Note: Descriptions are shown in the official language in which they were submitted.
CA 02537526 2006-02-23
SYSTEM AND METHOD FOR MONITORING BATTERY STATE
Field of the Invention
[001] The present invention relates to battery monitoring for determining
battery state to obtain a better estimation of battery lifetime.
Backcround of the Invention
[002] Batteries are generally well understood devices that convert stored
chemical energy into electrical energy. With changes in electronics and power
requirements such as changes from analog to digital loads, battery technology
is
ever evolving and improving technically in chemistry, process control,
material and
control electronics. With the proliferation of digital electronics and
wireless
communications, batteries are becoming increasingly vital in modern
technology.
In many cases, the battery is the limiting factor for the size, shape and run
time (or
life) of the device which it serves.
[003] Batteries have a limited lifetime, which is nonlinearly related to the
chemical composition, application, environmental factors, and use/maintenance.
Because batteries are a consumable, it is understood that battery performance
declines over the lifetime until the battery eventually stops working. A
problem lies
in identifying when the battery will stop working, in advance of such a
failure. This
is vital, especially in critical applications that are dependant on battery
power. For
example, factories or even hospitals that require battery power cannot afford
battery failure in certain applications.
[004] Voltage measurement is commonly used to determine the state of a
battery. When the measured voltage is below a predetermined threshold value,
the battery is considered to be at or near the end of the battery lifetime and
is
replaced. While this is a simple measurement technique, it suffers from many
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disadvantages. For example, the battery must be charged prior to testing. If
the
battery is not fully charged, an incorrect determination that the battery
requires
changing is made. Further, this measurement does not estimate lifetime of the
battery and does not consider load conditions during use or the pattern of use
of
the battery.
[005] In another prior art battery testing method, a known load is applied to
a battery while the voltage discharge is measured and used as an indication of
battery state. This method requires that the battery be taken out of service
for
testing and charged prior to testing. Further, this test does not accurately
reflect
the load conditions during use and pattern of use of the battery, thereby
resulting
in inaccurate or unreliable results.
[006] Other prior art systems use Electrochemical Impedance
Spectroscopy (EIS) in conjunction with trending software to monitor the
battery
chemistry. Regular maintenance of battery systems (ie telecomm UPS systems)
includes the use of an EIS device to quantify the state of the battery. These
prior
art systems for estimating the state of batteries suffer many disadvantages.
For
example, many systems estimate battery lifetime without consideration of the
application, the load conditions during use or the pattern of use. Thus, the
estimated lifetime is inaccurate or unreliable.
[007] A better system and technique for estimating the state of a battery
and an estimated lifetime is therefore desirable.
Summary of the Invention
[008] In accordance with one aspect of the present invention, there is
provided a method of monitoring a battery that includes measuring an open
circuit
voltage (Voc) on the battery to determine a measured state of charge (SOCm)
after
at least one use, obtaining a calculated state of charge (SOC,) based on a
state of
charge prior to the at least one use and a discharge of the battery during the
at
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least one use, and determining a state of the battery based at least in part
on a
difference between the calculated state of charge and the measured state of
charge, and a weighted average of the difference and previously determined
differences between the calculated state of charge and the measured state of
charge.
[009] In accordance with another aspect of the present invenfion, there
is provided a system for determining state of health of a battery. The system
includes a meter for measuring an open circuit voltage on the battery to
determine a measured state of charge after at least one use and a calculation
device for determining a calculated state of charge based on a state of charge
prior to the at least one use and a calculated discharge of the battery during
the
at least one use, and for determining a state the battery based at least in
part
on the difference between the calculated state of charge and the measured
state of charge, and a weighted average of the difference and previously
determined differences between the calculated state of charge and the
measured state of charge.
[010] In accordance with another aspect of the present invention, there
is provided a method of monitoring a battery. The method includes measuring
an electrical characteristic after at least one use to provide a measured
electrical characteristic, obtaining a calculated electrical characteristic
based on
the electrical characteristic prior to the at least one use and a discharge of
the
battery during the at least one use, and determining a state of the battery
based
at least in part on a difference between the measured electrical
characteristic
and the calculated electrical characteristic, and a weighted average of the
difference and previously determined differences between the calculated
electrical characteristic and the measured electrical characteristic
[011] According to an aspect of the present invention, there is provided
a method of monitoring state of health using battery parameters obtained while
in use to develop a battery model. Therefore, the battery is not taken off-
line
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for measurement or calculations. Instead, the state of health determination is
related to the particular application and pattem of use of the battery.
[012] Although an accurate indication of battery condition or State of
Health (SOH) cannot be directly measured, the internal impedance of a battery
reflects the intemal electrochemical condition of the battery. A direct
relation
does not exist between intemal impedance and SOH. However, in one aspect
of the
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present invention, the evolution of internal impedance over the lifetime of
the
battery is used as an indication of the SOH.
[013] Advantageously, an embedded controller measures state of health
and provides an estimation of battery lifetime. In a particular aspect of the
present
invention, a warning is provided for the user to replace the battery in
advance of
failure. The embedded controller provides greatly increased reliability of the
system as a more accurate indication of battery lifetime is provided.
Brief Description of the Drawings
[014] An embodiment of the present invention will now be described, by
way of example only, with reference to the attached Figure, wherein:
[015] Figure 1 is a schematic representation of a system for monitoring
battery state of health according to one embodiment of the present invention;
[016] Figure 2 is a table illustrating an exemplary series of measurements
and calculations for monitoring the state of health of a battery, according to
the
embodiment of Figure 1; and
[017] Figure 3 is a graph for determining a measured state of charge in the
exemplary series of Figure 2.
Detailed Description of the Invention
[018] Reference is first made to Figure 1 to describe a system and method
of determining state of a battery, the system indicated generally by the
numeral 10.
An open circuit voltage (Vac) of the battery 12 is measured to determine a
measured state of charge (SOCm). A calculated state of charge (SOCc) is
determined based on a state of charge prior to at least one use and a
calculated
discharge of the battery during the at least one use. The state of the battery
is
determined based at least in part on the difference between the calculated
state of
CA 02537526 2006-02-23
charge and the measured state of charge. Thus, a measured electrical
characteristic (SOC,,,) is compared to a calculated electrical characteristic
(SOC.)
to determine a state of health of the battery.
[019] The system 10 according to one embodiment includes a main board
14 with a controller and a current sensor for measuring current, as well as a
voltage divider from which open circuit voltage Voc of the battery is measured
in
order to determine the measured state of charge (SOCm). The controller
calculates the calculated state of charge (SOC,,) based on the state of charge
prior
to the at least one use and the calculated discharge of the battery during the
at
least one use. The controller also determines the state of health of the
battery
based at least in part on the difference between the calculated state of
charge and
the measured state of charge.
[020] Setup and connection of the system 10 is well within the grasp of
one skilled in the art. The main board 14 including the controller, the
current
sensor for measuring current and the voltage divider is connected to the lead
acid
battery 12 through the connectors 16. An LCD display 18 is connected directly
to
the main board 14 for displaying, for example, the last determined state of
health
of the battery 12 or a voltage measurement. A charger 20 is operably connected
through the connectors 16 and a charge control relay 22 to the main board 14
and
the battery 12, for charging the battery 12. An output inverter 24 is operably
connected through the connectors 16 and a discharge control relay 26 to the
main
board 14 and the battery 12, for using and thereby discharging the battery 12.
It
will be appreciated that the system 10 is portable along with the battery 12.
[021] The following portion of the disclosure is intended to provide
definitions for terms used herein in order to aid in understanding the
embodiment
disclosed herein.
[022] The State of Charge (SOC) is the percentage of electrochemical
potential remaining in a battery. Because batteries are not ideal, a
calculated state
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of charge, that is, the State of Charge approximated during use, is not always
equivalent to the ideal measured state of charge.
[023] The Measured State of Charge (SOCm) is the SOC obtained via
measurement of the open circuit voltage (Voc) - voltage without load - of the
battery.
[024] The Depth of Discharge (DOD) is the percentage of capacity
removed from battery. DOD is calculated during use of the battery via coulomb
counting.
[025] The Calculated State of Charge (SOC,,) is the SOC obtained by
using the DOD of the battery during use. Ideally, SOC = 1-DOD.
[026] The State of Health (SOH) is the percentage of capacity a battery is
able to provide, with respect to its rated value.
[027] Ideally, SOCm = 1-DOD and SOCm = SOCc. Unfortunately, however,
battery performance declines over the lifetime of the battery and therefore,
SOCm
# 1-DOD. Similarly, SOCm # SOCc. This discrepancy is accounted for by the SOH
which is adjusted according to the errors produced between the determined DOD
and the SOCm. In particular, a scaling factor 0 is used to relate SOCm and
SOCc.
To summarize, the SOH decreases over the lifetime of the battery and therefore
the calculated or expected State of Charge SOCc is not equal to the measured
State of Charge.
[028] To determine the SOH of the battery, SOCm is obtained after a
discharge period, while SOCc is cumulatively calculated during that period via
coulomb counting, and taking into account an expected state of charge prior to
the
discharge period. For example, SOCc of the battery can be determined after at
least one discharge of the battery and prior to charging. In order to
calculate
SOCc, SOCõ is first calculated as follows: SOC',, = SOCCn-, + IC~ , where
1SOC'n is
the ideal state of charge (calculated using coulomb counting as indicated). It
will
be appreciated that n refers to a cycle number. Thus, SOC' is calculated
taking
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into account SOCc for the previous cycle and by coulomb counting. C' is the
capacity and is related to the discharge rate of the battery during use. The
value
of C' is determined as follows: C' = Co I , where C. is the rated capacity
(amp*hours), I is the actual current discharged (amps), and I, is the rated
current
(amps). The value of y is based on battery discharge curves using battery run
time
data. The ideal state of charge SOC'n is then used to determine SOCc, taking
the
SOH into account as follows: SOCcn =~3 x SOCõ where R is a fraction that
represents the SOH of the battery.
[029] As shown, each new SOCc is calculated taking into account the
previously calculated SOCc and the value of P. In the first SOCc, R= 1 (given
that
the battery is new). The SOCm is measured by measurement of the Voc when the
battery is not discharging and after a rest period for the Vo(; to settle. An
error
value is produced based on the difference between SOCc and SOCm:
~= SOCc - SOC,n , where ~ denotes the error.
[030] The error is used to adjust R, thereby diminishing errors calculated in
subsequent cycles and therefore producing an accurate SOCc. A positive error
indicates that there is less available energy than calculated and P is
therefore
decreased. Conversely, a negative error indicates that there is more energy
than
indicated by the SOCc and R is therefore increased. The adjustments are
proportional to the errors, and implemented with a weighted moving average
kernel.
[031] The change in R is calculated by calculating a weighted average of
n-1
n
the errors calculated as follows: a/3n =, n I E~x + n~n , where ~. is a
scaling
factor. The value of (3 is then calculated based on the previously calculated
value
of (3 and the change in R, as follows: Qn+l
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[032] The resulting calculated 0 is a value between 0 and 1. The SOH is
100XR.
[033] It will be appreciated that the determined SOH provides an indication
of battery life taking into account the load conditions and pattern of use of
the
battery. Using this number for SOH, limitations can be set for replacement of
a
battery in a particular application. For example, for a particular
application, a
battery may be replaced at a SOH value of 70(%).
[034] The following exampie is intended to further illustrate an embodiment
of the present invention. Referring to Figure 2, there is shown a table
illustrating
an exemplary series of measurements in monitoring the state of health of a
battery. For the purpose of the present example the battery 12 is a 100Ah
battery.
[035] The 100Ah battery 12 is connected to the system for determining the
state of health of the battery shown in Figure 1. Because the battery is new,
the
SOH is assumed to be 100%. The open circuit voltage measured on the 100Ah
battery 12.5V. Using the graph for determining the measured state of charge
shown in Figure 3, the SOCm is 80% (see Cycle 0, Step 0 in Figure 2). The
graph
of Figure 3 shows the SOCm as a function of Voc. In general, a lead acid
battery is
considered to be at 100% State of Charge at approximately 13V and at 0% State
of Charge at approximately 10.5V as shown in Figure 3.
[036] During a first cycle of use of the battery, it is determined that 5Ah is
removed from the battery (discharged) using coulomb counting. It will be
appreciated that with a 100Ah battery, a discharge of 5Ah from the battery
should
result in a 5% decrease in the state of charge 100Ah * 100% ). Thus, the ideal
State of Charge SOC' ) is 80%-5% = 75%. The calculated State of Charge SOCc
is determined by multiplying SOC' by R. In this cycle, R is 1.00 and thus,
SOC' _
SOCc.
[037] The open circuit voltage (Voc) is then measured to determine the
SOCrr, and the error is obtained from the difference between SOCc and SOCm. As
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indicated above, the Voc is measured when the battery is not discharging (or
charging) and after a rest period for the Voc to settle. In Cycle 1 of the
present
example, the Voc is measured at 12.33V and the SOCm is determined to be 73.2%.
Thus, the error is determined to be 1.8 using the formula ~= SOC, - SOC,,, .
The
error registers, El, E2 and E3 are updated by shifting the error values so
that the
current error is entered into El, and the previous error from El is moved to
E2
while the previous error from E2 is moved to E3. The new (3 value is then
calculated based on the error values and the previous value of P, using the
n-1
formulas: O~n =A nn 1~~x + n and ~3n+, =/3n - o/3n , as indicated above.
x
Thus (3 is determined to be 0.98. The SOH is 100% X(3 and is calculated to be
98.2%.
[038] During a second cycle of use, it is determined that 5Ah is again
removed from the battery. As shown above, 5Ah removed from the battery should
result in a 5% decrease in the state of charge. Thus, SOC' is 75%-5% = 70%.
SOCc is then determined by multiplying SOC' by P. In this cycle, (3 is 0.98
and
thus, SOCc = 68.7. Voc is then measured to determine SOCm and the error is
obtained from the difference between SOCc and SOCm. In Cycle 2 of the present
example, Voc is measured at 12.20V and the SOCm is determined to be 68%.
Thus, the error is determined to be 0.7. The error registers, El, E2 and E3
are
updated by shifting the error values so that the current error is entered into
El.
The new R value is then calculated based on the error values and the previous
value of P.
[039] During a third cycie of use, it is determined that 5Ah is again
removed from the battery. As shown above, 5Ah removed from the battery should
result in a 5% decrease in the state of charge. Thus, SOC' is 68.7% - 5% =
63.7%. SOCc is determined by multiplying SOC' by P. In this cycle, R is 0.96
and
thus, SOCc = 61Ø Voc is then measured to determine SOCm and the error is
obtained from the difference between SOCc and SOCm. In Cycle 3 of the present
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example, Voc is measured at 12.02V and SOCm is determined to be 60.8%. Thus,
the error is determined to be 0.2. The error registers, El, E2 and E3 are
updated
by shifting the error values so that the current error is entered into El. The
new R
value is then calculated based on the error values and the previous value of
P.
[040] As shown in Figure 2, the determination of SOH continues for a total
of 6 cycles of the battery 12 with the SOH reaching 92.73 after Cycle 6. It
will be
appreciated that the values shown in Figure 2 and referred to above are for
exemplary purposes only. It will also be appreciated that after the SOH is
accurately determined, a decline in the SOH is likely to be observed only over
a
large number of cycles.
[041] As discussed, the determination of SOH is carried out over a number
of cycles of operation of the battery. The battery is not removed from use for
off-
line testing. Instead, the SOH is determined using measurements and
calculations
relating to the particular application and pattern of use of the battery.
[042] When the SOH declines to a certain level, the system 10 gives off a
warning to alert the user to the low state of health and to advise that the
battery
should be replaced. The warning can be for example an alert on the LCD display
18. For example, a warning can be displayed on the LCD display when the SOH
reaches 70%. Other warnings can be given, however. Further, such warnings can
be transmitted for remote monitoring. Such transmission can be carried out in
any
suitable manner for monitoring, for example, at a remote computing device. The
hardware and software for transmission and remote monitoring is well within
the
grasp of one skilled in the art.
[043] It will be appreciated that, although embodiments of the invention
have been described and illustrated in detail, various modifications and
changes
may be made. For example, rather than being limited to a lead acid battery, it
is
conceivable that the present invention can be extended to other battery
chemistries such as a lithium ion battery. Still further alternatives and
modifications may occur to those skilled in the art. All such alternatives and
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modifications are believed to be within the scope of the invention as defined
by the
claims appended hereto.
