Estimating of the state of charge of a battery
A method for estimating a state of charge of a battery including plural cells connected in series, including: determination, at a given time, of a minimum cell voltage and of a maximum cell voltage from among the cell voltages, and calculation of a physical quantity analytically depending on the minimum and maximum cell voltages according to an equation including weighting elements ensuring the weight associated with the maximum cell voltage increases when the state of charge of the associated cell increases, and the weight associated with the minimum cell voltage increases when the state of charge of the associated cell decreases.
1. A method for estimating a state of charge of a battery in an electric or hybrid vehicle, the battery including plural electrochemical cells connected in series, a voltage across terminals of the battery corresponding to a sum of cell voltages across terminals of each electrochemical cell, the method comprising:
determining, using processing circuitry, at a given moment in time, a minimum cell voltage and a maximum cell voltage from amongst the cell voltages across the terminals of the electrochemical cells connected in series without measuring all of the cell voltages of the plural electrochemical cells at the given moment in time;
determining, using the processing circuitry, a physical quantity on which the state of charge of the battery directly or indirectly depends, the physical quantity depending analytically, directly or indirectly, on the determined minimum cell voltage and on the determined maximum cell voltage according to an equation that includes weighting elements ensuring that a weighting associated with the determined maximum cell voltage increases when a state of charge of an associated electrochemical cell increases, and a weighting associated with the determined minimum cell voltage increases when the state of charge of the associated electrochemical cell decreases; and
estimating, using the processing circuitry, the state of charge of the battery from the determined physical quantity to measure current flowing through the electrochemical cells at the given moment in time, and to measure temperature of the battery at the moment in time.
2. The estimation method as claimed in claim 1 , wherein the physical quantity is a weighted mean voltage depending analytically on the minimum cell voltage (U Cmin ) and on the maximum cell voltage (U Cmax ) according to equation:
U
m
p
(
k
)
=
V
high
_
threshold
U
Cmin
(
k
)
-
V
low
_
threshold
U
Cmax
(
k
)
(
V
high
_
threshold
-
V
low
_
threshold
)
-
(
U
Cmax
(
k
)
-
U
Cmin
(
k
)
)
,
in which U Cmin (k) and U Cmax (k) are respectively samples of the determined minimum cell voltage and the determined maximum cell voltage at the given moment in time k, V low _ threshold is a predetermined minimum voltage threshold in use corresponding to a minimum state of charge of the associated electrochemical cell and V high _ threshold is a predetermined maximum voltage threshold in use corresponding to a maximum state of charge of the associated electrochemical cell.
3. The estimation method as claimed in claim 1 , wherein said estimating comprises filtering using a Kalman filter.
4. The estimation method as claimed in claim 1 , further comprising:
a first estimation of the state of charge SOC Cmin (k) of the electrochemical cell associated with the determined minimum cell voltage using the minimum cell voltage, a measurement of current flowing through the electrochemical cells at the given moment in time, and a first measurement of temperature at the given moment in time; and
a second estimation of the state of charge SOC Cmax (k) of the electrochemical cell associated with the determined maximum cell voltage using the maximum cell voltage, the measurement of current flowing through the electrochemical cells at the given moment in time, and a second measurement of temperature at the given moment in time,
wherein the physical quantity is directly the state of charge (SOC BAT ) of the battery at the given moment in time, is calculated according to equation
SOC
BAT
(
k
)
=
SOC
Cmin
(
k
)
1
-
(
SOC
Cmax
(
k
)
-
SOC
Cmin
(
k
)
)
.
5. The estimation method as claimed in claim 4 , wherein the first and second estimations implement Kalman filtering.
6. The estimation method as claimed in claim 4 , wherein the first and second measurements of temperature are one and a same measurement representative of the temperature of the battery.
7. The estimation method as claimed in claim 4 , wherein the first and second measurements of temperature are two different measurements respectively taken in association with the electrochemical cell associated with the determined minimum cell voltage and in in association with the electrochemical cell associated with the determined maximum cell voltage.
8. A device for estimating a state of charge of a battery in an electric or hybrid vehicle, the battery comprising plural electrochemical cells connected in series, a voltage across terminals of the battery corresponding to sum of cell voltages across terminals of each electrochemical cell, the device comprising:
circuitry configured to
determine, at a given moment in time, a minimum cell voltage and a maximum cell voltage from amongst the cell voltages across the terminals of the electrochemical cells connected in series without measuring all of the cell voltages of the plural electrochemical cells at the given moment in time,
calculate a physical quantity on which the state of charge of the battery directly or indirectly depends, the physical quantity depending analytically, directly or indirectly, on the determined minimum cell voltage and on the determined maximum cell voltage according to an equation that includes weighting elements ensuring that a weighting associated with the determined maximum cell voltage increases when a state of charge of an associated electrochemical cell increases, and a weighting associated with the determined minimum cell voltage increases when the state of charge of the associated electrochemical cell decreases, and
estimate the state of charge of the battery from the calculated physical quantity to measure current flowing through the electrochemical cells at the given moment in time, and to measure temperature of the battery at the moment in time.
9. The estimation device as claimed in claim 8 , wherein the circuitry includes an analog component configured to perform a MIN-MAX function to determine, at the given moment in time, the minimum cell voltage and the maximum cell voltage without measuring all of the cell voltages of the plural electrochemical cells at the given moment in time.