Automatic load shed control for spacecraft power system

• US 4,575,679 A
• Filed: 05/10/1983
• Issued: 03/11/1986
• Est. Priority Date: 05/10/1983
• Status: Expired due to Fees

• Current Assignee: Lockheed Martin Corporation (Martin Marietta Corporation)
• Original Assignee: General Electric Company
• Inventors: Mazzocco, Carl F., Chung, Albert S.
• Primary Examiner(s): Karlsen, Ernest F.
• Assistant Examiner(s): O'Shea, Kevin D.
• Application Number:
  US06/493,227
• Time in Patent Office: 1,036 Days
• Field of Search: 324/426-429, 324/433, 324/434, 320/13, 320/14, 320/39, 320/40, 320/43, 320/44, 320/48, 340/636, 330/279, 307/38, 307/39, 323/906
• US Class Current: 324/427
• CPC Class Codes: G01R 19/16542 for batteries charge condition monitoring in G01R31/36
• CPC Class Codes: G01R 31/382 Arrangements for monitoring battery or accumulator variables, e.g. SoC
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements
• CPC Class Codes: H02J 1/14 Balancing the load in a network
• CPC Class Codes: H02J 7/00714 in response to battery charging or discharging current
• CPC Class Codes: H02J 7/34 Parallel operation in networks using both storage and other dc sources, e.g. providing buffering H02J7/14 takes precedence
• CPC Class Codes: H02J 7/35 with light sensitive cells
• CPC Class Codes: Y10S 323/906 Solar cell systems

Abstract

An automatic load shed control for a spacecraft power system employs a selectable one of a plurality of charging gains to approximate a state of charge in a secondary battery system. An ampere hour meter is initialized when the battery system is known to be fully charged and the inefficiencies of the charging system are lumped into a charging gain parameter. An output of the ampere hour meter is compared with a load shed threshold to determine the time for load shedding. The charging gain can be remotely changed as required to a value which causes the output of the ampere hour meter to track the true state of charge of the battery system closely enough for system satisfactory operation of load shedding. The charging gain may be a constant or may be a variable in dependence on the state of charge of the battery system. Charge which is delivered at a rate below a predetermined threshold is ignored by the ampere hour meter.

Claims

1. Apparatus for calculating a state of charge of batteries in a battery system of the type having at least one of a variable source capacity and a variable load, comprising:

means for measuring a charging current and a discharging current of said batteries;

means having a first gain for accumulating a first value related to said charging current;

means having a second gain for calculating a second value related to said discharging current;

means for differencing said first and second values to produce a calculated state of charge of said battery system;

means for initializing said means for differencing to a known value of said state of charge of said battery system; and

means for selecting a value of at least one of said first and second gains effective to produce an output of said means for differencing which is an approximation of a variable state of charge of said battery system.

2. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said known value is a state of full charge.

3. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said battery system includes at least first and second batteries and said known value is a state of full charge in at least one of said first and second batteries.

4. Apparatus for calculating a state of charge of a battery system according to claim 3 wherein said known value is a state of full charge in one of said first and second batteries and a state of full discharge in the other thereof.

5. Apparatus for calculating a state of charge of a battery system according to claim 3 wherein said known value is a state of full charge in all of said at least first and second batteries.

6. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said means for selecting a value includes means for selecting one of a family of charging efficiency curves of said battery system.

7. Apparatus for calculating a state of charge of a battery system according to claim 6 further comprising means for controlling said first gain according to said charging efficiency curve and said output of said means for differencing.

8. Apparatus for calculating a state of charge of a battery system according to claim 6 wherein said means for selecting a value includes remote means for selecting a value.

9. Apparatus for calculating a state of charge of a battery system according to claim 8 wherein said remote means includes a telemetry system.

10. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said means for selecting a value includes means for selecting one of a plurality of fixed values of said first gain.

11. Apparatus for calculating a state of charge of a battery system according to claim 10 wherein said means for selecting a value includes remote means for selecting said value.

12. Apparatus for calculating a state of charge of a battery system according to claim 11 wherein said means for selecting a value includes a telemetry system.

13. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said variable load includes an interruptible load and an uninterruptible load, said apparatus further comprises a load shed relay effective for shedding said interruptible load, a threshold generator effective to generate a threshold voltage related to a condition of said battery system at which said shedding should occur, and a comparator responsive to said threshold voltage and said approximation for producing a load shed command signal, said load shed command signal being effective to actuate said load shed relay and to thereby shed said interruptible load.

14. Apparatus for calculating a state of charge of a battery system according to claim 13 wherein said battery system includes at least first and second batteries and said threshold generator is responsive to a number of said at least first and second batteries which are active in said battery system.

15. Apparatus for calculating a state of charge of a battery system according to claim 14 wherein said number of said at least first and second batteries which are active includes all of said at least first and second batteries and said known value is a full state of charge in all of said batteries.

16. Apparatus for calculating a state of charge of a battery system according to claim 14 wherein said number of said at least first and second batteries which are active includes all of said at least first and second batteries and said known value is a full state of charge in at least one of said batteries and a full state of discharge in at least another of said batteries.

17. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said means having a first gain includes means for setting said first gain to zero for charging currents less than a predetermined value, said predetermined value being dependent upon a number of said batteries being charged.

18. Apparatus for calculating a state of charge of a battery system according to claim 1 wherein said means for differencing includes a voltage-to-frequency converter responsive to said first and second values for producing a variable frequency, an up-down counter effective for counting up and down in response to said variable frequency, gating means responsive to said first value for applying said variable frequency to one of an up and a down input of said up-down counter and responsive to said second value for applying said variable frequency to the other of said up and down inputs.

19. Apparatus for calculating a state of charge of a battery system according to claim 18 wherein said means for differencing includes a digital-to-analog converter effective to convert a count in said up-down counter to a related analog voltage.

20. A method for calculating a state of charge of batteries in a battery system of the type having at least one of a variable source capacity and a variable load, comprising:

measuring a charging current and a discharging current of said batteries;

calculating a first value related to said charging current at a first gain;

calculating a second value related to said discharging current at a second gain;

differencing said first and second values to produce a calculated state of charge of said battery system;

initializing said differencing to a known value of said state of charge of said battery system; and

selecting a value of at least one of said first and second gains effective to produce an output of said differencing which is an approximation of a variable state of charge of said battery system.

21. An automatic load shedding apparatus for a battery power system of the type having at least one battery and at least one of a variable source capacity and a variable load, said load including an interruptible load and an uninterruptible load, comprising:

means for producing a first signal at a first gain responsive to a charging current in said battery;

means for producing a second signal at a second gain responsive to a discharge current in said battery;

means for differencing said first and second signal to produce a state of charge signal;

means for initializing said state of charge signal to a predetermined fixed value;

means for selecting at least one of said first and second gains effective for making said state of charge signal representative of a variable state of charge of said at least one battery;

means for producing a load shed reference signal;

means for producing a load shed command signal in response to a predetermined relationship of said state of charge signal and said load shed reference signal; and

a load shed relay responsive to said load shed command signal for shedding said interruptible load.

22. An automatic load shedding apparatus according to claim 21 wherein said predetermined fixed value includes a state of full charge in at least one of said at least one battery.

23. An automatic load shedding apparatus according to claim 22 wherein said at least one battery includes at least first and second batteries and said predetermined fixed value includes a full state of charge in all of said at least first and second batteries.

24. An automatic load shedding apparatus according to claim 22 wherein said at least one battery includes at least first and second batteries and said predetermined fixed value includes a full state of charge in said first battery and a state of full discharge in said second battery.