U.S. patent application number 13/510935 was filed with the patent office on 2012-10-04 for charge redistribution method for cell arrays.
This patent application is currently assigned to SENDYNE CORPORATION. Invention is credited to Ioannis Milios.
Application Number | 20120249052 13/510935 |
Document ID | / |
Family ID | 45994483 |
Filed Date | 2012-10-04 |
United States Patent
Application |
20120249052 |
Kind Code |
A1 |
Milios; Ioannis |
October 4, 2012 |
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.
Inventors: |
Milios; Ioannis; (New York,
NY) |
Assignee: |
SENDYNE CORPORATION
New York
NY
|
Family ID: |
45994483 |
Appl. No.: |
13/510935 |
Filed: |
October 27, 2011 |
PCT Filed: |
October 27, 2011 |
PCT NO: |
PCT/IB2011/054789 |
371 Date: |
May 19, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61408505 |
Oct 29, 2010 |
|
|
|
Current U.S.
Class: |
320/106 |
Current CPC
Class: |
H02J 7/007 20130101;
H01M 10/441 20130101; Y02E 60/10 20130101; H02J 7/0021 20130101;
H02J 7/0014 20130101 |
Class at
Publication: |
320/106 |
International
Class: |
H02J 7/00 20060101
H02J007/00 |
Claims
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)
Description
[0001] This application claims the benefit of U.S. application Ser.
No. 61/408,505 filed Oct. 29, 2010, which application is
incorporated herein by reference for all purposes.
BACKGROUND
[0002] The challenge of balancing individual cells connected in
series is a well-known issue in the battery industry. Cells
connected in series to form a battery system, even if they were
"ideally" manufactured and identically characterized, over time
will exhibit deviating electrochemical behavior. These behavioral
differences are due, among other reasons, to manufacturing
tolerances that get accented over time, but also due to differences
in the environment within which each individual cell operates.
[0003] There are two major concerns regarding cell imbalance. The
first one regards maximizing the charge a battery system can
accept. The goal is to achieve 100% SOC for the whole battery
system. The second concern regards the ability of the battery to
provide the maximum amount of stored charge to the user. In an
optimal situation the battery shuts down with minimum residual
capacity having provided the maximum amount of its stored energy to
the task at hand.
Achieving 100% SOC
[0004] In a battery consisting of N number of cells the overall
battery SOC.sub.B can be expressed as the sum of the SOC.sub.n of
each cell.
SOC B = 1 N n = 1 N SOC n ( 1 ) ##EQU00001##
[0005] Since SOC is closely related to EMF (Electro Motive Force),
it is obvious that in order to maximize the SOC of the whole pack,
the open circuit voltage Voc of each cell after charging should be
at the maximum value allowed by the chemistry and dictated by the
operating guidelines of the cell manufacturer. In specific cell
chemistries, like Lithium Ion, the charging process is terminated
whenever any cell reaches its manufacturer defined maximum voltage.
The maximum voltage is set at a value that ensures the battery
safety and long term health.
[0006] Cells connected in series receive the same amount of
charging current. Due to differences in their internal impedance,
the cell with the highest impedance will reach the cutoff voltage
earlier than other cells, forcing the control electronics to bah
the charging process. As a result specific cells may not reach
their optimum State of Charge (SOC) and the battery will not
perform at its maximum potential. The percentage of SOC that the
battery system achieves will depend on the value of the internal
impedance, the charging method (pulse, DC, etc.) and the value of
the charging current, among other factors.
[0007] FIG. 1 shows the equivalent circuit of a battery cell being
charged with a slowly changing current. The voltage as measured at
the cell terminals V.sub.cell=V.sub.oc+I.sub.chgR.sub.s. The
R.sub.s value depends among other things on manufacturing
variances, aging and cycle history, present SOC and
temperature.
[0008] Several methods have been proposed to address this
challenge. The most commonly used method in the industry today
"bleeds" the cell whose V.sub.cell value has reached the V.sub.MAX
value and repeats the process of charging until all cells have
reached the V.sub.MAX value. The shortcomings of this method are
well known and have been referenced in several patents, such as
U.S. Pat. No. 5,710,504 "Switched Capacitor System for automatic
Battery Equalization" and U.S. Pat. No. 6,518,725 "Charge balancing
system". Published Japanese patent application 09-084275 "Method
and Apparatus for Controlling Charging of Assembly Battery Pack"
introduces a bypass mechanism combined with current reduction at
the end of charge in order to avoid overheating cells, with the
drawback of prolonged charge time. Sendyne's own U.S. Pat. No.
7,936,150 entitled "Cell protection and conditioning circuit and
system" addresses also this same issue proposing a conditioning
circuit that takes over charging of individual cells when their
V.sub.cell value starts approaching their V.sub.MAX.
Delivering the Stored Charge to the Load
[0009] Assuming the battery has been charged optimally, the next
and probably most important performance issue is its ability to
deliver the stored energy to the active load. If the load is
constant or the variation is slow the equivalent circuit looks like
the one used for charging with the only difference being the
direction of the current.
[0010] The voltage in this case as measured at the cell terminals
would be:
V.sub.cell=V.sub.oc-I.sub.dchgR.sub.2 or
V.sub.cell=EMF-I.sub.dchgR.sub.s (2)
[0011] As can be seen in the above equation, cells with the same
EMF connected in series can exhibit different terminal voltages
V.sub.cell depending on the value of their respective internal
resistance.
[0012] If the load is changing dynamically as in the case of
electric vehicles, instead of the internal resistance we can use
the internal impedance Z in a similar relationship:
V.sub.cell=V.sub.oc-i.sub.dchgZ.sub.s (3)
[0013] The cell impedance among other factors depends on the
frequency content of the load current. A Nyquist representation of
the frequency dependence for Lead Acid and LiIon cells is shown in
FIGS. 3 and 4.
[0014] The high frequency portion on the left side of FIG. 4 is
attributed to conductance of wires, connections, etc., the
mid-section semi-circle to charge transfer and the electrochemical
double layer, and in general the kinetics of the electrochemical
cell reactions. The straight line segment on the right is
attributed to limitations in mass charge transfer, also referred to
as the diffusion limited part.
[0015] It can be appreciated that a dynamic load applied to a
battery system will employ, to a different extent, all mechanisms
of charge transfer. As a result, cell impedances and resulting cell
terminal voltages will vary according to the frequency content of
the load current.
[0016] The impedance Nyquist plot for each cell varies according to
its SOC, temperature, aging and cycle history but also due to
manufacturing variances as it is shown on the following
figures.
[0017] The practical implication is that even if cells
participating in a battery system, start with the same SOC and are
operating at exactly the same conditions they will still exhibit
differences in their internal impedances and the resulting terminal
voltages due to manufacturing variances. Because the impedance
differences are frequency dependent, the voltage differences among
cells will depend on the load current frequencies. As a result,
during discharge of a cell array, cells with the highest composite
impedance at the specific load frequency spectrum, will reach the
cell cutoff voltage first, even if their SOC is not the smallest
one in the array.
[0018] The alert reader will appreciate that in order to prolong
the battery operation, charge should not be distributed equally to
every cell as it is the common concept and practice today, but it
should be distributed in a manner that boosts the cells with the
highest exhibited internal impedance based on the actual load
frequency content. So practically in order to prolong battery life
cell charge should be actively unbalanced.
[0019] Saying the same thing in a different way, while many
investigators have expended much energy and ingenuity to attempting
to make the charge of each cell as close as possible to being
identical to each of the other cells, the invention as will be
discussed below actually pursues the very different end of
unbalanced cell charge among cells.
A Typical Cell Array
[0020] FIG. 6 shows part of a typical battery cell array.
[0021] In a typical implementation N-oumber of cells (2) is
connected in series to form a cell-array. The array (1) can form
the whole or part of a battery system. The voltage of each cell is
monitored typically by a Cell Voltage Monitor circuit (3).
Individual cells may have their own voltage monitoring device, or
they can time-share one. Among other functions, the "Cell Voltage
Monitor" device compares the cell voltage with a set of fixed
values specific to the type of cell, a maximum voltage value
V.sub.OVC (overcharge voltage) and a minimum Vutc (undercharge
voltage) value. A control unit (4) controls a set of switches (5)
that will selectively open if any of the cell voltages exceeds the
V.sub.OVC value or becomes lower than the V.sub.UNC value. The set
of switches is designed in such a way so they will prevent
discharging but allow charging if the undercharge voltage is
detected and they will prevent further charging while allowing
discharging if the overvoltage is detected. The set of protection
switches (5) may be one for the whole array as shown in FIG. 6, or
one per cell if every cell employs its own protection circuit.
The Issue of Unbalanced Cells and Prior Art
[0022] It is appreciated that the first cell whose terminal voltage
is detected to reach the V.sub.UNC (undercharge voltage) value will
force the Control Unit (4) to open the protection switch (5)
forcing the cell array to cease providing charge to the load. The
cutoff value V.sub.UNC is provided by the cell manufacturer and
adherence to guarantees that no irreversible damage will occur to
the cell, due to super-saturation of the cathode or for other
reasons. It is common knowledge in the industry that due to
electrochemical differences among the cells, it is likely that not
all the cells will reach simultaneously the V.sub.UNC (undercharge
voltage) value. So when a cell reaches this value, the rest of the
cells in the battery array may still have residual charge that is
remaining unused, in order to avoid this situation, battery
manufacturer employ different methods, such as: [0023] Cell
matching. Cells are measured during manufacturing and they are
grouped according to their electrical characteristics which may
include capacity, internal impedance, etc. so they will exhibit
similar behavior under the same load [0024] The thermal environment
is controlled so all cells in the cell array, operate under the
same temperature [0025] Cells are "actively balanced", usually
during periods of inactivity in order to match their EMF. The goal
of balancing is to equate the SOC of all the cells participating in
the array, so they hold the same amount of charge. [0026] Batteries
are used along with super-capacitors that isolate the load
variations from the battery cells.
The Role of Cell Impedance in Battery Pack Performance
[0027] When a battery is connected to a dynamic load, such as the
load of an electric car, individual cells connected in series
within the battery, will exhibit differences in their terminal
voltage V.sub.n, which is caused by two factors, as it is shown in
the following equations: [0028] The difference in their EMF [0029]
The difference tn their impedance under the specific load
[0029] V.sub.1=EMF.sub.1+I.sub.LOAD*Z.sub.1
V.sub.2=(EMF.sub.1.+-..DELTA.EMF)+I.sub.LOAD*(Z.sub.1.+-..DELTA.Z)
(4)
Or
V.sub.2-V.sub.1=.+-..DELTA.EMF.+-.I.sub.LOAD*.DELTA.Z (5)
[0030] From (5) it may be seen that the difference in terminal
voltage between two cells depends not only on differences of their
respective EMF, but also on the I.sub.LOAD current value and the
dynamic difference of their internal impedance .DELTA.Z. From (5)
it can be appreciated that even if .DELTA.EMF=o, which means the
cells are "balanced" (in the traditional sense of the term), they
will still exhibit dynamic voltage difference that depending on the
current value, and the load, among other things, may cause the cell
with the highest impedance to reach first the voltage cutoff value
V.sub.UNC.
SUMMARY OF THE INVENTION
[0031] A method for charge redistribution that prolongs battery
operation. A dynamic charge re-distribution method is proposed for
a cell array of N-cells, where cell voltages V.sub.1, V.sub.2, . .
. , V.sub.N are constantly monitored for variances underload
signals of various frequency and intensity. In one implementation,
the algorithm may detect the voltage difference between the cell
with the highest voltage in the array and the cell with the lowest
voltage in the array. In another implementation the supervising
circuit may monitor the standard deviation of the distribution of
cell voltages. Other algorithms may be used to detect load current
frequency and intensity conditions where voltages of individual
cells exhibit their widest spread.
[0032] When depending on the method employed one of the above
conditions is met, the charge stored within the cells is
re-distributed in such a way, so the following expression becomes
true:
EMF.sub.1-I*Z.sub.1=EMF.sub.2-I*Z.sub.2= . . .
=EMF.sub.n-I*Z.sub.n= . . . =EMF.sub.N-I*Z.sub.N (6)
[0033] The goal of the charge re-distribution method is to "boost"
proactively the cell with the lowest EMF, highest impedance
combination in order to prevent it from triggering an array
shutdown condition, as well as achieve optimum power output for the
cell array under the specific load conditions.
[0034] It should he appreciated that the internal impedance of each
cell depends on the cell's EMF so there is no assurance of, let
alone a likelihood of, a linear relationship between the charge
transfer and the impedance change.
DESCRIPTION OF THE DRAWING
[0035] The invention will be described with respect to a drawing in
several figures, of which:
[0036] FIG. 1 is an equivalent circuit of a cell being charged with
a slowly changing current;
[0037] FIG. 2 is an equivalent circuit of a cell being discharged
through a constant or slowly varying load;
[0038] FIG. 3 is a Nyquist representation of LA cell impedance
dependence on load signal frequency;
[0039] FIG. 4 is a typical Nyquist plot of 40 Ah Li-tec cell, 50%
State-of-Charge (SoC);
[0040] FIG. 5 shows impedance spectra measured at 3.750 V, from 1
kHz to 1 mHz, for 50 new cells from two manufacturers; and
[0041] FIG. 6 shows a cell array.
DETAILED DESCRIPTION
[0042] A first insight is to choose to model each cell not merely
with an effective internal resistance but with an effective
internal impedance.
[0043] If a series array of cells were driving a DC load (for
example an incandescent bulb or an array of LEDs) then there is no
reason to pay any attention to the imaginary component (if any) of
cell impedance.
[0044] Impedance, in distinction to a pure ohmic resistance, is
dependent among other things on the load frequency. In battery
cells, impedance increases significantly in low frequencies. In the
case of a cell stack operating under a given load current, cells
with the higher impedance will exhibit lower terminal voltage and
subsequently lower power output. The same cells will also reach
earlier the cutoff voltage, causing the whole battery to cease
operation.
[0045] A second insight is to appreciate that some loads have a
substantial non-DC component. This happens for example in electric
and hybrid cars.
[0046] A third insight is to appreciate the impedance dependence on
the SOC (State-of-charge) of the cell. Impedance among cells varies
due to manufacturing tolerances, age, use, etc. During each
charge/discharge cycle impedance also changes, so a cell will
exhibit a lower impedance when it is in a charged state (high SOC)
and a higher impedance when it is in a discharged state (low SOC).
Thus charging a cell will result in lowering its impedance.
[0047] In specific battery chemistries, such as the Lithium Iron
Phosphate (LiFePO.sub.4), the change in impedance as related to the
SOC, is the most significant indication of the State of Charge, as
such cells maintain an almost constant voltage through most of the
discharge process.
[0048] This prompts us to look not merely at the effective internal
resistance of a cell but at the effective internal impedance.
[0049] In response to these insights, we choose to infer the Z
(effective internal impedance) of each cell. Having worked out
which cells have larger Z than others, we then "top up" in a
proactive way a cell that has a lower EMF and greater Z as compared
with at least one of the other cells in the array.
[0050] There is the possibility that two cells with the same EMF
will have different internal impedances due to manufacturing
variances or/and temperature or/and ageing effects. Charging a cell
with a high impedance will again have the effect of lowering its
internal impedance which may be desirable in situations where this
cell is reaching its cutoff voltage.
[0051] It is then helpful to review some possible ways of inferring
the impedance Z of a particular cell. It will be appreciated,
however, that the teachings of the invention offer their benefits
regardless of the particular impedance-inferring approach that is
adopted in a particular embodiment.
[0052] One example is that you may have an estimate of Voc through
another method (Coulomb counting) or direct measurement, which case
it is possible to infer Z. What we are really interested in this
method is to "force" the revelation of large voltage differences
that depend on Z, then, use this information to transfer charge so
the differences in the frequency operating region of the battery
can disappear.
[0053] There are at least two methods we can employ to "reveal"
Z.
[0054] Returning to FIG. 4, we can divide the plotted values into
three areas--an area I which is nearly a straight line, an area II
which is roughly a half circle, and an area III which extends
upwards and to the right in FIG. 4.
[0055] One approach to force the cell to reveal its Z is to perform
a Fast Fourier Transform (FFT), as well as achieve optimum power
output for the cell array under the specific load conditions upon
the actual load current I. Since the battery system is not a linear
system, and most likely the load variation will fall outside the
range of any linear behavior, the FFT can give us only a rough idea
of the impedance range. A "high" frequency load signal (some
kilohertz, depending on cell technology) will reveal "ohmic"
resistance in Area I, which will remain constant (independently of
State of Charge or load current or age). A "medium" frequency load
signal (in the range of a few Hertz to some kiloHertz) will reveal
impedances in Area II (which will depend on State-of-Charge, load
current, age, etc.). A "low" frequency signal (mHz/uHz) will reveal
impedance in Area III with impedance values changing along the same
parameters with Area II.
[0056] It is in areas II and III where the impedance dependence on
SOC is pronounced, and subsequently voltage differences among cells
with differing SOC become apparent. The lower the frequency the
more apparent the voltage and SOC difference.
[0057] A second method to force the cell to reveal its Z is to
impose a small voltage signal (or current) and then measure the
current variation (or Voltage). The input signal could be of a
single frequency, of multiple frequencies or of a step signal
according to the practices used in Electronic Impedance
Spectroscopy or in Frequency Response Analyzers. This method may
produce more accurate estimates of the impedance at the expenses of
greater processing time and greater system complexity.
[0058] The chief significance of the plot of FIG. 4 is not so much
as a way of arriving at actual impedance values, but as a way of
(a) reminding us that the modeled internal impedance is frequency
dependent, and (b) giving us a bit of insight as to how much
bandwidth we will require our current meter and our voltmeter to be
able to analyze. In an exemplary embodiment, we are actually
interested more in the lower frequency spectrum than the
higher.
Picking the Bandwidth at Which to Carry Out Z Measurements
[0059] We can determine the bandwidth within which measurements are
carried out by the bandwidth the battery system will have to
operate within (from signal history data, general experience or
theoretical projections). Some economy of effort may be achieved by
limiting the measurement bandwidth to some region of bandwidth
within which the cells exhibit their maximum impedances. This may
be influenced by particular cell chemistry and the topology of the
cell structure, among other things.
Optimally Unbalancing a Cell Array
[0060] Dynamic balancing involves transfers of charge among cells.
Anytime a charge transfer occurs, part of the energy is lost. It is
appreciated, that in a battery system the number of such transfers
should be minimized.
[0061] When the frequency or the intensity of the load current
changes, the equality of (6) achieved through charge
re-distribution will seize to exist and (7) will be true.
EMF.sub.1.about.I*Z.sub.1.noteq.EMF.sub.2.about.I*Z.sub.2.noteq. .
. . .noteq.EMF.sub.n.about.I*Z.sub.n.noteq. . . .
.noteq.EMF.sub.N.about.I*Z.sub.N (7)
[0062] Under the proposed algorithm this fact will not necessarily
trigger another charge redistribution process.
[0063] The process will be repeated only when the signal frequency
and intensity conditions exist to maximize the spread of the cell
voltage values (for example a high intensity step signal--such as
in acceleration).
[0064] It is understood that when the load current becomes zero
(for example during a "rest" period), then the I*Z.sub.x terms in
(6) become zero and the following becomes true:
EMF.sub.1.noteq.EMF.sub.2 . . . EMF.sub.n . . . .noteq.EMF.sub.N
(8)
[0065] That is, the Open Circuit Voltage (OCV) of the cells is
different and subsequently their SOC is different and the cells are
unbalanced, by the classical definition.
[0066] Nevertheless, with this method in future load conditions
cells will match much closer their output voltages and thus will
use their collective energy and power optimally before triggering
the under-voltage cutoff mechanism.
Monitoring Individual Cell Impedances and Projecting Future
Values
[0067] By monitoring cell impedance frequency dependent data,
projections can be made regarding the relationship of impedance
with each cell's SOC and signal frequency content. Based on these
projections charge re-distribution can be performed in a way that
ensures that the "weakest" cells (higher impedance, lower EMF), can
he boosted to withstand future dynamic loads, specific to the
application, without entering the Voltage undercharge region.
What-If Scenarios
[0068] Based on load conditions, for example a car maybe driving or
being parked, the Battery System Manager processor may make
decisions on when and whether to redistribute the charge for
compensating either for impedance (driving) or EMF (parked).
[0069] Having also the knowledge of the impedance condition of each
cell, the BMS can optimize cell charge redistribution either for
endurance or for performance.
[0070] Finally the BSM can answer "what if" questions regarding
future battery performance under different load scenarios.
[0071] Charge re-distribution can be implemented either selectively
(from some cells to one or more cells) or with the simultaneous
participation of all cells.
[0072] What has been described, then, is a method for use with a
series array of a plurality of electrochemical cells, each cell
having a respective state of charge. We measure discharge current
during a first measurement interval, said current measurement
carried out across a predetermined bandwidth. We measure cell
terminal voltage for a first one of the cells during the first
measurement interval, said voltage measurement carried out across
the predetermined bandwidth. We measure discharge current during a
second measurement interval, said current measurement carried out
across a predetermined bandwidth. We measure cell terminal voltage
for a second one of the cells during the second measurement
interval, said voltage measurement carried out across the
predetermined bandwidth. We derive 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. We derive 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. We identify
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. Finally, we top
up the state of charge of the identified cell.
[0073] In this method, the first measurement interval and the
second measurement interval can be the same, in which case 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.
[0074] Alternatively, in this method the first measurement interval
and the second measurement interval can be one after another, in
which case 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.
[0075] In this method, where the measurements are carried out with
respect to N cells, N greater than two, it may develop that what is
identified is a cell having the lowest effective internal cell
voltage and the highest magnitude of effective internal
impedance.
[0076] Speaking more generally, we may carry out a method that
starts with estimating the internal impedance of each of the cells,
and identifying a cell with higher impedance than that of at least
one other cell, in which case we boost the charge of the identified
cell. The outcome is that the boosted cell finishes with a higher
EMF than that of the at least one other cell.
[0077] Suitable apparatus may be employed to carry out these
methods.
[0078] The alert reader will have no difficulty devising myriad
obvious improvements and variants upon the invention as described
and claimed herein. All such obvious improvements and variants are
intended to be encompassed within the claims which follow.
* * * * *