U.S. patent application number 16/283361 was filed with the patent office on 2020-08-27 for system and method to improve range and fuel economy of electrified vehicles using life balancing.
The applicant listed for this patent is FORD GLOBAL TECHNOLOGIES, LLC. Invention is credited to Richard Dyche ANDERSON, Xiaohong Nina DUAN.
Application Number | 20200269704 16/283361 |
Document ID | / |
Family ID | 1000003955778 |
Filed Date | 2020-08-27 |
United States Patent
Application |
20200269704 |
Kind Code |
A1 |
DUAN; Xiaohong Nina ; et
al. |
August 27, 2020 |
SYSTEM AND METHOD TO IMPROVE RANGE AND FUEL ECONOMY OF ELECTRIFIED
VEHICLES USING LIFE BALANCING
Abstract
A vehicle includes a traction battery comprising a plurality of
cells. The vehicle also includes a plurality of power converters
each electrically coupled between a corresponding group of cells
and an electrical bus. A controller is programmed to allocate
current demand to the power converters and operate the power
converters to minimize energy consumption and losses.
Inventors: |
DUAN; Xiaohong Nina;
(Canton, MI) ; ANDERSON; Richard Dyche; (Plymouth,
MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FORD GLOBAL TECHNOLOGIES, LLC |
Dearborn |
MI |
US |
|
|
Family ID: |
1000003955778 |
Appl. No.: |
16/283361 |
Filed: |
February 22, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B60L 53/20 20190201;
B60L 2210/14 20130101; B60L 2210/44 20130101; B60L 50/50 20190201;
B60L 2240/529 20130101; B60L 50/00 20190201; B60L 58/15 20190201;
B60L 15/2045 20130101; B60L 53/64 20190201 |
International
Class: |
B60L 15/20 20060101
B60L015/20; B60L 58/15 20060101 B60L058/15; B60L 53/20 20060101
B60L053/20; B60L 53/64 20060101 B60L053/64 |
Claims
1. A vehicle comprising; a traction battery comprised of cells;
power converters, each electrically coupled between a corresponding
group of cells and an electrical bus; and a controller programmed
to allocate current demand to the power converters, responsive to
currents allocated to a subset of the power converters having
magnitudes less than a threshold, redistribute a sum of the
currents to power converters not in the subset and operate the
subset to provide no current, and increase current magnitude of
each of the power converters not in the subset by a factor that is
a ratio of the current demand to a total current already allocated
to each of the power converters not in the subset.
2. The vehicle of claim 1 wherein the controller is further
programmed to initially allocate current demand to the power
converters based on states of charge of the groups of cells such
that a first power converter coupled to a first group of cells
having a state of charge greater than a state of charge of a second
group of cells coupled to a second power converter receives a
greater current allocation.
3. The vehicle of claim 1, wherein current allocated to one or more
of the power converters is a negative current that flows to the
cells.
4. The vehicle of claim 1 wherein the threshold is a current level
corresponding to an efficiency being less than a predetermined
converter efficiency.
5. (canceled)
6. The vehicle of claim 1 wherein the currents include positive
currents flowing from the cells and negative currents flowing to
the cells, and the controller is further programmed to redistribute
a sum of the positive currents to power converters with positive
current allocation and redistribute a sum of the negative currents
to power converters with negative current allocation.
7. The vehicle of claim 1 wherein the controller is further
programmed to, responsive to currents allocated to all of the power
converters having magnitudes less than the threshold, redistribute
the current demand to a group of the power converters such that
current is supplied by a set of power converters and that each of
the power converters provides current that exceeds the
threshold.
8. The vehicle of claim 1 wherein the controller is further
programmed to redistribute the current such that a total current
provided by the power converters is same before and after
redistribution of the currents.
9. A method comprising: operating, by a controller, power
converters electrically coupled between corresponding groups of
battery cells and an electrical bus to satisfy a current demand of
the electrical bus by allocating a portion of the current demand to
each of the power converters, responsive to a subset of the power
converters being allocated currents having magnitudes less than a
threshold, redistributing a sum of the currents to power converters
not in the subset and operating the subset to provide no current,
wherein the currents include positive currents and negative
currents; redistributing a sum of the positive currents to power
converters with positive current allocation; and redistributing a
sum of the negative currents to power converters with negative
current allocation.
10. The method of claim 9 further comprising allocating the current
demand based on a state of charge of the corresponding groups of
battery cells.
11. The method of claim 9 further comprising redistributing current
by increasing current to each of the power converters not in the
subset by a factor that is a ratio of the current demand to a total
current already allocated to each of the power converters not in
the subset.
12. The method of claim 9 further comprising, responsive to all of
the power converters being allocated currents having magnitudes
less than the threshold, redistributing the current demand to a
group of power converters such that each provides current that
exceeds the threshold.
13. The method of claim 9 further comprising redistributing the sum
of the currents such that a total current provided by the power
converters is same before and after redistribution of the
currents.
14. (canceled)
15. The method of claim 9 wherein the threshold is a current level
corresponding to an efficiency being less than a predetermined
converter efficiency.
16. A vehicle electrical system comprising: a plurality of power
converters each electrically coupled between a group of battery
cells and an electrical bus; and a controller programmed to
allocate current demand to the power converters, responsive to
currents allocated to a subset of the power converters having
magnitudes less than a threshold, redistribute a sum of the
currents to power converters not in the subset such that a total
current provided by the power converters is same before and after
redistribution of the currents and operate the subset to provide no
current, and responsive to currents allocated to all of the power
converters having magnitudes less than the threshold, redistribute
the current demand to a group of the power converters such that
current is supplied by a set of power converters that each provide
current that exceeds the threshold.
17. The vehicle electrical system of claim 16 wherein the
controller is further programmed to increase current magnitudes to
each of the power converters not in the subset by a factor that is
a ratio of the current demand to a total current already allocated
to each of the power converters not in the subset.
18. The vehicle electrical system of claim 16 wherein the currents
include positive currents and negative currents, and the controller
is further programmed to redistribute a sum of the positive
currents to power converters with positive current allocation and
redistribute a sum of the negative currents to power converters
with negative current allocation.
19. (canceled)
20. The vehicle electrical system of claim 16 wherein the threshold
is a current level corresponding to an efficiency being less than a
predetermined converter efficiency.
Description
TECHNICAL FIELD
[0001] This application generally relates to a system for powering
a low-voltage bus by one or more power converters arranged in
parallel.
BACKGROUND
[0002] Electrified vehicles include hybrid-electric and electric
vehicles and are configured to provide propulsion with a powertrain
including an electric motor. Electrified vehicles distribute energy
to various components that are connected to a high-voltage
electrical bus and a low-voltage electrical bus. Sources of energy
include a battery that is connected to the high-voltage bus and a
generator that is also connected to the high-voltage bus. The
vehicles generally include a single power converter for
transferring energy from the high-voltage electrical bus to the
low-voltage electrical bus. Fuel economy of electrified vehicles
depends, at least in part, on an amount of energy drawn from the
power grid during charging.
SUMMARY
[0003] A vehicle includes a traction battery comprised of cells and
power converters, each electrically coupled between a corresponding
group of cells and an electrical bus. The vehicle further includes
a controller programmed to allocate current demand to the power
converters and, responsive to currents allocated to a subset of the
power converters having magnitudes less than a threshold,
redistribute a sum of the currents to power converters not in the
subset and operate the subset to provide no current.
[0004] The controller may be further programmed to initially
allocate current demand to the power converters based on states of
charge of the groups of cells such that a first power converter
coupled to a first group of cells having a state of charge greater
than a state of charge of a second group of cells coupled to a
second power converter receives a greater current allocation. The
current allocated to one or more of the power converters may be a
negative current that flows to the cells. The threshold may be a
current level corresponding to an efficiency being less than a
predetermined converter efficiency. The controller may be further
programmed to increase current magnitude of each of the power
converters not in the subset by a factor that is a ratio of the
current demand to a total current already allocated to each of the
power converters not in the subset. The currents may include
positive currents flowing from the cells and negative currents
flowing to the cells, and the controller may be further programmed
to redistribute a sum of the positive currents to power converters
with positive current allocation and redistribute a sum of the
negative current to power converter with negative current
allocation. The controller may be further programmed to, responsive
to currents allocated to all of the power converters having
magnitudes less than the threshold, redistribute the current demand
to a group of the power converters such that current is supplied by
a set of power converters such that each provides current that
exceeds the threshold. The controller may be further programmed to
redistribute the current such that a total current provided by the
power converters is the same before and after redistribution of the
currents.
[0005] A method includes operating, by a controller, power
converters electrically coupled between corresponding groups of
battery cells and an electrical bus to satisfy a current demand of
the electrical bus by allocating a portion of the current demand to
each of the power converters and, responsive to a subset of the
power converters being allocated currents having magnitudes less
than a threshold, redistribute a sum of the currents to power
converters not in the subset and operate the subset to provide no
current.
[0006] The method may further include allocating the current demand
based on a state of charge of the corresponding groups of battery
cells. The method may further include redistributing current by
increasing current to each of the power converters not in the
subset by a factor that is a ratio of the current demand to a total
current already allocated to each of the power converters not in
the subset. The method may further include, responsive to all of
the power converters being allocated currents having magnitudes
less than the threshold, redistribute the current demand to a group
of power converters such that each provides current that exceeds
the threshold. The method may further include redistributing the
sum of the currents such that a total current provided by the power
converters is the same before and after redistribution of the
currents. The currents may include positive currents and negative
currents and the method may further include redistributing a sum of
the positive currents to power converters with positive current
allocation and redistributing a sum of the negative current to
power converter with negative current allocation. The threshold may
be a current level corresponding to an efficiency being less than a
predetermined converter efficiency.
[0007] A vehicle electrical system includes a plurality of power
converters each electrically coupled between a group of battery
cells and an electrical bus. The vehicle electrical system further
includes a controller programmed to allocate current demand to the
power converters and, responsive to currents allocated to a subset
of the power converters having magnitudes less than a threshold,
redistribute a sum of the currents to power converters not in the
subset such that a total current provided by the power converters
is the same before and after redistribution of the currents and
operate the subset to provide no current.
[0008] The controller may be further programmed to increase current
magnitudes to each of the power converters not in the subset by a
factor that is a ratio of the current demand to a total current
already allocated to each of the power converters not in the
subset. The currents may include positive currents and negative
currents, and the controller may be further programmed to
redistribute a sum of the positive currents to power converters
with positive current allocation and redistribute a sum of the
negative current to power converter with negative current
allocation. The controller may be further programmed to, responsive
to currents allocated to all of the power converters having
magnitudes less than the threshold, redistribute the current demand
to a group of the power converters such that current is supplied by
a set of power converters that each provide current that exceeds
the threshold. The threshold may be a current level corresponding
to an efficiency being less than a predetermined converter
efficiency.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a diagram of an electrified vehicle illustrating
drivetrain and energy storage components including an electric
machine.
[0010] FIG. 2 is a diagram of a vehicle electrical system including
bypass converters.
[0011] FIG. 3 is a block diagram of a possible configuration for
operating the bypass converters.
[0012] FIG. 4 is a graph of a possible converter efficiency curve
with respect to converter input current.
[0013] FIG. 5 is a flowchart for a possible sequence of operations
for operating the bypass converters to satisfy current demand on an
electrical bus.
DETAILED DESCRIPTION
[0014] Embodiments of the present disclosure are described herein.
It is to be understood, however, that the disclosed embodiments are
merely examples and other embodiments can take various and
alternative forms. The figures are not necessarily to scale; some
features could be exaggerated or minimized to show details of
particular components. Therefore, specific structural and
functional details disclosed herein are not to be interpreted as
limiting, but merely as a representative basis for teaching one
skilled in the art to variously employ the present invention. As
those of ordinary skill in the art will understand, various
features illustrated and described with reference to any one of the
figures can be combined with features illustrated in one or more
other figures to produce embodiments that are not explicitly
illustrated or described. The combinations of features illustrated
provide representative embodiments for typical applications.
Various combinations and modifications of the features consistent
with the teachings of this disclosure, however, could be desired
for particular applications or implementations.
[0015] FIG. 1 depicts an electrified vehicle 112 that may be
referred to as a plug-in hybrid-electric vehicle (PHEV). A plug-in
hybrid-electric vehicle 112 may comprise one or more electric
machines 114 mechanically coupled to a hybrid transmission 116. The
electric machines 114 may be capable of operating as a motor or a
generator. In addition, the hybrid transmission 116 is mechanically
coupled to an engine 118. The hybrid transmission 116 is also
mechanically coupled to a drive shaft 120 that is mechanically
coupled to the wheels 122. The electric machines 114 can provide
propulsion and deceleration capability when the engine 118 is
turned on or off. The electric machines 114 may also act as
generators and can provide fuel economy benefits by recovering
energy that would normally be lost as heat in a friction braking
system. The electric machines 114 may also reduce vehicle emissions
by allowing the engine 118 to operate at more efficient speeds and
allowing the hybrid-electric vehicle 112 to be operated in electric
mode with the engine 118 off under certain conditions. An
electrified vehicle 112 may also be a battery electric vehicle
(BEV). In a BEV configuration, the engine 118 may not be
present.
[0016] A traction battery or battery pack 124 stores energy that
can be used by the electric machines 114. The traction battery 124
may be electrically couplable to a high-voltage electrical bus 154.
The high-voltage bus 154 may include power and return conductors.
The vehicle battery pack 124 may provide a high voltage direct
current (DC) output. One or more contactors 142 may isolate the
traction battery 124 from the high-voltage bus 154 when opened and
connect the traction battery 124 to the high-voltage bus 154 when
closed. The traction battery 124 may be electrically coupled to one
or more power electronics modules 126 (may also be referred to as a
traction inverter). The power electronics module 126 is also
electrically coupled to the electric machines 114 and provides the
ability to bi-directionally transfer energy between the traction
battery 124 and the electric machines 114. For example, a traction
battery 124 may provide a DC voltage while the electric machines
114 may operate with a three-phase alternating current (AC) to
function. The power electronics module 126 may convert the DC
voltage to a three-phase AC current to operate the electric
machines 114. In a regenerative mode, the power electronics module
126 may convert the three-phase AC current from the electric
machines 114 acting as generators to the DC voltage compatible with
the traction battery 124.
[0017] The vehicle 112 may include a variable-voltage converter
(VVC) (not shown) electrically coupled between the traction battery
124 and the power electronics module 126. The VVC may be a DC/DC
boost converter configured to increase or boost the voltage
provided by the traction battery 124. By increasing the voltage,
current requirements may be decreased leading to a reduction in
wiring size for the power electronics module 126 and the electric
machines 114. Further, the electric machines 114 may be operated
with better efficiency and lower losses.
[0018] In addition to providing energy for propulsion, the traction
battery 124 may provide energy for other vehicle electrical
systems. The vehicle 112 may include a bypass converter module 128
that converts the high voltage DC output of the traction battery
124 to a low voltage DC supply that is compatible with low-voltage
vehicle loads 152. The bypass converter module 128 may be coupled
between the high-voltage bus 154 and a low-voltage electrical bus
156. An output of the bypass converter module 128 may be
electrically coupled to the low-voltage electrical bus 156 and to
an auxiliary battery 130 (e.g., 12V battery) for charging the
auxiliary battery 130. The low-voltage systems and loads 152 may be
electrically coupled to the low-voltage electrical bus 156. The
low-voltage bus 156 may include power and return conductors. One or
more electrical loads 146 may be coupled to the high-voltage bus
154. The electrical loads 146 may have an associated controller
that operates and controls the electrical loads 146 when
appropriate. Examples of electrical loads 146 may be a fan, an
electric heating element and/or an air-conditioning compressor.
[0019] The electrified vehicle 112 may be configured to recharge
the traction battery 124 from an external power source 136. The
external power source 136 may be a connection to an electrical
outlet. The external power source 136 may be electrically coupled
to a charger or electric vehicle supply equipment (EVSE) 138. The
external power source 136 may be an electrical power distribution
network or grid as provided by an electric utility company. The
EVSE 138 may provide circuitry and controls to regulate and manage
the transfer of energy between the power source 136 and the vehicle
112. The external power source 136 may provide DC or AC electric
power to the EVSE 138. The EVSE 138 may have a charge connector 140
for plugging into a charge port 134 of the vehicle 112. The charge
port 134 may be any type of port configured to transfer power from
the EVSE 138 to the vehicle 112. The charge port 134 may be
electrically coupled to a charger or on-board power conversion
module 132.
[0020] The power conversion module 132 may condition the power
supplied from the EVSE 138 to provide the proper voltage and
current levels to the traction battery 124. The power conversion
module 132 may interface with the EVSE 138 to coordinate the
delivery of power to the vehicle 112. The EVSE connector 140 may
have pins that mate with corresponding recesses of the charge port
134. Alternatively, various components described as being
electrically coupled or connected may transfer power using a
wireless inductive coupling.
[0021] One or more wheel brakes 144 may be provided for
decelerating the vehicle 112 and preventing motion of the vehicle
112. The wheel brakes 144 may be hydraulically actuated,
electrically actuated, or some combination thereof. The wheel
brakes 144 may be a part of a brake system 150. The brake system
150 may include other components to operate the wheel brakes 144.
For simplicity, the figure depicts a single connection between the
brake system 150 and one of the wheel brakes 144. A connection
between the brake system 150 and the other wheel brakes 144 is
implied. The brake system 150 may include a controller to monitor
and coordinate the brake system 150. The brake system 150 may
monitor the brake components and control the wheel brakes 144 for
vehicle deceleration. The brake system 150 may respond to driver
commands and may also operate autonomously to implement features
such as stability control. The controller of the brake system 150
may implement a method of applying a requested brake force when
requested by another controller or sub-function.
[0022] Electronic modules in the vehicle 112 may communicate via
one or more vehicle networks. The vehicle network may include a
plurality of channels for communication. One channel of the vehicle
network may be a serial bus such as a Controller Area Network
(CAN). One of the channels of the vehicle network may include an
Ethernet network defined by Institute of Electrical and Electronics
Engineers (IEEE) 802 family of standards. Additional channels of
the vehicle network may include discrete connections between
modules and may include power signals from the auxiliary battery
130. Different signals may be transferred over different channels
of the vehicle network. For example, video signals may be
transferred over a high-speed channel (e.g., Ethernet) while
control signals may be transferred over CAN or discrete signals.
The vehicle network may include any hardware and software
components that aid in transferring signals and data between
modules. The vehicle network is not shown in FIG. 1, but it may be
implied that the vehicle network may connect to any electronic
module that is present in the vehicle 112. A vehicle system
controller (VSC) 148 may be present to coordinate the operation of
the various components.
[0023] Electrified vehicles (e.g., BEV, PHEV) distribute power via
the high-voltage bus 154 and the low-voltage bus 156. Prior
arrangements typically utilize a single DC/DC converter
electrically coupled between the high-voltage bus 154 and the
low-voltage bus 156 to provide power to the low-voltage bus 156.
The DC/DC converter may be configured to reduce the voltage of the
high-voltage bus 154 to a voltage level (e.g., 12V) of the
low-voltage bus 156. The system described herein replaces the
traditional DC/DC converter with the bypass converter module 128 to
provide power to the low-voltage bus 156 and provide additional
benefits to be described herein.
[0024] Vehicle performance for conventional internal combustion
engine (ICE) vehicles may be evaluated in terms of fuel economy.
However, electrified vehicles may be evaluated in term of energy
economy. Analysis of the energy economy for electrified vehicles
may consider the total amount of energy provided during charging.
Since electrical energy may be used for propulsion, an energy
economy term similar to fuel economy may be computed using
equivalent representations for the electrical energy. Energy
economy may be represented as a travel range or distance provided
by the traction battery 124. Electrical energy may be provided to
the high-voltage electrical bus 154 and the low-voltage electrical
bus 156. During operation, energy provided by the traction battery
124 is used for propulsion and for low-voltage loads 152. The
low-voltage loads 152 may be similar to those in conventional
vehicles. In the electrified vehicle 112, the bypass converter 128
may perform functions similar to the alternator of the conventional
vehicle.
[0025] Fuel and/or energy economy of electrified vehicles may be
affected by efficiency of the bypass converter 128. The efficiency
of a power converter may vary with the amount of current or power
passing through the converter. The power output by the bypass
converter 128 may be defined as the product of the voltage at the
converter output and the current provided by the converter. The
current passing through the bypass converter 128 may be determined
by the current drawn by the low-voltage loads 152 coupled to the
low-voltage electrical bus 156. The bypass converter 128 may be
configured to satisfy the maximum current requirement of the
low-voltage electrical bus 156. The current demand of the
low-voltage electrical bus 156 may vary significantly based on
operating conditions of the vehicle 112. For example, during a
drive cycle, the current demand may be hundreds of Amperes. During
battery charging or idle conditions, the current demand may ten
Amperes or less. The bypass converter 128 may operate over a large
current range. A conventional power converter designed to be highly
efficient at high current levels may be significantly less
efficient at lower current levels.
[0026] The efficiency of a power converter may be affected by the
conversion ratio. The conversion ratio may be expressed as the
ratio of the input voltage to the output voltage of the power
converter. For example, in some electrified vehicle configurations,
the input voltage may be 300 Volts and the output voltage may be 12
Volts. Efficiency improvements may be realized by decreasing the
conversion ratio. Energy economy and/or efficiency may also be
affected by energy that is wasted or lost (e.g., as heat) in the
system.
[0027] Some solutions to improve energy economy may incorporate a
second power converter that is optimized for high efficiency at
lower current levels. However, such solutions can add additional
cost and complexity to the system. Further, additional control
logic is needed to manage switching between the converters.
[0028] FIG. 2 depicts a possible configuration of a vehicle
electrical system 200. The traction battery 124 may be coupled to
the high-voltage electrical bus 156 through a positive terminal 210
and a negative or return terminal 216. The traction battery 124 may
be comprised of a plurality of battery cells 202. The battery cells
202 may be constructed from a variety of chemical formulations.
Typical battery pack chemistries may be lead acid, nickel-metal
hydride (NIMH) or Lithium-Ion. The traction battery 124 may be
comprised of a series configuration of n battery cells 202. Other
configurations are possible, however, and the traction battery 124
may be composed of any number of individual battery cells 202
connected in series or parallel or some combination thereof. The
system may include one or more controllers, such as a Battery
Energy Control Module (BECM) 208, that are configured to monitor
and control the performance of the traction battery 124. The BECM
208 may monitor several traction battery characteristics such as
pack current, pack voltage and pack temperature. The BECM 208 may
include non-volatile memory such that data may be retained when the
BECM 208 is in an off condition. Retained data may be available
upon the next key cycle.
[0029] In addition to the pack level characteristics, there may be
battery cell level characteristics that are measured and monitored.
For example, the terminal voltage, current, and temperature of each
of the battery cells 202 may be measured. The vehicle electrical
system 200 may use one or more sensor modules 204 to measure the
characteristics of the battery cells 202. The sensor modules 204
may include voltage sensors configured to measure the voltage
across each of the battery cells 202. Depending on the
capabilities, the sensor modules 204 may measure the
characteristics of one and/or groups of the battery cells 202. The
traction battery 124 may utilize multiple sensor modules 204 to
measure the characteristics of all the battery cells 202. Each
sensor module 204 may transfer the measurements to the BECM 208 for
further processing and coordination. The sensor modules 204 may
transfer signals in analog or digital form to the BECM 208. In some
configurations, the sensor module 204 functionality may be
incorporated internally to the BECM 208. That is, the sensor module
204 hardware may be integrated as part of the circuitry in the BECM
208 and the BECM 208 may handle the processing of raw signals.
[0030] The BECM 208 may be configured to compute various
characteristics of the traction battery 124 and/or battery cells
202. Quantities such a battery power capability and battery state
of charge may be useful for controlling the operation of the
traction battery 124 as well as any electrical loads 146 receiving
power from the traction battery 124. Battery power capability is a
measure of the maximum amount of power the traction battery 124 can
provide or the maximum amount of power that the traction battery
124 can receive. Each of the battery cells 202 may be characterized
by a battery power capability. Knowing the battery power capability
allows electrical loads 146 to be managed such that the power
requested is within limits that the traction battery 124 can
handle.
[0031] Battery state of charge (SOC) gives an indication of how
much charge remains in the battery. The battery SOC may be output
to inform the driver of how much charge remains in the traction
battery 124, similar to a fuel gauge. The battery SOC may also be
used to control the operation of an electrified vehicle.
Calculation of battery SOC can be accomplished by a variety of
methods. One possible method of calculating battery SOC is to
perform an integration of the battery pack current over time. This
is well-known in the art as ampere-hour integration. The SOC of
each of the battery cells 202 may be computed in a similar
manner.
[0032] Another feature of the traction battery 124 may be the
ability to balance the battery cells 202. The traction battery 124
may include hardware and software features to perform balancing of
the battery cells 202. Balancing the battery cells 202 may include
charging or discharging individual battery cells 202 so that the
states of charge or power capability of the battery cells 202 are
equalized. This operation is useful to prevent overcharging and/or
undercharging of individual battery cells. To facilitate cell
balancing, the traction battery 124 and/or sensor modules 204 may
include a switching element 214 and a resistor 212 coupled to each
of the battery cells 202. The switching element 214 and the
resistor 212 may be arranged so that the resistor 212 may be
coupled in parallel with a battery cell 202 to cause current to
flow from the battery cell 202 through the resistor 212. By
activating the switching element 214, current may flow from the
battery cell 202 through the resistor 212 to cause the cell SOC to
decrease. The switching element 214 may be controllable by the
sensor module 204 via a control signal. The switching element 214
may be in a normally open position. The switching element 214 may
be a solid-state element (e.g., metal-oxide semiconductor
field-effect transistor (MOSFET)). The cell balancing achieved by
the resistor may be referred to as passive cell balancing as it
only affects a discharge of the battery cells. Passive cell
balancing may contribute to wasted energy since the energy is lost
as heat in the resistor 212.
[0033] The BECM 208 may be programmed to manage life balancing of
the traction battery 124. Life balancing may include active and
passive balancing procedures. Various methods for managing cell
balance may be available. For example, cell balancing may be
performed after a charging event. The SOC of each of the battery
cells 202 may be compared. Battery cells 202 having a higher SOC
may be discharged to match the SOC of the remaining cells by
activating the associated switching element 214 to discharge
current from the cell. Life balancing allows cell balancing by
operating cells or groups of cells during drive cycles. Life
balancing may achieve balance between groups of cells 202 while
passive cell balancing may achieve balance between individual cells
by discharging current through the resistor 212. Other criteria for
managing cell balance may be used. For example, the life balancing
may be configured to balance the cells 202 to maximize range, life,
and/or available power during a drive cycle. Each strategy may
result in a different balancing strategy.
[0034] The bypass converter module 128 may include a plurality of
DC/DC bypass converters 206 that are configured to provide an
output voltage compatible with the low-voltage bus 156. The voltage
outputs of the bypass converters 206 may be connected in parallel
such that each of the bypass converters 206 may contribute to the
current flowing through the low-voltage bus 156. The voltage input
to each of the bypass converters 206 may be coupled across a
predetermined number of battery cells 202. For example, the input
to the bypass converters 206 may be across m battery cells 202. The
m battery cells may be referred to as a group of battery cells. The
voltage input to the bypass converters 206 may be the sum of the
voltages across the m battery cells 202. The voltage input may also
be referred to as the voltage across the group of battery cells. In
addition, each group of battery cells may have an associated SOC
that may be derived from the individual battery cells that make up
the group. For example, the group SOC may be an average cell SOC, a
median cell SOC, a lowest cell SOC of the battery cells in the
group, or a highest SOC of the battery cells of the group.
[0035] Depending on the voltage input, the bypass converters 206
may be configured to increase (boost) or decrease (buck) the
voltage to provide the output voltage. In some configurations, the
bypass converters 206 may include a bypass mode to couple the input
to the output with a high efficiency when the input and output
voltage levels are similar. For example, the bypass mode may
increase efficiency by avoiding switching losses within the
converter. The bypass converters 206 may include an internal
controller to manage the voltage conversion operation. As there are
multiple bypass converters 206, the BECM 208 may manage and
coordinate the operation of the bypass converters 206 to supply a
total current provided to the low-voltage bus 156. For example,
each of the bypass converters 206 may communicate with the BECM 208
over the vehicle network or a dedicated communication channel.
[0036] Each of the bypass converters 206 may be operated to provide
current to the low-voltage bus 156. Current demand on the
low-voltage bus 156 may be satisfied by operation of the bypass
converters 206. As there are multiple bypass converters 206 the
current may be distributed or proportioned among each of the bypass
converters 206. There may be a predetermined number of converters
with each assigned an integer index in the range of 1 to max. The
BECM 208 may manage the total current provided to the low-voltage
bus 156 by the bypass converters 206. The BECM 208 may be
programmed to distribute a portion of the total current to each of
the bypass converters 206. The BECM 208 may implement a current
distribution function. The current distribution function may
distribute the total current to the individual bypass converters
206 based on the state of charge of the battery cells 202 (cell
unit) associated with each of the bypass converters 206.
[0037] The bypass converters 206 may be in communication with an
associated sensor module 204. For example, the bypass converters
206 and the sensor modules 204 may be connected to a serial
peripheral interface (SPI) bus. The SPI bus may be a dedicated
communication link between the bypass converter 206 and associated
sensor module 204. Voltage and current values from the
corresponding group of battery cells 202 may be transferred from
the sensor module 204 to the bypass converters 206.
[0038] The BECM 208 may be in communication with the sensor modules
204 and the bypass converters 206. For example, the BECM 208 and
the sensor modules 204 may communicate via a CAN communication
channel. In some configurations, the communication between the BECM
208 and the bypass converters 206 may be indirect and pass through
the sensor modules 204. For example, the BECM 208 may communicate
via the CAN communication channel to the sensor modules 204. The
sensor modules 204 may then transfer messages for the bypass
converters 206 to the SPI bus.
[0039] The bypass converters 206 may include one or more current
sensors to measure the input and/or output current of the bypass
converter 206. The current measurement may be used for control of
the conversion operation. In addition, the current measurement
values may be communicated to the BECM 208. The bypass converters
206 or associated sensor modules 204 may include one or more
voltage sensors to measure the input and/or output voltage of the
bypass converter 206. In some configurations, the output voltage
may be measured by the BECM 208 and the output voltage value may be
communicated to the bypass converters 206. In some configurations,
the sensor module 204 may measure or calculate the voltage across
the group of battery cells 202 and communicate the value to the
bypass converters 206 as the input voltage.
[0040] The total current supplied from traction battery 124 to the
low-voltage bus 156 may be varied with the current demanded by the
LV loads 152 to regulate the LV bus voltage to a target voltage.
The current demand on the low-voltage bus 156 may vary from a few
Amperes to over hundreds of Amperes in an electrified vehicle. As
such, the total current draw of the bypass converter 128 varies in
same range. The vehicle electrical system with a plurality of DC/DC
bypass converters 206 is not only able to individually adjust the
current flow through each of the bypass converters 206 but can also
reduce the voltage conversion ratio between the bypass converter
input and output. The ratio may be made close to one in order for
the converters to work with higher converting efficiency (e.g.,
engaging a bypass mode of the converter). As shown in FIG. 2, the
input voltage of bypass converters 206 may be determined by a
predetermined number of battery cells 202, and the output voltage
of converters may be determined by LV bus voltage which is
nominally around 12V for a vehicle. Considering the effect of the
voltage converting ratio on the converter efficiency, the ratio may
be selected to be in the range of 1-2. For this ratio range, there
may be 3 to 6 battery cells 202 that are serially connected on the
battery or input side of the bypass converters 206.
[0041] The BECM 208 may be programmed to implement a strategy for
allocating or proportioning the current among the bypass converters
206. When the current demand of the low-voltage electrical bus 156
is high, the current may be proportioned equally between the bypass
converters 206. Each of the bypass converters 206 may operate with
a current greater than a current threshold resulting in operation
above a selected limit of efficiency.
[0042] A factor that affects converter efficiency may be the
current flow through the bypass converter 206 after the ranges of
input and output voltages are determined. As an example, a graph
400 of a possible efficiency curve 402 of a bypass converter 206 is
shown in FIG. 4. At current magnitudes above a certain threshold
the efficiency generally exceeds 85%. The converter efficiency
decreases when the input current magnitude falls within a window
defined within a certain current boundary, and approaches zero as
the input current magnitude approaches to zero. When viewed from an
efficiency standpoint, it is desired to operate the bypass
converters 206 at higher current levels (e.g., >2 A) to achieve
efficiencies greater than a predetermined efficiency level 408. The
converter efficiency may also be expressed as a function of power
provided by the bypass converter 206. For example, the current axis
of FIG. 4 may also be expressed as a power axis. A power threshold
may be defined at which the efficiency exceeds a predetermined
efficiency (e.g., 85%). In the following discussion, current or
power may be used for selecting the bypass converters 206 and
associated operating points.
[0043] When a plurality of DC/DC bypass converters 206 is used to
supply power from traction battery 124 to the low-voltage bus 156
of an electrified vehicle, the number of bypass converters 206 that
are used may be determined by the maximum allowed current of the
bypass converters 206 and the number of battery cells 202 that are
connected to the bypass converters 206 after considering the
voltage conversion ratio and balance requirements. For example, if
the nominal voltage of the high-voltage bus 154 is about 300V, the
system may need 13-26 converters when the voltage conversion ratio
is in a range of 1-2. One mode of operation may be to distribute
the current load equally among the DC/DC bypass converters 206. As
an example, a system may include fourteen bypass converters having
a conversion ratio of 2 (e.g., input/output is 2) and the
converters may be operated such that each of the converters provide
the same level of current. In this example, the average input
current of a bypass converter may be in the range between less than
0.5 A to greater than 7 A when current loads of the low-voltage bus
156 that are connected to the output of the bypass converters
varies in the range of 10 A to 200 A. When distributing the current
equally, the converter efficiency may be below 75% when the
low-voltage bus 156 current load is less than about 30 A. The
efficiency may fall below 50% if the low-voltage bus 156 current
load is less than about 15 A. Lower working efficiency of the
converters not only affects equivalent fuel economics of vehicle,
but also results in higher heat generation.
[0044] The total current supplied from the high-voltage electrical
bus 154 to the low-voltage electrical bus 156 may vary based on the
current demand of the low-voltage loads 152. The current flow
through each of the DC/DC bypass converters 206 may be individually
adjusted according to the corresponding cell unit state. The cell
unit state may include a cell voltage, a voltage across a group of
cells 202, a battery cell capacity, a capacity of a group of
battery cells, a cell state of charge, and/or a state of charge of
a group of cells 202. Current flow through some of the bypass
converters 206 may be low (e.g., <1 Amp) even though the total
current supplied to the low-voltage electrical bus 156 and/or the
average converter current may be much higher. The current
distribution may be determined by the balance state of the
associated battery cells 202. The SOC of the battery cells or
groups of battery cells may be considered when selecting the bypass
converter current levels. The SOC differences between the groups of
battery cells may be used to adjust a proportion of current
distributed to each of the bypass converters 206. Considering the
SOC differences may decrease divergence of the battery SOC from a
reference SOC. The strategy may provide active balancing of the
battery cells without intentional losses from dissipating energy in
passive resistors.
[0045] Using the battery cell unit state may result in each of the
bypass converters 206 passing a different amount of current. To
achieve cell balance during battery discharging, cell groups having
a higher state of charge may have a larger current flow than cell
groups with lower states of charge. In this situation, the current
flow for some of the bypass converters 206 may cause the associated
converters to operate in a range of lower efficiency. Current
demand may be allocated to the bypass converters 206 based on
states of charge of the groups of battery cells 202 such that a
first bypass converter coupled to a first group of battery cells
having a state of charge greater than a state of charge of a second
group of battery cells coupled to a second bypass converter
receives a greater current allocation.
[0046] The relationship between the converter efficiency and the
converter input current may be known for the bypass converters 206
as shown in FIG. 4. From this relationship, the minimum magnitude
of the input current to achieve an efficiency above a predetermined
efficiency value 408 can be determined. The predetermined
efficiency value 408 may be based on vehicle performance
requirements. The predetermined efficiency value 408 may be
selected as a minimum operating efficiency for the bypass
converters 206. When the limit of efficiency is selected, the
current and/or power limits may be determined.
[0047] A minimum accepted efficiency limit for the bypass
converters 206 may be selected as the predetermined efficiency
level 408. The predetermined efficiency level 408 may be a
calibratable value based on system design requirements.
Corresponding to the predetermined efficiency level 408, a
zero-current range 410 may be defined by a high limit zero-current
bound 404 and a low limit zero-current bound 406. If current is
allocated to one of the bypass converters 206 in the zero-current
range 410 of current between a high limit zero-current bound 404
and a low limit zero-current bound 406, then the current may be
restricted to zero. Outside of the zero-current range 410, the
current may be allowed to be passed. When power is used, a
zero-power range may be defined with corresponding high limit
zero-power bound and low limit zero-power bound.
[0048] FIG. 3 depicts a block diagram 300 for processes for
managing and operating the bypass converters 206. A timing process
302 may generate signals for triggering the other processes. The
timing process 302 may be interrupt driven or polled. The timing
process 302 may generate a plurality of trigger signals for
triggering execution of processes. The timing process 302 may
generate trigger signals at different rates. The timing process 302
may generate a fast-speed trigger 314 that is configured to trigger
execution of high-speed processes. The high-speed processes may be
those processes that are to be executed most frequently. The timing
process 302 may generate a slow-speed trigger 320 that is
configured to trigger execution of low-speed processes. The
low-speed processes may be those processes that are to be executed
least frequently. The timing process 302 may generate a first
medium-speed trigger 318 and a second medium-speed trigger 316 that
are configured to trigger execution of medium-speed processes. The
first medium-speed trigger 318 and the second medium-speed trigger
316 may occur at rates between the low-speed trigger 320 and the
high-speed trigger 314.
[0049] The trigger signals may cause execution of different
processes. For example, high-speed processes may include low-level
control routines. Low-speed processes may include high-level or
background operations such as setting overall parameters.
Medium-speed processes may include intermediate control decisions
that set reference or setpoint values for the low-level control
routines.
[0050] A target current distribution process 304 may be configured
to determine a current distribution ratio for each of the bypass
converters 206. The target current distribution process 304 may be
triggered by the slow-speed trigger 320. For example, the target
current distribution process 304 may determine the current
allocation to each of the bypass converters 206 based on the states
of charge of associated battery cell groups.
[0051] A total current demand process 306 may be configured to
determine the total current demand for the low-voltage electrical
bus 156. The total current demand process 306 may receive
information from low-voltage loads 152. The total current demand
process 306 may receive a voltage of the low-voltage electrical bus
156. The total current demand process 306 may be triggered by the
first medium-speed trigger 318. For example, the total current
demand process 306 may include a closed-loop control strategy based
on a reference voltage and the actual voltage of the low-voltage
electrical bus 156.
[0052] A current distribution process 308 may be configured to
generate the current distribution for the bypass converters 206.
The current distribution process 308 may receive the total current
demand from the total current demand process 306 and distribution
ratios from the target current distribution process 304. The
current distribution process 308 may be triggered by the second
medium-speed trigger 316. The current distribution process 308 may
adjust the current allocation to improve overall efficiency of the
vehicle electrical system.
[0053] The current distribution process 308 may accumulate the
currents that are restricted or limited. The restricted currents
may be identified as positive or negative. A positive current may
be defined as a current flow from the traction battery cells 202 to
the low-voltage electrical bus 156. A negative current may be
defined as a current flow from the low-voltage electrical bus 156
to the traction battery cells 202. Positive and negative currents
may be separately accumulated. For example, a first variable may be
used to accumulate negative currents and a second variable may be
used to accumulate positive currents. The accumulated currents may
be redistributed to the bypass converters 206 that are not being
limited or restricted. Bypass converters that are not limited may
have an initial target current allocation that is outside of the
zero-current range 410 (e.g., greater than the high limit
zero-current bound 404 or less than the low limit zero-current
bound 406). The accumulated positive currents may be redistributed
to the bypass converters having current that is greater than the
high limit zero-current bound 404. The accumulated negative
currents may be distributed to the bypass converters having current
that is less than the low limit zero-current bound 406.
[0054] A bypass converter management process 310 may be configured
to operate the bypass converters 206 to achieve the selected target
current. The bypass converter management process 310 may be
triggered by the fast-speed trigger 314. The bypass converter
management process 310 may include a converter control process 312
for each of the bypass converters 206. The bypass converter
management process 310 may receive a target current for each of the
bypass converters 206. The converter control process 312 may
receive the target current and operate the bypass converter to
achieve the target current.
[0055] FIG. 5 depicts a flow chart 500 for a possible sequence of
operations for allocating current to the bypass converters 206. At
operation 502, the distribution ratios may be determined. Operation
502 may be implemented as part of the target current distribution
process 304. For example, the distribution ratios may be determined
based on the state of charge of the cell groups associated with the
bypass converters 206 as previously described herein. The
distribution ratios may be selected to cause the battery cells 202
to move toward a balance condition such that all of the battery
cells 202 have the same SOC after a predetermined time. The active
balancing strategy may achieve balance between the groups of
battery cells 202 that are coupled to corresponding bypass
converters 206. Passive balancing using the resistor 212 may be
used to achieve balance between battery cells 202 with each of the
groups. The active balancing strategy may reduce the current
discharged in the resistors 212 during passive balancing.
[0056] At operation 504, the total current or power demand may be
determined. Operation 504 may be implemented as part of the total
current demand process 306. A closed-loop voltage control strategy
may be implemented in which the total current demand is based on an
error between the bus voltage and a reference voltage. Operation
502 and operation 504 may be performed in parallel or
sequentially.
[0057] At operation 506, an initial current allocation may be
determined. A target current, I(i), for each of the bypass
converters 206 may be computed as a product of the total current
demand and the corresponding distribution ratios. The result may be
an initial estimate of the target current for each of the bypass
converters 206. The allocation may also be a power allocation.
However, further processing may be performed to account for
converter efficiencies.
[0058] It may be desired to operate the bypass converters with a
low initial current allocation at zero current and reallocate the
current to the other bypass converters 206. The initial target
current allocations may each be compared to the zero-current range
410. If the initial target current allocation falls within the
zero-current range 410, the current allocated to the corresponding
bypass converter may be limited to zero.
[0059] At operation 508, the target currents may be compared to the
high limit zero-current bound 404 and a low limit zero-current
bound 406 to determine if the target current falls within the
zero-current range 410. If none of the target currents fall outside
of the zero-current range 410, operation 510 may be performed. For
example, the initial current distribution may result in all of the
target currents falling within the zero-current range 410. Under
this condition, the bypass converters 206 could potentially operate
at lower efficiency levels unless the target currents are
distributed in a different manner.
[0060] At operation 510, the target current may be distributed to a
subset of the bypass converters 206. Operation of the bypass
converters 206 when all currents are within the zero-current range
410 would result in poor efficiency. In this condition, the current
demand may be redistributed such that a group of the total number
of bypass converters 206 is operated above the predetermined
efficiency value 408. Some of the bypass converters 206 may be
operated at zero current. In this condition, the total current
demand may be distributed to a group of m bypass converters such
that each of the m bypass converters are operating at or above the
predetermined efficiency value 408. The m bypass converters may be
selected according the battery cell unit parameters. For example,
the number m may be estimated as the total current divided by the
high limit zero-current bound 404.
[0061] Operation 510 may also be performed when the total current
demand falls within the zero-current range 410. Under this
condition, the distributed current for all of the bypass converters
206 may be set to zero. In this case, the total current before and
after the current redistribution may differ. In some
configurations, one of the bypass converters 206 operating below
the predetermined efficiency value 408 may be operated to supply
the current demand. Operating in this manner, the total current
before and after distribution may be the same. This mode of
operation results in lower efficiency.
[0062] If at least one of the initial target currents is outside of
the zero-current range 410, operation 512 may be performed. Each of
the initial current allocations may be compared to the zero-current
range 410. Current estimates that fall between the low-limit
zero-current bound 406 and the high-limit zero-current bound 404
represent converter operation at low efficiency levels. At
operation 512, the initial current allocation may be checked to
determine if the value is between zero and the high limit
zero-current bound 404 (I.sub.LimH). This indicates a small
positive current (e.g., flowing to the low-voltage electrical bus
156). If the condition is satisfied, operation 518 may be
performed. At operation 518, the corresponding initial current
allocation may be accumulated or summed in a first variable
(I.sub.p). At operation 520, the corresponding final current
allocation, I.sub.f(i), is set to zero.
[0063] If the condition is not satisfied, operation 514 may be
performed. At operation 514, the initial current allocation may be
checked to determine if the value is between zero and the low limit
zero-current bound 406 (I.sub.LimL). This indicates a small
negative current (e.g., flowing from the low-voltage electrical bus
156). If the condition is satisfied, operation 516 may be
performed. At operation 516, the corresponding initial current
allocation is accumulated or summed in a second variable (I.sub.n).
At operation 520, the final current allocation, I.sub.f(i), is set
to zero.
[0064] At operation 522, the index, i, may be compared to the
maximum index (e.g., the total number of converters). The index
being equal to the maximum index may be indicative of all of the
current allocations being processed. If the index is less than the
maximum index, operation 524 may be performed. At operation 524,
the index, i, may be incremented and operations starting at 512 may
be performed for the next current allocation. If the initial
current allocations have all been processed, operation 526 and
operation 528 may be performed.
[0065] The previous operations determine a subset of the power
converters that includes zero-limited bypass converters that have a
magnitude (e.g., current or power) less than a threshold (e.g., the
low-limit zero-current bound 406 and the high-limit zero-current
bound 404). The total current allocated to the subset may be
redistributed to the bypass converters that are not part of the
subset (e.g., having current allocation outside of the zero-current
range 410). Bypass converters that are members of the subset are
operated to provide no current.
[0066] At operation 526, positive currents may be redistributed to
bypass converters that are not in the subset that includes the
zero-limited bypass converters. The positive current may be
redistributed to those bypass converters having a positive current
allocation. One strategy for redistributing the positive currents
that are accumulated in the first variable (4) may be to equally
distribute the accumulated current to those converters having a
positive current allocation. The current may also be redistributed
as follows:
I.sub.f(i)=I(i)*I.sub.ptotal/(I.sub.ptotal-I.sub.p)
where I.sub.ptotal is an accumulation of all the positive initial
current allocations, and I(i) is the initial current allocation.
The current allocation to each of the bypass converters may be
increased by a factor that is a ratio of the total current demand
(represented by I.sub.ptotal) to a total current already allocated
(represented by I.sub.ptotal-I.sub.p) to each of the bypass
converters having current above the threshold.
[0067] At operation 528, negative currents may be redistributed.
The negative current may be redistributed to those converters
having a negative current allocation. One strategy for
redistributing the negative currents that are accumulated in the
second variable (I.sub.n) may be to equally distribute the
accumulated current to those converters having a negative current
allocation. The current may be redistributed as follows:
I.sub.f(i)=i(i)*I.sub.ntotal/(I.sub.ntotal-I.sub.n)
where I.sub.ntotal is an accumulation of all the negative initial
current allocations, and I(i) is the initial current allocation.
The current allocation to each of the bypass converters may be
increased by a factor that is a ratio of the total current demand
(represented by I.sub.ntotal) to a total current already allocated
(represented by I.sub.ntotal-I.sub.n) to each of the bypass
converters having current above the threshold.
[0068] At operation 530, the converters may be controlled to the
target current I.sub.f(i). Each of the bypass converters 206 may be
provided a target current and may operate the switching devices
within the bypass converter 206 to achieve the target current.
[0069] After the current cutoff and redistribution, the total
current of all of the bypass converters 206 may remain unchanged.
The total current allocated to the bypass converters 206 remains
the same before and after the efficiency limits have been applied.
The difference may be in the allocation of the currents to the
bypass converters. Further, the bypass converters that are limited
or restricted are operated at zero current.
[0070] The operating strategy disclosed improves overall efficiency
of the system as the bypass converters are operated at higher
levels of efficiency. In addition, the strategy provides
opportunities for active balancing the battery cells during vehicle
operation without unnecessary losses.
[0071] The processes, methods, or algorithms disclosed herein can
be deliverable to/implemented by a processing device, controller,
or computer, which can include any existing programmable electronic
control unit or dedicated electronic control unit. Similarly, the
processes, methods, or algorithms can be stored as data and
instructions executable by a controller or computer in many forms
including, but not limited to, information permanently stored on
non-writable storage media such as ROM devices and information
alterably stored on writeable storage media such as floppy disks,
magnetic tapes, CDs, RAM devices, and other magnetic and optical
media. The processes, methods, or algorithms can also be
implemented in a software executable object.
[0072] Alternatively, the processes, methods, or algorithms can be
embodied in whole or in part using suitable hardware components,
such as Application Specific Integrated Circuits (ASICs),
Field-Programmable Gate Arrays (FPGAs), state machines, controllers
or other hardware components or devices, or a combination of
hardware, software and firmware components.
[0073] While exemplary embodiments are described above, it is not
intended that these embodiments describe all possible forms
encompassed by the claims. The words used in the specification are
words of description rather than limitation, and it is understood
that various changes can be made without departing from the spirit
and scope of the disclosure. As previously described, the features
of various embodiments can be combined to form further embodiments
of the invention that may not be explicitly described or
illustrated. While various embodiments could have been described as
providing advantages or being preferred over other embodiments or
prior art implementations with respect to one or more desired
characteristics, those of ordinary skill in the art recognize that
one or more features or characteristics can be compromised to
achieve desired overall system attributes, which depend on the
specific application and implementation. These attributes may
include, but are not limited to cost, strength, durability, life
cycle cost, marketability, appearance, packaging, size,
serviceability, weight, manufacturability, ease of assembly, etc.
As such, embodiments described as less desirable than other
embodiments or prior art implementations with respect to one or
more characteristics are not outside the scope of the disclosure
and can be desirable for particular applications.
* * * * *