A method can be used to perform a state of health (SoH) estimation for a rechargeable battery energy storage system during operation of the rechargeable battery energy storage system. The rechargeable battery energy storage system includes a plurality of parallel strings that are individually controllable. The method includes selecting at least one string from the plurality of parallel strings, placing the selected at least one string into a SoH calibration mode for performing a SoH calibration while concurrently maintaining at least one other string of the plurality of parallel strings in operative mode, and causing the selected at least one string to return to the operative mode after the SoH calibration has been completed for the selected at least one string.