of the cell.

15. An apparatus for discharging a cell, comprising:

a means for detecting when a fault condition exists during charging of the cell;

a means for stopping charging when the fault condition exists; and

a means for discharging the cell to a safe level when the fault condition exists;

wherein the safe level relates to a charge level of the cell such that the cell may be discarded safely.

16. The apparatus of claim 15, wherein the means for detecting when the fault condition exists during charging of the cell, comprises a means for detecting when a charging signal is above a predetermined threshold.

17. The apparatus of claim 16, wherein the means for detecting when the fault condition exists during charging of the cell further comprises a means for detecting when a temperature of the cell is above a predetermined threshold.