Methods of determining a relative state of charge of a battery for a surgical instrument and surgical instruments and computer-readable media implementing the same

A surgical instrument, comprising:

• an end effector configured to treat tissue with ultrasonic energy;
• a user interface for receiving an input;
• a generator coupled to the end effector;
• a battery in communication with the generator;
• and
• a controller coupled to the battery, the controller including:
• a processor;
• and
• a memory coupled to the processor, the memory storing a plurality of discharge profiles and having instructions stored thereon which, when executed by the processor, cause the processor to:
• detect an input at the user interface,
• in response to the detected input at the user interface, match a discharge profile among the plurality of discharge profiles to the detected input, the discharge profile corresponding to a discharge curve representing current discharge over time,
• identify a relative state of charge from the discharge curve based on usage of the surgical instrument during a surgical procedure, and
• provide the identified relative state of charge as an output.