Charge redistribution method for cell arrays

Abstract

Cell balancing aims to prolong the battery operating life by equalizing the Electro Motive Force (or Open Circuit Voltage) of the participating cells. Even perfectly balanced cells though will exhibit different output voltages because of differences in their internal impedances. The difference in voltage will depend on the load current frequency and intensity. A method is described for re-distributing charge in such a way so when the worst (from the point of view of voltage spread) possible load conditions occur, cells will have similar outputs and none will cross the under-voltage threshold causing a premature shut down of the battery.

1 . A method for use with a series array of a plurality of electrochemical cells, each cell having a respective state of charge, the method comprising the steps of:
 measuring discharge current during a first measurement interval, said current measurement during the first measurement interval carried out across a predetermined bandwidth;   measuring cell terminal voltage for a first one of the cells during the first measurement interval, said voltage measurement during the first measurement interval carried out across the predetermined bandwidth;   measuring discharge current during a second measurement interval, said current measurement during the second measurement interval carried out across a predetermined bandwidth;   measuring cell terminal voltage for a second one of the cells during the second measurement interval, said voltage measurement during the second measurement interval carried out across the predetermined bandwidth;   deriving information indicative of a respective effective internal impedance for each of the first one of the cells and the second one of the cells, said derived effective internal impedance having not only a pure ohmic   component but also frequency dependent component, said derived effective internal impedance defining a magnitude greater than that of the pure ohmic component taken alone;   deriving information indicative of a respective effective internal cell voltage for each of the first one of the cells and the second one of the cells;   identifying a particular one of the first one of the cells and the second one of the cells having a lower effective internal cell voltage and a higher magnitude of effective internal impedance; and   topping up the state of charge of the identified cell.   
     
     
         2 . The method of  claim 1  wherein the first measurement interval and the second measurement interval are the same, and wherein the measurements of cell terminal voltage for the first one of the cells and for the second one of the cells are carried out simultaneously by separate voltage measurement devices. 
     
     
         3 . The method of  claim 1  wherein the first measurement interval and the second measurement interval are one after another, and wherein the measurements of cell terminal voltage for the first one of the cells and for the second one of the cells are carried out in turn by a single voltage measurement device multiplexed to the first one of the cells and to the second one of the cells. 
     
     
         4 . The method of  claim 1  wherein the measurements are carried out with respect to N cells, N greater than two, and wherein the step of identifying a particular one of the cells is further characterized in that what is identified is a cell having the lowest effective internal cell voltage and the highest magnitude of effective internal impedance. 
     
     
         5 . A method for use with a series array of electrochemical cells, each cell having a respective internal impedance, the method comprising the steps of:
 estimating the internal impedance of each of the cells;   identifying a cell with higher impedance than that of at least one other cell; and   boosting the charge of the identified cell;   whereby the boosted cell finishes with a higher EMF than that of the at least one other cell.   
     
     
         6 . The method of  claim 5  wherein the series array of electrochemical cells further comprises a shutdown mechanism responsive to the event of a cell terminal voltage dropping below a predetermined shutdown threshold for shutting down the array. 
     
     
         7 . The method of  claim 5  further comprising the step of estimating the EMF of each of the cells; and wherein the identified cell has the lowest EMF and highest impedance of any of the cells of the array. 
     
     
         8 . The method of  claim 5  wherein the step of estimating the internal impedance of each of the cells is carried out at frequencies between one millihertz and one kilohertz. 
     
     
         9 . The method of  claim 5  further comprising the step of estimating the EMF of each of the cells; and wherein the identified cell has a lower EMF than that of the at least one other cell. 
     
     
         10 . Apparatus for use with a series array of a plurality of electrochemical cells, each cell having a respective state of charge, the apparatus comprising:
 means measuring discharge current during a first measurement interval, said current measurement during the first measurement interval carried out across a predetermined bandwidth;   means measuring cell terminal voltage for a first one of the cells during the first measurement interval, said voltage measurement during the first measurement interval carried out across the predetermined bandwidth;   means measuring discharge current during a second measurement interval, said current measurement during the second measurement interval carried out across a predetermined bandwidth;   means measuring cell terminal voltage for a second one of the cells during the second measurement interval, said voltage measurement during the second measurement interval carried out across the predetermined bandwidth;   means deriving information indicative of a respective effective internal impedance for each of the first one of the cells and the second one of the cells, said derived effective internal impedance having not only a pure ohmic component but also a frequency dependent component, said derived effective internal impedance defining a magnitude greater than that of the pure ohmic component taken alone;   means deriving information indicative of a respective effective internal cell voltage for each of the first one of the cells and the second one of the cells;   means identifying a particular one of the first one of the cells and the second one of the cells having a lower effective internal cell voltage and a higher magnitude of effective internal impedance; and   means topping up the state of charge of the identified cell.   
     
     
         11 - 12 . (canceled)