Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: SELF-DIAGNOSIS SYSTEM FOR AN ENERGY
STORAGE DEVICE
FIELD OF THE INVENTION
The present invention generally relates to the field of energy storage
devices, such as
rechargeable batteries. More specifically, the invention relates to a self
diagnosis
system for a rechargeable battery capable to estimate the short term and long
term
capacity of the battery as well as its life expectancy.
BACKGROUND OF THE INVENTION
Rechargeable batteries are used in a wide variety of applications, for example
as a
propulsion power source for electric and hybrid vehicles or as a power reserve
in
telecommunications network stations. In any application, it is important to
monitor the
discharge capacity of the battery at any given time, as well as to monitor the
state of
health of the battery as it ages through degradation of its chemistry over
repeated
floating/charging/discharging cycles. This information helps to manage the
timely
replacement of a battery approaching the end of its useful life.
A battery can be approximated as a voltage source with an internal resistance.
The
internal resistance of a battery varies with battery age, but remains
relatively constant
over a short time period. As the battery ages during floating, charging and
discharging,
its internal resistance increases. The increase of the internal resistance is
caused by the
degradation of the battery's chemistry, which in turn reduces the battery's
ability to
hold a charge. The performance of a battery is characterized by its discharge
curve,
which is a curve of the battery voltage as a function of time at a
predetermined
discharge rate or as a function of the percentage of the remaining charge of
the battery.
As the internal resistance of the battery increases, the discharge curve
drops, indicating
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a reduction of the battery capacity. The battery discharge curve may vary with
the
internal resistance of the battery, its discharge rate and its temperature.
Various systems, methods and apparatus have been devised to estimate the
battery
capacity and the battery remaining life based on the internal resistance of
the battery.
US Patent No. 5,404,106 discloses an electronic control unit, switching means
for
connecting and disconnecting the positive and negative terminals of the
battery from an
electrical load, first and second voltage measurement means for measuring
battery
voltage in the connected and disconnected states, discharge current
measurement means
and .battery electrolyte temperature sensing means. The voltage and current
measurements are relayed to the control unit, which calculates the internal
resistance of
the battery. The remaining capacity level of the battery is then estimated
from stored
values corresponding to the calculated internal resistance, and a correction
coefficient
associated with the electrolyte temperature reading is applied to the
remaining capacity
level of the battery to more accurately estimate the remaining battery
capacity.
US Patent No. 6,087,808 discloses a system and circuit means that measure the
internal
resistance of the battery and an output current at varying battery loads.
These
measurements are relayed to a computer having stored in its memory an array of
discharge curves specific to the battery type. Software running on the
computer
employs the internal resistance and output current readings to select from its
memory a
discharge curve most closely associated with the state of the battery as
characterized by
the internal resistance and output current values. The selected discharge
curve is then
used to estimate the relative remaining life of the battery.
US Patent No. 6,167,309 discloses a system or process for estimating the level
of
power depletion in a cardiac pacing system having a lithium battery, by
measuring and
correlating the rate of charge of a charge storage capacitor connected to the
lithium
battery to a value of the internal resistance of the battery, which provides
an estimation
of the remaining battery capacity and the replacement time for the battery.
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Each of the systems or methods described above provide an estimation of the
remaining
battery capacity based on a correlation of voltage measurements taken at the
battery
terminals to a discharge curve. Unfortunately, these voltage measurements
prove
inaccurate when a battery discharge curve comprises a mild slope and are
inadequate
when a battery discharge curve comprises a plateau wherein the voltage of the
battery is
not sufficient to establish the remaining battery capacity. The systems or
methods
described above also measure an output current to determine the internal
resistance of
the battery. However, the measurement of an output current to determine the
internal
resistance renders the value of the internal resistance somewhat approximate.
Also, the
internal resistance is calculated based on measurements taken at the battery
terminals,
and provides little information on the state of the individual cells of the
battery.
Furthermore, none of the systems or methods described above provide an
accurate
evaluation of the state of health of a battery, in order to predict how long
the battery
may be able to perform before reaching a level of performance (battery
capacity) at
which replacement is required.
Against this background, it clearly appears that a need exists in the industry
for an
improved system for accurately predicting the remaining capacity of an energy
storage
device, as well as for accurately evaluating the state of health of the energy
storage
device.
SUMMARY OF THE INVENTION
According to a broad aspect, the invention provides a self diagnosis system
for an
energy storage device, the energy storage device including a plurality of
electrochemical cells forming a cell string. The self diagnosis system
includes a
current source providing a constant current, a cell selector switch operative
to select a
particular one of the electrochemical cells within the cell string, and
voltage
measurement means for measuring an initial voltage and a second voltage of the
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particular cell. The self diagnosis system also includes a processing unit
coupled to the
voltage measurement means, the processing unit being operative to calculate an
internal
resistance of the selected cell on a basis of the constant current value and
the initial and
second voltages. The electronic self diagnosis system is capable to determine
a state of
health of the energy storage device, on the basis of the internal resistance
of each cell of
the cell string.
According to another broad aspect, the invention provides an energy storage
device
comprising a housing, a plurality of electrochemical cells each having a
positive, a
negative electrode and an electrolyte separator therebetween. The plurality of
electrochemical cells are connected in series or parallel to form a cell
string within the
housing. The energy storage device also includes an electronic self diagnosis
system,
which correlates a measured internal resistance of the cell string to a state
of health
value representative of a capacity fade of the cell string. The electronic
self diagnosis
system selects an initial capacity of the energy storage device corresponding
to the state
of health value, and monitors a state of charge of the energy storage device
by
measuring energy flowing in or out of the energy storage device and adding or
subtracting the energy to determine an exact battery capacity from the
selected initial
capacity.
In a preferred embodiment of the invention, the energy flowing in is
calculated as a
current drawn from the energy storage device over time during a discharge and
the
energy flowing out is calculated as a current received by the energy storage
device over
time during a charge. The measured energy is representative of the battery
capacity
flow.
According to a further broad aspect, the invention also provides an energy
storage
device comprising a housing, a plurality of electrochemical cells each having
a
positive, a negative electrode and an electrolyte separator therebetween. The
plurality
of electrochemical cells are connected in series or parallel to form a cell
string within
the housing. The energy storage device also includes an electronic self
diagnosis
system comprising a processing unit, a current source, a voltage measurement
means
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and a cell selector switch adapted to select a single electrochemical cell
within the cell
string. The electronic self diagnosis system measures an internal resistance
of each cell
of the cell string, one cell at a time, to determine a state of health of the
energy storage
device, wherein the cell selector switch selects a first cell of the cell
string, the voltage
measurement means measures an initial voltage and a second voltage of the
selected
cell and the current source provides a constant current. The constant current
value, the
initial and second voltage measurement are processed by the processing unit
wherein
the processing unit calculates the internal resistance of the selected cell
using Ohm's
law, the processing unit further comprising a memory for storing
electrochemical cell's
capacity fade as a function of its internal resistance. The processing unit
correlates the
highest calculated internal resistance of the cell string to a corresponding
state of health
value stored in the memory to define an overall state of health of the energy
storage
device.
According to yet another broad aspect, the invention provides a method for
determining
the state of health of an energy storage device.
BRIEF DESCRIPTION OF THE DRAWINGS
A detailed description of examples of implementation of the present invention
is
provided hereinbelow with reference to the following drawings, in which:
Figure 1 is a partial perspective view illustrating an energy storage module
according to
an example of implementation of the invention;
Figure 2 is an exploded perspective view illustrating an energy storage module
according to an example of implementation of the invention;
Figure 3 is a schematic representation of a voltage source with an internal
resistance;
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Figure 4 is a typical discharge curve of a Lithium-metal-polymer battery;
Figure 5 is a graphical representation of the evolution of a battery capacity
as a function
of its internal resistance;
Figure 6 is a graph of voltage variations relative to current variation
according to a first
method of calculation of the internal resistance of individual cells;
Figure 7 is a graph of voltage variations relative to current variations
according to a
second method of calculation of the internal resistance of individual cells;
Figure 8 is a graph of voltage variations relative to current variations
according to a
third method of calculation of the internal resistance of individual cells;
Figure 9 is a graph of voltage variations relative to current variations
according to a
fourth method of calculation of the internal resistance of individual cells;
and
Figure 10 is a schematic diagram of a diagnosis system according to an example
of
implementation of the invention.
In the drawings, embodiments of the invention are illustrated by way of
example. It is
to be expressly understood that the description and drawings are only for
purposes of
illustration and as an aid to understanding, and are not intended to be a
definition of the
limits of the invention.
DETAILED DESCRIPTION
Referring now to the drawings, and more particularly to FIG. 1, there is
provided a
partial illustration of an example of implementation of an energy storage
module or
battery 10 which utilizes a number of rechargeable solid-state thin-film
electrochemical
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cells 12 for storing electrical energy. Such rechargeable thin-film
electrochemical cells
are particularly well-suited for use in the construction of high-current, high-
voltage
energy storage modules and batteries, such as those used to power electric
vehicles or
as back-up electricity supply for telecommunication networks for example.
As shown in FIG. 1, the energy storage module 10 includes a number of
individual
electrochemical cells 12 which are arranged in a stack configuration 14 and
situated in
a housing 16. It will be appreciated that a generic stack 14 of
electrochemical cells 12
may be interconnected in various parallel and series relationships to achieve
desired
current and voltage ratings. To facilitate selective series or parallel
connectivity within
the stack 14 of electrochemical cells 10, an interconnect board 20 is situated
within the
housing 16.
The interconnect board 20 includes a connection pattern which, when the board
20 is
installed within the housing 16, interconnects the electrochemical cells 12 in
accordance with a pre-established connection configuration. The board 20 may
be
connected to a bus bar itself connected to each individual cell 12 or directly
connected
to the electrochemical cells 12 through a connection pattern typically affixed
or
otherwise bonded to a sheet of insulating material 22, such as a substantially
rigid
plastic or laminate material. A number of electrical and electro-mechanical
components may also be mounted on the interconnect board 20.
As shown in the example of FIG. 1, the interconnect board 20 includes a number
of
fuse packs 24, equalizer and bypass devices 26, and positive and negative
power
terminals 28 and 29. It is understood that any or all of the components
populating the
interconnect board 20 may be mounted on boards or platforms other than the
interconnect board 20, and situated internal to or externally of the module
housing 16.
In one embodiment, the interconnect board 20 shown in FIG. 1 and the
electrochemical
cells 12 are disposed in a hermetically sealed housing 16.
In FIG. 2, there is illustrated an exploded view of one example of a complete
energy
storage module 10 that includes an inner shell 15 which contains a stack 14 of
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electrochemical cells 12 and various electronic boards, including an
interconnect board
20 as previously discussed. An inner shell cover 32 incorporates a hermetic
seal 34,
that seals various feed-through provided in the inner shell cover 32.
In accordance with the particular example of implementation shown in Figure 2,
the
module 10 includes a stack 14 of electrochemical cells 12 which are
interconnected
through use of the interconnect board 20. . The stack 14 of electrochemical
cells 12 are
subjected to a continuous compressive force generated by use of the bands 36,
end
plates 38 and a foam or spring-type element disposed within or adjacent each
of the
cells 12.
The interconnect board 20 is situated above the cell stack 14 and includes
control
circuitry for all electrochemical cells 12 constituting the cell stack 14. The
control
circuitry includes a short circuit protection device such as a fuse pack , a
bypass device,
and an equalizer circuit, which control the operation of the cell pack 14
while charging
and discharging, as well as an electronic self diagnosis system. Accordingly,
each of
the cells 12 is monitored and controlled by a control circuit. A control board
40,
situated above the interconnect board 20, includes a processor that monitors
and
controls each cell 12. As such, the control board 40 provides for module level
monitoring and control during charging and discharging and floating.
A pair of quick connectors 42 pass through corresponding holes provided in an
inner
shell cover 32 and serve as the main power terminals of the module 10. The
quick
connectors 42 are hermetically sealed to the inner shell cover 32 using a
sealing
apparatus 34. When an outer shell cover 44 is positioned onto the inner shell
cover 32,
the quick connectors 42 are received into mating sockets 28 and 29 mounted on
the
interconnect board 20. Communication connectors 46, which pass through the
inner
shell cover 32 and are similarly hermetically sealed thereto, provide external
access to
the control board 40 and other electronic boards of the module 10.
A hermetic seal is provided between the inner shell 15 and inner shell cover
32 by
welding the inner shell cover 32 to the top of the inner shell 15. The
hermetically sealed
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inner shell 15 is then inserted into an outer shell 16. The outer shell 16 may
be
fabricated from glass filled polypropylene through use of an injection molding
process,
and may be characterized by a thickness of approximately 2 mm.
A battery or a electrochemical cell can be approximated as a voltage source
with an
internal resistance as illustrated in Figure 3. The voltage source is
characterized by its
discharge curve. This curve mainly depends on the electrolyte type used in the
battery.
Figure 4 illustrates a typical discharge curve for a Lithium-metal-polymer
battery. The
discharge curve of a particular battery is affected by the temperature of the
battery and
the load current. An increase in load current will produce a temporary drop of
the
discharge curve, thereby reducing the battery capacity to hold its charge,
whereas an
increase in temperature will raise the discharge curve. The discharge curve is
also
affected by the internal resistance of the battery or of the electrochemical
cells making
up the battery. As the battery ages during floating, charging and discharging,
its
internal resistance increases causing the discharge curve to drop which
effectively
reduces the battery capacity. The State of Health of a battery is defined as a
fraction or
percentage of the initial battery capacity when the battery was new for a
complete
discharge under given conditions (rate of discharge).
A battery end of life is arbitrarily set at a point when the battery reaches a
battery
capacity expressed in Ampere-Hour (Ah) of about 80% of its initial (100%)
capacity,
under a specific rate of discharge at a specific temperature. Once the battery
no longer
meets the set requirement of battery capacity, the battery should be replaced
and/or
disposed of. Depending on applications, the battery end of life threshold may
be set at
a higher or a lower percentage of its initial capacity.
Experiments have shown that the overall internal resistance of a battery is
defined by
the internal resistance of each electrochemical cell making up the battery,
and may be
correlated directly to the capacity fade of the battery or battery state of
health. As
shown in Figure 5, the capacity of the battery decreases with the increase of
overall
internal resistance of the battery in a predictable manner. The relation
between the two
parameters is almost linear and may be expressed as an equation of the type:
Y=mX +b.
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In this example of implementation of the present invention, this data has been
tabulated
such that the measured internal resistance of the battery may be correlated to
a
percentage value included in this table. This correlation enables the self
diagnosis
system of the battery to determine the battery state of health and to
extrapolate the
battery end of life, and this is accomplished no matter what the state of
charge of the
battery is. By measuring, for each individual electrochemical cell, an initial
voltage
and a second voltage for a given current, at a predetermined cell temperature,
the self
diagnosis system of the battery calculates the overall internal resistance of
the battery to
determine exactly the state of health of the battery (updated fizll charge
battery
capacity).
In one example, the self diagnosis system sets a specific precise current,
measures a
voltage difference according to Ohm's law R = ~ (the relation between the
voltage
and the current) and calculates the exact internal resistance of each cell
constituting the
battery. Since the initial current value is zero, the equation is simplified
to R = ~~
The use of a precise set current value eliminates the inaccuracy of current
measurements and enables an accurate calculation of the internal resistance.
The
theoretical fixed current value is however measured to validate the real
current value
and allow for fine tuning of the fixed current for repeatability purposes. The
self
diagnosis system measures the internal resistance of each cell one at a time
and stores
the results in memory. The state of health of the battery is defined as the
weakest link
in the cell string; therefore the highest internal resistance is retained to
determine the
exact percentage of battery capacity remaining in the battery or its state of
health.
From the value of the state of health of the battery correlated to the
internal resistance
of the weakest cell, the self diagnosis system retrieves an initial capacity
based on a
corresponding discharge curve of the battery, which takes into account the
internal
resistance of the battery. With the selected initial capacity of the battery
according to
its actual state of health, the self diagnosis system is able to provide an
accurate
indication of the state of charge of the battery at all time by calculating
the current
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drawn from the battery over time as the energy or capacity is drawn from the
battery,
which represents the area under the discharge curve. Similarly, when the
battery is
being charged, the self diagnosis system calculates the energy or capacity
returned into
the battery by monitoring the charge current over time. The state of charge
indicates
the level of charge of the battery, which enables to precisely evaluate the
remaining
back-up time the battery can provide under a measured rate of discharge until
the
battery is fully discharged.
There are several ways to evaluate the internal resistance. The self diagnosis
system
may use any one of the following methods with similar accuracy, among other
possible
evaluation methods. Each method measures voltages and calculates the internal
resistance of each individual electrochemical cell making up the battery, one
cell at a
time.
The first method of calculation is to measure internal resistance of each
electrochemical
cell during a small charge at constant current as represented by Figure 6. As
shown in
Figure 6, a constant current is applied to the electrochemical cell and the
voltage
increases. The diagnosis system measures an initial voltage Vi prior to the
application
of the current and measures a second voltage Vf after the application of the
current. The
instantaneous increase in voltage divided by the current indicates the
internal resistance
of the individual cell. This measurement is stored in memory and repeated for
each cell
of the battery. The overall internal resistance of the battery is calculated
by a processor
and correlated to a state of health value, itself correlated to a battery
initial capacity
representative of the battery state of health.
The second method of calculation is to measure internal resistance on each
cell after a
small charge starting at a initial voltage. As shown in Figure 7, after a
charge at
constant current, the voltage relaxation creates a drop of the cell voltage.
This drop is
measured at a specific time and divided by the current to calculate the
internal
resistance of the cell. This measurement is stored in memory and repeated for
each cell
of the battery. The overall internal resistance of the battery is calculated
by a processor
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and correlated to a state of health value, itself correlated to a battery
initial capacity
representative of the battery state of health.
The third method of calculation is to measure internal resistance of each cell
during a
small discharge at constant current starting at an initial voltage. As shown
in Figure 8,
the instantaneous decrease in voltage divided by the current indicates the
internal
resistance of the cell. This measurement is stored in memory and repeated for
each cell
of the battery. The overall internal resistance of the battery is calculated
by a processor
and correlated to a state of health value, itself correlated to a battery
initial capacity
representative of the battery state of health.
The fourth method of calculation is to measure internal resistance on each
cell after a
small discharge at constant current starting at an initial voltage. As shown
in Figure 9,
after a discharge at constant current, the voltage relaxation creates a bump
of the
voltage. This bump is measured at a specific time and divided by the current
to evaluate
the internal resistance of the cell. This measurement is stored in memory and
repeated
for each cell of the battery. The overall internal resistance of the battery
is calculated
by a processor and correlated to a state of health value, itself correlated to
a battery
initial capacity representative of the battery state of health.
Although small, these repetitive discharges are routed into the application
network
connected to the battery or batteries to avoid wasting any energy while
performing
diagnosis routine. This feature of the testing procedure enables the system to
monitor
the state of health of the battery at any time with minimal energy wastage.
Any one of these methods of calculation may be implemented through the self
diagnosis system to obtain an accurate value of the internal resistance of the
battery,
which can be correlated to the actual battery capacity or state of health.
Figure 10 is a diagram of a self diagnosis system 100 of an energy storage
device,
according to an example of implementation of the present invention. The self
diagnosis system 100 comprises a main processing unit 102, a cell selector
sequencer
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104, a current source commands processor 106, a bi-directional current source
108, a
voltage cell signal processor 118, current signal processor 119, and a cell
selector
switch 110 coupled to each cell of the electrochemical cells string 112 of the
battery.
The cells string module 112 comprises at least one temperature sensor
connected to a
temperature processing unit 114. Self diagnosis system 100 further comprises
an A/D
signal conversion unit 116 adapted to transform analog signals received from
various
sources into readable digital signals for processing unit 102.
Processing unit 102 comprises a memory 120 for storing its own executing
program,
the various curves and capacity data in table form and data received from
individual
cells, and a external communication port 122 for sending or receiving data to
an
external station through Internet or telephone links. Processing unit 102
commands the
diagnosis routine for evaluating the state of health of the battery which may
be
scheduled at regular intervals or when a specific situation occurs such as
full discharge,
or may be ordered by a remote user.
When the routine begins, processing unit 102 first selects the test to be
performed on
each individual cell from one of the four tests outlined above and sends a
corresponding signal to cell selector microsequencer 104. Microsequencer 104
sets the
current direction according to the test sequence (charge or discharge) and
feeds the
corresponding signals to cell selector switch 110. According to the test being
performed, the processing unit 102 sends a signal to current source commands
processor 106 to either feed or retrieve a specific constant current to or
from the ceh
selected by cell selector switch 110 through bi-directional current source
108, which
relays the current to or from the selected cell. Whatever the test being
performed, the
current is fixed and a first and second voltage are measured. The fixed
current value
and the voltage measurements taken at the selected cell are relayed to A/D
signal
conversion unit 116 through voltage cell signal processor 118 and current
signal
processor 119. The average temperature of the cells is measured at regular
intervals by
at least one temperature sensor connected to a temperature processing unit
114, which
relays the temperature signals to A/D signal conversion unit 116. The voltage,
current
and temperature signals are converted from analog to digital signals and sent
to
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processing unit 102, which in turn calculates precisely the internal
resistance of the
selected cell and stores the internal resistance value into memory. The test
is repeated
for each individual cell of the battery following the routine established by
processing
unit 102 and cell selector switch 110.
Once the internal resistance values for all of the cells have been calculated
and stored
into memory 120, the processing unit 102 correlates the highest measured
internal
resistance value of the electrochemical cells of the cell string with the data
from the
graph shown in Figure 5. This data, whether in table form or as an equation of
the type
Y=mX+b, represents the internal resistance vs. capacity of the battery, and is
also
stored in memory 120. From that correlation, processing unit 102 extrapolates
an
overall state of health of the battery or energy storage device. With the
outlined cell by
cell evaluation of the internal resistance, diagnosis system 100 is able to
provide
specific data on the overall state of health of the battery and raise alarm
flags if one
particular cell shows an abnormally high internal resistance and is found
defective.
Based on the graph of Figure S, processing unit 102 is able to provide an
accurate
evaluation of the battery remaining life in numbers of cycles; this value can
be
extrapolated in months or years based on the history of battery use i.e.
number of cycles
over time.
With an established battery state of health, processing unit 102 retrieves
from memory
120 the battery initial capacity corresponding to the updated battery state of
health.
With this specific initial capacity, the diagnosis system 100 is able to keep
tabs on the
exact level of charge of the battery and evaluates the remaining back-up time
the
battery can provide under various conditions. When the battery is solicited
and is under
discharge, the discharge current is monitored over time and the energy or
capacity
extracted from the battery, calculated as I8t (current x time), which
represents the area
under the discharge curve, is subtracted such that the diagnosis system 100
knows at all
time the remaining capacity of the battery and can evaluate the remaining time
the
battery can provide a given current.
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Note that, in a variant, the final steps of determining the exact level of
charge of the
battery and evaluating the remaining back-up time based on the state of health
of the
individual cells may be carried out by an external system (not shown) or by a
remote
user linked to self diagnosis system 100, which retrieves the internal
resistance values
of each cell through external communication port 122. The external system or
remote
user similarly retrieves a battery initial capacity corresponding to the
updated battery
state of health and determines the exact level of charge of the battery and
evaluates the
remaining back-up time the battery can provide under given conditions.
If the battery is under load, the self diagnosis system 100, the external
diagnosis system
or the remote user will calculate the amount of remaining back-up time under a
measured instantaneous rate of discharge until the battery is discharged. This
information is stored into memory for reference and for trend analysis.
Although the present invention has been described in relation to particular
variations
thereof, other variation and modifications are contemplated and are within the
scope of
the present invention. Therefore the present invention is not to be limited by
the above
description but is defined by the appended claims.
