Method of and apparatus for monitoring the state of charge of a rechargeable battery

• US 4,677,363 A
• Filed: 02/26/1986
• Issued: 06/30/1987
• Est. Priority Date: 06/30/1984
• Status: Expired due to Fees

• Current Assignee: Udo Kopmann
• Original Assignee: Udo Kopmann
• Inventors: Kopmann, Udo
• Primary Examiner(s): HICKEY, ROBERT
• Application Number:
  US06/833,842
• Time in Patent Office: 489 Days
• Field of Search: 320/20, 320/21, 320/39, 320/40, 320/44, 320/45, 320/48, 320/32
• US Class Current: 320/131
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm
• CPC Class Codes: G01R 31/374 with means for correcting the measurement for temperature or ageing
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements
• CPC Class Codes: G01R 31/386 using test-loads
• CPC Class Codes: Y10S 320/21 State of charge of battery

Abstract

A method of and an apparatus for monitoring the state of charge of a rechargeable battery are disclosed which use a reference value corresponding to a defined state of charge stored in a memory. During each battery charging and discharging cycle, the terminal voltage, the current and time are measured and are converted into energy delivered to the battery or by the battery depending on the direction of current flow. The actual state of charge is then calculated from the reference value stored. The characteristic terminal voltage curve during charging and discharging is used for minimizing the differences between the quantities measured and the actual state of charge of the battery so monitored by monitoring the rate of change in the terminal voltage during the charging and the discharging cycles, by determining, after a phase of a relatively steady and small rate of change in voltage, at least one point at which the rate of change in voltage is higher and by adjusting the actual state of charge determined at this point by coulometry to the predetermined reference value.

Claims

1. A method of monitoring the state of charge chargeable battery and, more particularly, a nickel-cadmium storage battery comprising the storage of a reference value corresponding to a certain state of charge, the measurement of the terminal voltage, the current and time during each battery charging and discharging cycle and the conversion of the quantities so measured into a quantity corresponding to the energy charged or discharged, accounting for the direction of current flow, said quantity being used for computing the actual state of charge based on said reference value, characterized in that the rate of change in said terminal voltage is monitored during said charging and discharging cycles and in that at least one point, following a phase of relatively steady and low rate of change in said terminal voltage, is determined, at which point a characteristic higher rate of change in said terminal voltage occurs and in that the computed actual state of charge at said point or at another point delayed with respect to said point by a predetermined margin is adjusted to said predetermined reference value.

3. A method according to claim 1 wherein the actual states of charge are determined at a first such adjustment point in the charging cycle and at a second such adjustment point in the discharging cycle and in that the difference between the energies stored at said two points is used to determine the capacity of the battery.

4. A method according to claim 1 wherein said point at which a characteristic rate of change in voltage occurs represents a point at which a progressive change in voltage commences.

5. A method according to claim 1 wherein said point at which said characteristic rate of change in voltage occurs represents a point of inflection in the curve plotting the increase in voltage during a charging cycle.

6. A method according to claim 1 wherein said terminal voltage is monitored and in that the state of charge determined by coulometry at each adjustment point is only adjusted to said predetermined reference value for said adjustment point if a certain limit voltage is exceeded during a charging or a discharging cycle.

7. A method according to claim 1 characterized in that the battery temperature is also measured and in that the temperature so measured is accounted for in the identification of said adjustment points and in the computation of the states of charge.

8. A method according to claim 1 wherein changes in the voltage-versus-time behavior of any such battery are compensated for by computing means in response to changes in the charge or discharge current.

9. A method according to claim 1 wherein changes in the voltage-versus-time behavior of any such battery are compensated for in response to the battery temperature.

10. A method according to claim 1 wherein said quantities are measured and processed at a predetermined scanning frequency which may be varied and wherein said scanning frequency is increased close to said adjustment points to improve the accuracy of measurement.

11. A method according to claim 1 wherein the rate of change in terminal voltage is determined by computing voltage over time derivatives and wherein, by way of the intermediate storage of derivatives, higher order derivatives are obtained for identifying said adjustment points.

2. A method of monitoring the state of charge of a rechargeable battery having terminals for providing a terminal voltage during charge and discharge cycles, said terminal voltage subjected to a relatively steady and low rate of charge during a substantial phase of said charge and discharge cycles and to a characteristic higher rate of charge outside said substantial phase,

said method comprising the steps of:

(a) providing a reference value representative of a certain state of charge of said battery and storing said reference value in a storage means;

(b) measuring the terminal voltage, the current flow through the battery and the time, during each battery charge and discharge cycle;

(c) converting said measured quantities into a further quantity, said further quantity corresponding in response to the direction of current flow to the energy charged or discharged;

(d) computing in response to said further quantity and actual state of charge based on said stored reference value;

(e) monitoring the rate of change in said terminal voltage during each of said charge and discharge cycles and determining at least one point following said phase of relatively steady and low rate of change in said terminal voltage at which point a characteristic higher rate of change in said terminal voltage occurs; and

(f) adjusting the computed actual state of charge at one of said points and of any other point delayed with respect to said one point by a predetermined margin to said stored reference value.

12. A method of monitoring the state of charge of a rechargeable battery having terminals for providing a terminal voltage during charge and discharge cycles, said terminal voltage subjected to a relatively steady and low rate of charge during a substantial phase of said charge and discharge cycles and to a characteristic higher rate of charge outside said substantial phase,

said method comprising the steps of:

(a) providing a reference value representative of a certain state of charge of said battery and storing said reference value in a storage means;

(b) measuring the terminal voltage, the battery temperature, the current flow through the battery and the time, during each battery charge and discharge cycle;

(c) converting said measured quantities into a further quantity, said further quantity corresponding in response to the direction of current flow to the energy charged or discharged;

(d) computing in response to said further quantity an actual state of charge based on said stored reference value;

(e) monitoring the rate of change in said terminal voltage and battery temperature during each of said charge and discharge cycles and determining at least one point in which the temperature so measured is accounted for, said at least one point following said phase of relatively steady and low rate of change in said terminal voltage at which point a characteristic higher rate of change in said terminal voltage commences; and

(f) adjusting the computed actual state of charge at said at least one point and of any other point delayed with respect to said at least one point by a predetermined margin to said stored reference value, wherein changes in the voltage-versus-time behavior of such battery are compensated for by computing means in response to changes in the charge of discharge current and in response to the battey temperature.

13. An apparatus for monitoring the state of charge of a rechargeable battery having terminals for providing a terminal voltage during charge and discharge cycles, said terminal voltage subjected to a relatively steady and low rate of charge during a substantial phase of said charge and discharge cycles and to a characteristic higher rate of charge outside said substantial phase, said apparatus comprising,

means for measuring the terminal voltage of the battery, the direction and the amplitude of the charging and discharging currents and the battery temperature,

a program-controlled central processing unit coupled to said measuring means and controlled to monitor the rate of change in the terminal voltage during all charging and discharging cycles of said battery and to determine an actual state of charge when, following a substantially constant phase, said rate of change undergoes one of a substantial increase and substantial decrease,

memory means coupled to said central processing unit for storing reference values and the actual energy input and energy output values computed by said central processing unit, and

an interface and driving circuit for coupling said central processing unit with peripheral input and output units.

14. An apparatus according to claim 13 wherein said measuring means are coupled to said central processing unit by way of multiplexing means, an analog-to-digital converter and a bus.

15. An apparatus according to claim 14 wherein the scanning frequency of said analog-to-digital converter may be controlled by said central processing unit.

16. An apparatus according to claim 13, wherein said measuring means comprises means for measuring current having a resistor included in the current circuit of the battery and maintained at a predetermined potential at one end which is coupled to said battery.

17. An apparatus according to claim 13 wherein a program storage means for controlling said central processing unit is provided, said program storage means containing a program for correcting the actual state of charge determined and the adjustment points in response to changes in temperature and current.

18. An apparatus according to claim 13 wherein said interface and driving circuit is provided with actuating and switch means for interrupting the battery charge and discharge circuits if predetermined limits for the terminal voltage are exeeded.