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

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;
• scheduling the battery to be disconnected from the external device at the future time;
• operating a switch to disconnect the battery at the scheduled future time;
• and
• measuring an open circuit voltage of the battery when the battery is disconnected from the external device.

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,
• schedule at least one of the first battery string and the second battery string to be disconnected from the external device at the future time,
• disconnect the at least one of the first battery string and the second battery string at the scheduled future time;
• 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.