Electrical energy storage system with battery state-of-charge estimation

1. An electrical energy storage system comprising:

a battery configured to store electrical energy and discharge the stored electrical energy to an external device;

a switch electrically connected to the battery and operable to connect the battery to the external device and disconnect the battery from the external device;

a sensor configured to measure an open circuit voltage of the battery; and

a controller configured to:

predict usage of the battery at a future time,

determine a time of minimum battery usage based on the predicted usage of the battery at the future time;

schedule the battery to be disconnected from the external device at the time of minimum battery usage;

operate the switch to disconnect the battery at the time of minimum battery usage; and

obtain a measurement of the open circuit voltage of the battery using the sensor when the battery is disconnected from the external device.

2. The electrical energy storage system of claim 1 , wherein the controller is configured to estimate a state-of-charge of the battery based on the measured open circuit voltage of the battery.

3. The electrical energy storage system of claim 2 , wherein the controller is configured to estimate an uncertainty in the state-of-charge of the battery.

4. The electrical energy storage system of claim 1 , wherein the external device is a part of an energy grid.

5. The electrical energy storage system of claim 1 , wherein the controller is configured to use the battery to shift an energy load of a building by storing electrical energy in the battery at a first time and discharging the stored electrical energy to the building at a second time.

6. The electrical energy storage system of claim 5 , wherein the controller is configured to operate the battery to store and discharge electrical energy to satisfy the energy load of the building or campus.

7. The electrical energy storage system of claim 1 , wherein the controller is configured to estimate a state-of-charge of the battery based on battery power setpoints.

8. The electrical energy storage system of claim 7 , wherein the controller is configured to estimate a change in the state-of-charge of the battery resulting from the battery power setpoints after the open circuit voltage is measured.

9. The electrical energy storage system of claim 1 , wherein the controller is configured to estimate a remaining life of the battery based on the measurement of the open circuit voltage of the battery.

10. A method for operating an electrical energy storage system, the method comprising:

storing electrical energy in a battery and discharging the stored electrical energy to an external device;

predicting usage of the battery at a future time;

determining a time of minimum battery usage based on the predicted usage of the battery at the future time;

scheduling the battery to be disconnected from the external device at the time of minimum battery usage;

operating a switch to disconnect the battery at the time of minimum battery usage; and

measuring an open circuit voltage of the battery when the battery is disconnected from the external device.

11. The method of claim 10 , further comprising estimating a state-of-charge of the battery based on the measured open circuit voltage of the battery.

12. The method of claim 11 , further comprising estimating an uncertainty in the state-of-charge of the battery.

13. The method of claim 10 , wherein the external device is a part of an energy grid.

14. The method of claim 10 , further comprising shifting an energy load of a building by storing electrical energy in the battery at a first time and discharging the stored electrical energy to the building at a second time.

15. The method of claim 14 , further comprising operating the battery to store and discharge electrical energy to satisfy the energy load of the building.

16. An electrical energy storage system comprising:

a first battery string and a second battery string, each of the first battery string and the second battery string including one or more batteries configured to store electrical energy and discharge the stored electrical energy to an external device;

one or more sensors configured to measure open circuit voltages of the first battery string and the second battery string when the first battery string or the second battery string is disconnected from the external device; and

a controller configured to:

predict usage of the first battery string and the second battery string at a future time,

determine a time of minimum usage of the first battery string and the second battery string based on the predicted usage of the battery at the future time,

schedule at least one of the first battery string and the second battery string to be disconnected from the external device at the time of minimum usage,

disconnect the at least one of the first battery string and the second battery string at the time of minimum usage; and

obtain a measurement of the open circuit voltage of the at least one of the first battery string and the second battery string when the at least one of the first battery string and the second battery string is disconnected from the external device.

17. The electrical energy storage system of claim 16 , wherein the controller is configured to estimate a state-of-charge of the at least one of the first battery string and the second battery string based on the measured open circuit voltage of the at least one of the first battery string and the second battery string.

18. The electrical energy storage system of claim 16 , wherein the controller is configured to estimate a remaining life of the at least one of the first battery string and the second battery string based on the measurement of the open circuit voltage of the at least one of the first battery string and the second battery string.