Abstract: A method for determination of battery parameters comprises the following steps: the determination of multiple roundtrip battery for a number of different pairs of charging and efficiencies discharging battery C-rates/P-rates, solving a nonlinear optimisation problem to obtain one-way efficiencies, and finding a charging and discharging characteristics for selected charging and discharging C-rates/P-rates. The obtained characteristic curves can reveal the actual current/power that is charged/discharged into/from the battery when the battery is charged/discharged with selected current/power from/to an external source/sink. The characteristic charging/discharging curves used can be for determination of battery charge capacity, battery energy capacity, state-of-charge (SOC), state-of-energy (SOE), state-of-health (SOH), and other battery parameters.