Protection of batteries

• US 4,255,698 A
• Filed: 01/26/1979
• Issued: 03/10/1981
• Est. Priority Date: 01/26/1979
• Status: Expired due to Term

• Current Assignee: Littelfuse Incorporated
• Original Assignee: Raychem Limited (TE Connectivity Limited)
• Inventors: Simon, Henry I.
• Primary Examiner(s): Hickey, Robert J.
• Application Number:
  US06/006,773
• Time in Patent Office: 774 Days
• Field of Search: 320/35, 320/36, 320/20, 320/2, 338/22, 338/31, 361/106
• US Class Current: 320/134
• CPC Class Codes: H01M 10/443 in response to temperature
• CPC Class Codes: H01M 10/486 for measuring temperature
• CPC Class Codes: H01M 2200/10 Temperature sensitive devices
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

Improved methods of battery charge and/or discharge control make use of an electrical device which is connected in series with the cell or cells of the battery and which is preferably a permanent part of the battery, so that when the battery terminals are connected in order to charge or discharge the battery, the device provides an automatic guard against excessive battery temperatures and/or current discharges. The device comprises a PTC element which is preferably composed of a conductive polymer composition, and which is in a low resistance state under normal operating conditions but which changes to a high resistance state (and thus reduces the charging current or the discharge current) when the temperature and/or current become excessive.

Claims

1. A method of recharging a battery comprising one or more secondary cells and having a capacity of C ampere hours, which method comprises passing current through the cell or cells while they are part of a circuit which also comprises a source of DC power and an electrical device which

(i) is connected in series with the cell or cells of the battery;

(ii) is in thermal contact with at least one cell of the battery;

(iii) comprises a PTC element and at least two electrodes which are placed so that current flows through said PTC element; and

(iv) has an electrical power/temperature relationship and is capable of dissipating heat at a rate such that

(A) when the current is initially passed through the circuit, the circuit is in a rapid-charging condition in which there is a rapid-charging current through the cell or cells of the battery of at least 0.25C, and there is a stable equilibrium between the rate at which the device generates heat by I² R heating and the rate at which heat is lost from the device; and

(B) when the rapid-charging current has recharged the cell or cells to a certain state of charge and in consequence the cell or cells and the device have reached elevated temperatures, the rate at which the device generates heat by I² R heating exceeds the rate at which heat is dissipated from the device and thus causes the temperature and the resistance of the device to rise rapidly, and thereafter the circuit reaches a trickle-charging condition, in which condition there is a trickle-charging current through the cell or cells which is less than half said rapid-charging current and is from 0.03C to 0.2C amps and in which condition the rate at which the device generates heat by I² R heating is equal to the rate at which heat is dissipated from the device, said trickle-charging condition being maintained so long as current continues to pass through the circuit.

2. A method according to claim 1 wherein the rapid-charging current is at least 0.5C amps.

3. A method according to claim 2 wherein the rapid-charging current is from 0.75C to 4C amps.

4. A method according to claim 1 wherein said trickle-charging current is from 0.05C to 0.2C amps.

5. A method according to claim 4 wherein said trickle-charging current is from 0.05C to 0.1C amps.

6. A method according to claim 1 wherein the temperature around the device, at the time when the rate at which the device generates heat first exceeds the rate at which heat is dissipated from the device, T_crit, is 40°

to 75°

C.

7. A method according to claim 6 wherein T_crit is 40°

to 50°

C.

8. A method according to claim 1 wherein said device

(a) comprises a PTC element which is composed of a PTC conductive polymer composition having a resistivity of less than 10 ohm.cm when said circuit is in said rapid-charging condition and through which current flows over an area of equivalent diameter d with an average path length t such that d/t is at least 2;

and

(b) has a resistance which is less than 1 ohm and less than 0.1×

R_L ohm when said circuit is in said rapid-charging condition, where R_L is the resistance of the circuit in series with the device.

9. A method according to claim 8 wherein said PTC conductive polymer composition has a resistivity of less than 4 ohm.cm when said circuit is in said rapid-charging condition.

10. A method according to claim 8 wherein said PTC conductive polymer composition has a resistivity of at least 10⁴ ohm.cm when said circuit is in said trickle-charging condition.

11. A method according to claim 8 wherein said ratio d/t is at least 10.

12. A method according to claim 1 wherein the temperature of said device when said circuit is in said trickle-charging condition is T_d latch and the resistance of the device at T_d latch is less than the resistance of the device at all temperatures between T_d latch and (T_d latch +10)°

C.

13. A method according to claim 12 wherein the resistance of the device at T_d latch is less than the resistance of the device at all temperatures between T_d latch and (T_d latch +50)°

C.

14. A method according to claim 1 wherein said battery comprises only two terminals and said device is connected between said terminals so that it is connected in series with the cell or cells of the battery when it is being charged and when it is being discharged.

15. A method according to claim 1 wherein said battery comprises three or more terminals and said device is connected between said terminals so that it is connected in series with the cell or cells only when the battery is being charged.

16. A method according to claim 1 wherein said battery comprises a plurality of cells and said device is placed between the cells.

17. A method according to claim 1 wherein said cells are nickle-cadmium cells.

45. A method according to claim 1, 3 or 8 wherein the current which passes through the device and the current which passes through the cell or cells are equal to each other when the circuit is in the rapid charging condition and when the circuit is in the trickle charging condition.

46. A method according to claim 1, 3 or 8 wherein the circuit comprises a fixed resistor in parallel with the device, said fixed resistor having a resistance which is higher than the resistance of the device when the circuit is in the rapid charging condition and which is lower than the resistance of the device when the circuit is in the trickle charging condition.

18. A battery comprising one or more secondary cells and an electrical device which

(i) is connected in series with the cell or cells of the battery when the battery is being charged;

(ii) is in thermal contact with at least one cell of said battery;

(iii) comprises a PTC element and at least two electrodes which are placed so that, when the battery is being recharged, current flows through said PTC element; and

(iv) has an electrical power/temperature relationship and is capable of dissipating heat at a rate such that

(A) when the battery has been discharged and is being recharged in a recharging circuit with the battery being placed in still air at 20°

C., there is at least one rapid-charging current through the cell or cells of the battery which is in the range 0.25C to 4C amps, where C is the capacity of the battery in amp. hours, and which produces in the circuit a rapid-charging condition in which there is a stable equilibrium between the rate at which the device generates heat by I² R heating and the rate at which heat is lost from the device; and

(B) when said rapid-charging current has recharged the cell or cells to a certain state of charge and in consequence the cell or cells and the device have reached elevated temperatures, the rate at which the device generates heat by I² R heating exceeds the rate at which heat is dissipated from the device and thus causes the temperature and the resistance of the device to rise rapidly, and in consequence the circuit thereafter reaches a trickle-charging condition, in which condition there is a trickle-charging current through the cell or cells which is less than half said rapid-charging current and is from 0.2C to 0.03C amps, and in which condition the rate at which the device generates heat by I² R heating is equal to the rate at which heat is dissipated from the device.

19. A battery according to claim 18 wherein said rapid-charging current is 0.5C amps.

20. A battery according to claim 19 wherein said trickle-charging current is 0.05C to 0.1C amps.

21. A battery according to claim 18 wherein said rapid-charging current is C amps.

22. A battery according to claim 21 wherein said trickle-charging current is 0.05C to 0.2C amps.

23. A battery according to claim 18 wherein said rapid-charging current is 3C amps.

24. A battery according to claim 18 wherein said device

(a) comprises a PTC element which is composed of a PTC conductive polymer composition having a resistivity of less than 10 ohm.cm when said circuit is in said rapid-charging condition and through which current flows over an area of equivalent diameter d with an average path length t such that d/t is at least 2;

and

(b) has a resistance which is less than 1 ohm and less than 0.1×

R_L ohm when said circuit is in said rapid-charging condition, where R_L is the resistance of the circuit in series with the device.

25. A battery according to claim 24 wherein said PTC conductive polymer composition has a resistivity of less than 4 ohm.cm when said circuit is in said rapid-charging condition.

26. A battery according to claim 25 wherein said PTC conductive polymer composition has a resistivity of at least 10⁴ ohm.cm when said circuit is in said trickle-charging condition.

27. A battery according to claim 24 wherein said ratio d/t is at least 10.

28. A battery according to claim 18 wherein the temperature of said device when said circuit is in said trickle-charging condition is T_d latch and the resistance of the device at T_d latch is less than the resistance of the device at all temperatures between T_d latch and (T_d latch +10)°

C.

29. A battery according to claim 28 wherein the resistance of said device when said circuit is in said trickle-charging condition is T_d latch and the resistance of the device at T_d latch is less than the resistance of the device at all temperatures between T_d latch and (T_d latch +50)°

C.

30. A battery according to claim 18 which comprises three or more terminals and said device is connected between said terminals so that it is connected in series with the cell or cells only when the battery is being charged.

31. A battery according to claim 18 wherein said rapid-charging current is in the range of 0.25C to 2C and said battery comprises only two terminals and said device is connected in series with at least one of the cells of the battery both when the battery is being charged and when it is being discharged.

32. A battery according to claim 18 which comprises a plurality of cells and said device is placed between the cells.

33. A battery according to claim 32 wherein said cells are nickel-cadmium cells.

47. A battery according to claim 18, 19, 21, 23 or 24 wherein, when the battery is being recharged, the current which passes through the device and the current which passes through the cell or cells are equal to each other when the circuit is in rapid charging condition and when the circuit is in the trickle charging condition.

48. A battery according to claim 18, 19, 21, 23 or 24 which comprises a fixed resistor in parallel with the device, said fixed resistor having a resistance which is higher than the resistance of the device when the circuit is in the rapid charging condition and which is lower than the resistance of the device when the circuit is in the trickle charging condition.

34. A battery comprising one or more cells and an electrical device which

(i) is connected in series with the cell or cells of said battery when current is being drawn from the battery;

(ii) is in thermal contact with at least one cell of said battery;

(iii) comprises a PTC element and at least two electrodes which are placed so that, when the battery is being discharged, current flows through said PTC element; and

(iv) has an electrical power/temperature relationship and is capable of dissipating heat at a rate of such that if said battery is placed in still air at 20°

C. and is used as a source of power, there is a critical current, i_crit, the device is in a low temperature stable equilibrium state in which there is a stable equilibrium between the rate at which the device generates heat by I² R heating and the rate at which heat is dissipated from the device, and when the current drawn from the battery is above i_crit, the rate at which the device generates heat by I² R heating exceeds the rate at which heat is dissipated from the device and thus causes the temperature and the resistance of the device to rise rapidly until the device reaches a high temperature stable equilibrium state in which

(a) the rate at which the device generates heat by I² R heating is equal to the rate at which heat is dissipated from the device, and

(b) the current drawn from the battery is less than 0.5C amps.

35. A battery according to claim 34 wherein i_crit is between C and 10C amps, where C is the capacity of the battery in amp. hours.

36. A battery according to claim 35 wherein i_crit is between 1.5C and 2.5C amps.

37. A battery according to claim 34 wherein said device

(a) comprises a PTC element which is composed of a PTC conductive polymer composition having a resistivity of less than 10 ohm.cm at 20°

C. and through which current flows over an area of equivalent diameter d with an average path length t such that d/t is at least 2; and

(b) has a resistance which is less than 1 ohm at 20°

C.

38. A battery according to claim 37 wherein said PTC composition has a resistivity of less than 4 ohm.cm at 20°

C.

39. A battery according to claim 37 wherein said PTC composition has a resistivity of at least 10⁴ ohm.cm in said high temperature equilibrium state.

40. A battery according to claim 35 wherein the temperature of said device in said high temperature equilibrium state is T_d latch and the resistance of the device at T_d latch is less than the resistance of the device at all temperatures between T_d latch and (T_d latch +10)°

C.

41. A battery according to claim 40 wherein the resistance of the device at T_d latch is less than the resistance of the device at all temperatures between T_d latch and (T_d latch +50)°

C.

42. A battery according to claim 34 which comprises a plurality of cells and said device is placed between the cells.

43. A battery according to claim 42 wherein said cells are nickel-cadmium cells.

44. A battery according to claim 34 which comprises three or more terminals and said device is connected between said terminals so that it is connected in series with the cell or cells only when current is being drawn from the battery.

49. A battery according to claim 34, 35, 36, 37, 38 or 41 wherein, when the battery is in still air at 20°

C. and is used as a source of power, the current which passes through the device and the current which passes through the cell or cells are equal to each other when the current is below i_crit and when the device is in the high temperature stable equilibrium state.

50. A battery according to claim 34, 35, 36, 37, 38 or 41 which comprises a fixed resistor in parallel with the device, said fixed resistor having a resistance which is higher than the resistance of the device in the low temperature stable equilibrium state and which is lower than the resistance of the device in the high temperature stable equilibrium state.