Method for determining a round-trip-efficiency indicator for an electric accumulator, this method comprising the following steps: - acquiring measurements of parameters representative at least of the current associated with the electric accumulator, and of a state-of-charge indicator of the electric accumulator, this acquisition being carried out over a measurement period (PM) comprising a set of operations of charging and discharging the electric accumulator; this method being characterized in that it comprises the following steps: - a) converting the set of charging and discharging operations of the measurement period (PM) into an equivalent set of symmetrical reconstructed cycles, each symmetrical reconstructed cycle comprising one charging operation and one discharging operation of same amplitude of the state-of-charge indicator; - b) selecting at least one symmetrical reconstructed cycle the amplitude of the state-of-charge indicator of which is greater than a predetermined value; - c) for a predetermined physical quantity involving at least the current associated with the electric accumulator, and for at least one selected symmetrical reconstructed cycle: determining a first magnitude of said predetermined physical quantity delivered to the electric accumulator during the charging operation, and determining a second magnitude of said predetermined physical quantity delivered by the electric accumulator during the discharging operation; - (d) determining the integral with respect to time of said first magnitude and of said second magnitude; - (e) determining a round-trip-efficiency indicator for the electric accumulator by dividing the integral of the second magnitude by the integral of the first magnitude. - acquiring measurements of parameters representative at least of the current associated with the electric accumulator, and of a state-of-charge indicator of the electric accumulator, this acquisition being carried out over a measurement period (PM) comprising a set of operations of charging and discharging the electric accumulator; - a) converting the set of charging and discharging operations of the measurement period (PM) into an equivalent set of symmetrical reconstructed cycles, each symmetrical reconstructed cycle comprising one charging operation and one discharging operation of same amplitude of the state-of-charge indicator; - b) selecting at least one symmetrical reconstructed cycle the amplitude of the state-of-charge indicator of which is greater than a predetermined value; - c) for a predetermined physical quantity involving at least the current associated with the electric accumulator, and for at least one selected symmetrical reconstructed cycle: determining a first magnitude of said predetermined physical quantity delivered to the electric accumulator during the charging operation, and determining a second magnitude of said predetermined physical quantity delivered by the electric accumulator during the discharging operation; - (d) determining the integral with respect to time of said first magnitude and of said second magnitude; - (e) determining a round-trip-efficiency indicator for the electric accumulator by dividing the integral of the second magnitude by the integral of the first magnitude. <EOS>