A method for determining an ageing function ( d Δ C dQ tot, Δ C ( t)) of an accumulator, or of a group of accumulators, the ageing function representing a variation in the capacity or resistance of the accumulator, or of the group of accumulators, as a function of variables representative of the operation of the accumulator, the method comprising: - a) carrying out a plurality of experimental cycles of charging and discharging at least one test accumulator representative of the accumulator or of the group of accumulators, each cycle being parameterised by accumulator operating parameters that vary as a function of time during the various cycles; - b) during the charging and discharging cycles, at various measurement times (t), determining experimental data comprising a value of each variable parameter; - c) on the basis of the experimental data resulting from b), determining the ageing function; wherein: - in step a), the variable parameters comprise at least the state of charge ( SOC ( t)) and a depth of discharge ( DOD ( t)); - step b) comprises determining a depth-of-discharge value ( DOD ( t)), at various measurement times ( t) of a given cycle, from a difference between the state of charge at the measurement time and the state of charge ( SOC ( t)) at a preceding measurement time ( SOC ( t - 1)); - c) is implemented by an optimisation algorithm, so as to minimise a deviation between: • the capacity or resistance, of the or of each test accumulator, at a plurality of measurement times; • and an estimate, determined by applying the ageing function, of the capacity or resistance of the or of each test accumulator at each measurement time. - such that, following step c), the variables of the ageing function comprise at least the state of charge and the depth of discharge. - a) carrying out a plurality of experimental cycles of charging and discharging at least one test accumulator representative of the accumulator or of the group of accumulators, each cycle being parameterised by accumulator operating parameters that vary as a function of time during the various cycles; - b) during the charging and discharging cycles, at various measurement times (t), determining experimental data comprising a value of each variable parameter; - c) on the basis of the experimental data resulting from b), determining the ageing function; - in step a), the variable parameters comprise at least the state of charge ( SOC ( t)) and a depth of discharge ( DOD ( t)); - step b) comprises determining a depth-of-discharge value ( DOD ( t)), at various measurement times ( t) of a given cycle, from a difference between the state of charge at the measurement time and the state of charge ( SOC ( t)) at a preceding measurement time ( SOC ( t - 1)); - c) is implemented by an optimisation algorithm, so as to minimise a deviation between: • the capacity or resistance, of the or of each test accumulator, at a plurality of measurement times; • and an estimate, determined by applying the ageing function, of the capacity or resistance of the or of each test accumulator at each measurement time. • the capacity or resistance, of the or of each test accumulator, at a plurality of measurement times; • and an estimate, determined by applying the ageing function, of the capacity or resistance of the or of each test accumulator at each measurement time. - such that, following step c), the variables of the ageing function comprise at least the state of charge and the depth of discharge.