A method for estimating state of charge (SOC) of an electrochemical cell (E) having a charge profile (PC) in which variation of voltage as a function of state of charge between a state of charge higher than a value MinSOCZone2 of state of charge corresponding to a predetermined voltage VmaxZone1 and a state of charge less than a value MaxSOCZone3 corresponding to a predetermined voltage VminZone4 is, on average, at least 10 times less rapid than variation in voltage for a state of charge greater than MaxSOCZone3, and
- wherein the charge profile further comprises an area in which variation of voltage of said cell as a function of state of charge for a state of charge less than value MinSOCZone2 corresponding to voltage VmaxZone1 is on average at least twice as rapid as variation in voltage for a state of charge between values of MinSOCZone2 and MaxSOCZone3, and
- wherein said charge profile for a state of charge between MinSOCZone2 and MaxSOCZone3 exhibits:
- a first sub-area ranging from MinSOCZone2 to a value of state of charge MaxSOCZone2;
- MaxSOCZone2 being a value corresponding to a predetermined voltage value VmaxZone2;
- a second sub-area ranging from a value of state of charge MinSOCZone3 to a value of state of charge MaxSOCZone3;
- said value of state of charge MinSOCZone3 corresponding to a predetermined voltage value VminZone3;
- said method comprising the steps of:
- a) providing a cell;
- connecting said cell to a device for charging or discharging it and measuring a current I through the cell;
- providing a calibration relationship between state of charge of the cell and voltage;
- b) providing an initial state of charge SOC(t0) of said cell at an initial time t0;
- c) measuring cell voltage V(tn) of said cell (E) and determining an adjusted state of charge SOC(tn) of said cell at a point in time tn later than t0 when V(tn) is greater than or equal to said voltage VminZone4, under the following conditions:
- i) if a current I(tn) lower in absolute value than a predetermined current threshold value Ithreshold has been passing through said cell for longer than a predetermined value t_Zone4, then a value SOCV(tn) is determined from said calibration relationship of voltage as a function of state of charge, and said value SOCV(tn) is assigned to SOC(tn) and a zero value is assigned to a parameter corresponding to a cumulative error in state of charge obtained by coulometry ErreurcumuléeSOCC(tn);
- ii) otherwise a value SOCC(tn) determined by coulometry is assigned to SOC(tn) and a cumulative measurement error ErreurcumuléeSOCC(tn) of the measurement of state of charge SOCC(tn) of said cell between time t0 and time tn is determined, and
- wherein step c) further comprises determining an adjusted state of charge SOC(tn) of said cell at a point in time tn greater than t0, when cell voltage V(tn) is less than or equal to said voltage VmaxZone1, under the following conditions:
- i) if a current I(tn) lower in absolute value than a predetermined current threshold value Ithreshold′ has been passing through said cell for longer than a predetermined period of time t_Zone1, then a value SOCV(tn) is determined from said calibration relationship of voltage as a function of state of charge SOC, said value SOCV(tn) is assigned to SOC(tn) and a value of zero is assigned to a parameter corresponding to cumulative error in state of charge obtained by coulometry ErreurcumuléeSOCC(tn);
- ii) otherwise a value SOCC(tn) determined by coulometry is assigned to SOC(tn) and a cumulative measurement error ErreurcumuléeSOCC(tn) in measurement of state of charge SOCC(tn) of said cell between time t0 and time tn is determined, and
- wherein step c) further comprises assigning a value SOC(tn) to said cell at a point in time tn greater than t0 when cell voltage V(tn) of said cell is greater than VmaxZone1 and less than VminZone4, under the following conditions:
- i) if the cell is being traversed by a current I(tn) lower in absolute value than a predetermined threshold current equal to min(Ithreshold, Ithreshold′) for a period of time greater than a predetermined value t_Zone23, value assigning is performed under the following conditions:
- if SOCC(tn)>MaxSOCZone3 then the value MaxSOCZone3 is assigned to SOC(tn) and the value (SOCC(tn)−MaxSOCZone3) is subtracted from the cumulative error in state of charge value determined by coulometry;
- if SOCC(tn)<MinSOCZone2 then the value MinSOCZone2 is assigned to SOC(tn) and the value (MinSOCZone2−SOCC(tn)) is subtracted from the cumulative error in state of charge determined by coulometry;
- if V(tn)<VmaxZone2 and if SOCC(tn)>MaxSOCZone2 then the value MaxSOCZone2 is assigned to SOC(tn) and the value (SOCC(tn)−MaxSOCZone2) is subtracted from the cumulative error in state of charge determined by coulometry;
- if V(tn)>VminZone3 and if SOCC(tn)<MinSOCZone3 then MinSOCZone3 is assigned to SOC(tn) and the value (MinSOCZone3−SOCC(tn)) is subtracted from the cumulative error in state of charge determined by coulometry;
- ii) otherwise a state of charge SOCC(tn) determined by coulometry is assigned to SOC(tn) and a cumulative measurement error ErreurcumuléeSOCC(tn) in measurement of state of charge SOCC(tn) of the cell E at time tn is determined.