Method and system for estimating state of charge of a lithium electrochemical cell having a lithium phosphate type positive electrode

摘要

A method and system are provided for estimating state of charge of a cell having a positive electrode, for example of a lithium phosphate type, the charge profile of which includes a range of state of charge between about 30 and about 90% in which variation of voltage is 10 times less rapid than for a state of charge higher than 90%. Determination of state of charge for a state of charge greater than 90% is based on either the use of a calibration relationship between cell voltage and state of charge when a current is passing through the cell that is less than a predetermined threshold value and its voltage has stabilized, or on the use of coulometry the result of which is corrected taking into account the accuracy of the current sensor.

主权项

1. 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)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)