Method for estimating the state of charge of a battery comprising an initialization phase of the state of charge of the battery that includes estimating an initial value (SOCini) of said state of charge, said initialization phase comprising the following steps: - reception of a memorized state-of-charge value of the battery (SOCmem) when the battery enters sleep mode, - calculation of a state-of-charge value (SOCocv) using a measurement of the no-load voltage (Ucell) at the terminals of the battery when the battery wakes, said initialization phase including a calculation step to calculate an error (Err) in the measurement of the no-load voltage due to the relaxation of the battery, in which said error is calculated as a function of the current temperature of the battery and a target relaxation time (Thresh) of the battery modelled for each temperature, the estimate of the initial value of said state of charge being calculated on the basis of a comparison between said memorized values (SOCmem) and calculated values (SOCocv) of the state of charge and as a function of said calculated error (Err), the method being characterized in that the calculation of the error (Err) caused by relaxation involves: - memorizing map data correlating a given state of charge with a given no-load voltage of the battery, for a given temperature of the battery, - determining a maximum error in the measurement of the no-load voltage corresponding to the error between the battery voltage on waking for a minimum relaxation time and the battery voltage on waking for said target relaxation time, - providing error calibration data describing a variation of a margin of error in the state of charge (SOC_error) in relation to a state-of-charge value provided by the map data, as a function of said maximum error, - calculating said error (Err) caused by relaxation using said no-load voltage measurement (Ucell) when the battery wakes and said calibration data. - reception of a memorized state-of-charge value of the battery (SOCmem) when the battery enters sleep mode, - calculation of a state-of-charge value (SOCocv) using a measurement of the no-load voltage (Ucell) at the terminals of the battery when the battery wakes, said initialization phase including a calculation step to calculate an error (Err) in the measurement of the no-load voltage due to the relaxation of the battery, in which said error is calculated as a function of the current temperature of the battery and a target relaxation time (Thresh) of the battery modelled for each temperature, the estimate of the initial value of said state of charge being calculated on the basis of a comparison between said memorized values (SOCmem) and calculated values (SOCocv) of the state of charge and as a function of said calculated error (Err), the method being characterized in that the calculation of the error (Err) caused by relaxation involves: - memorizing map data correlating a given state of charge with a given no-load voltage of the battery, for a given temperature of the battery, - determining a maximum error in the measurement of the no-load voltage corresponding to the error between the battery voltage on waking for a minimum relaxation time and the battery voltage on waking for said target relaxation time, - providing error calibration data describing a variation of a margin of error in the state of charge (SOC_error) in relation to a state-of-charge value provided by the map data, as a function of said maximum error, - calculating said error (Err) caused by relaxation using said no-load voltage measurement (Ucell) when the battery wakes and said calibration data. - memorizing map data correlating a given state of charge with a given no-load voltage of the battery, for a given temperature of the battery, - determining a maximum error in the measurement of the no-load voltage corresponding to the error between the battery voltage on waking for a minimum relaxation time and the battery voltage on waking for said target relaxation time, - providing error calibration data describing a variation of a margin of error in the state of charge (SOC_error) in relation to a state-of-charge value provided by the map data, as a function of said maximum error, - calculating said error (Err) caused by relaxation using said no-load voltage measurement (Ucell) when the battery wakes and said calibration data.