U.S. patent application number 16/928191 was filed with the patent office on 2021-02-25 for location-determinant fault monitoring for battery management system.
The applicant listed for this patent is Stafl Systems, LLC. Invention is credited to Erik Stafl.
Application Number | 20210055356 16/928191 |
Document ID | / |
Family ID | 1000005381759 |
Filed Date | 2021-02-25 |
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United States Patent
Application |
20210055356 |
Kind Code |
A1 |
Stafl; Erik |
February 25, 2021 |
LOCATION-DETERMINANT FAULT MONITORING FOR BATTERY MANAGEMENT
SYSTEM
Abstract
Battery management systems and methods for operation of same are
provided. A first switchable resistance may be connected between a
cell stack positive end and ground. A second switchable resistance
may be connected between a cell stack negative end and ground. The
switches for each resistance may be alternately opened and closed,
with comparison of the resulting currents through each resistance
being indicative of a location of isolation leakage current within
a battery system cell stack, and/or the magnitude of isolation
leakage current. Currents through the first and/or second
switchable resistances may also be indicative of Y capacitance. The
first and second switchable resistances may further be used to
reduce energy stored by Y capacitance.
Inventors: |
Stafl; Erik; (San Francisco,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stafl Systems, LLC |
South San Francisco |
CA |
US |
|
|
Family ID: |
1000005381759 |
Appl. No.: |
16/928191 |
Filed: |
July 14, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62891226 |
Aug 23, 2019 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 27/26 20130101;
G01R 31/006 20130101; G01R 31/371 20190101; G01R 31/396 20190101;
G01R 31/52 20200101; G01R 31/3842 20190101; B60L 58/18
20190201 |
International
Class: |
G01R 31/52 20060101
G01R031/52; B60L 58/18 20060101 B60L058/18; G01R 31/396 20060101
G01R031/396; G01R 31/371 20060101 G01R031/371; G01R 27/26 20060101
G01R027/26; G01R 31/3842 20060101 G01R031/3842; G01R 31/00 20060101
G01R031/00 |
Claims
1. A battery management system for monitoring the operation of a
battery pack containing a cell stack comprised of a plurality of
battery cells electrically connected in series, the battery
management system comprising: a first electronically-controlled,
switchable resistance comprising a first switch, electrically
connected between a positive end of the cell stack and ground; a
second electronically-controlled, switchable resistance comprising
a second switch, electrically connected between a negative end of
the cell stack and ground; a first current monitor configured to
measure a first current I.sub.1, through the first
electronically-controlled switchable resistance, when the first
switch is closed and the second switch is open; and a second
current monitor configured to measure a second current I.sub.2,
through the second electronically-controlled switchable resistance,
when the second switch is closed and the first switch is open;
wherein an isolation fault location within the cell stack is
determined based on a comparison of I.sub.1 and I.sub.2.
2. The battery management system of claim 1, in which: The first
and second electronically-controlled switchable resistances each
comprise a ladder of resistance comprising a number n bulk
resistors, where n is greater than one.
3. The battery management system of claim 2, which said ladders of
resistance each have an aggregate bulk resistance greater than
n/(n-1) times an isolation resistance specification against which
compliance is tested.
4. The battery management system of claim 2, in which: the first
electronically-controlled switchable resistance further comprises a
first measured resistance; the first current monitor comprises an
analog-to-digital converter measuring voltage across the first
measured resistance; the second electronically-controlled
switchable resistance further comprises a second measured
resistance; and the second current monitor comprises an
analog-to-digital converter measuring voltage across the second
measured resistance.
5. The battery management system of claim 1, further comprising: a
microprocessor configured to control operation of the first and
second switches, and to further calculate a ratio of
I.sub.1/(I.sub.1+I.sub.2) as an indication of the location of
isolation leakage current between the positive terminal and the
negative terminal.
6. The battery management system of claim 1, further comprising
said cell stack.
7. The battery management system of claim 6, further comprising an
electrically-powered object connected with said cell stack to
enable powering of said object by said cell stack.
8. The battery management system of claim 7, in which said
electrically-powered object comprises an electric vehicle.
9. The battery management system of claim 1, further comprising a
microprocessor, the microprocessor configured to: control operation
of the first and second switches; determine an indication of the
location of isolation leakage current between the positive terminal
and the negative terminal based on a comparison of a voltage V1
measured across at least a portion of the first
electronically-controlled switchable resistance, and a voltage V2
measured across at least a portion of the second
electronically-controlled switchable resistance; and determine a
magnitude of isolation leakage current based at least in part on V1
and V2.
10. The battery management system of claim 6, wherein: said cell
stack comprising a plurality of physically-separate,
electrically-connected cell substacks; and the isolation fault
location is indicative of one of said cell substacks being
associated with an isolation leakage current.
11. A method for monitoring the operation of a battery system
comprised of a cell stack having a plurality of battery cells
connected in series; a first switchable resistance comprising a
first electronically controlled switch in series with a first
resistance, connected between the cell stack positive and ground; a
second switchable resistance comprising a second electronically
controlled switch in series with a second resistance, connected
between the cell stack negative and ground; the method comprising
the steps of: measuring a first current through the first
switchable resistance when the first switch is closed and the
second switch is open; measuring a second current through the
second switchable resistance when the second switch is closed and
the first switch is open; determining location of an isolation
leakage current within the cell stack based at least in part on the
first current measurement and the second current measurement.
12. The method of claim 11, in which: the step of measuring a first
current through the first switchable resistance comprises measuring
voltage across a known resistor component within the first
resistance; and the step of measuring a second current through the
second switchable resistance comprises measuring voltage across a
known resistor component within the second resistance.
13. The method of claim 11, further comprising: determining a
magnitude of leakage current based at least in part on the first
current measurement and the second current measurement.
14. The method of claim 13, further comprising: initiating one of a
plurality of responses to the isolation leakage current based at
least in part upon characterization of an isolation fault condition
by mapping the isolation fault location and isolation fault
magnitude.
15. The method of claim 14, in which the step of initiating one of
a plurality of responses comprises: transmitting a fault condition
notification.
16. The method of claim 11, wherein: the battery system further
comprises a Y capacitance between a chassis ground and a motive
ground in an electrically-powered vehicle; with regard to the
method: the step of measuring a first current through the first
switchable resistance when the first switch is closed and the
second switch is open, further comprises sampling the first current
multiple times following closure of the first switch while the
first current settles towards a steady state; and/or the step of
measuring a second current through the second switchable resistance
when the second switch is closed and the first switch is open,
further comprises sampling the second current multiple times
following closure of the second switch while the second current
settles towards a steady state and the method further comprises
determining a magnitude of the Y capacitance based at least in part
on data recorded while sampling the first and/or second
current.
17. The method of claim 16, wherein: the step of sampling the first
current comprises sampling the first current at least from closure
of the first switch until the measured current has fallen by at
least 63% from a maximum value; and the step of determining a
magnitude of the Y capacitance comprises performing an exponential
curve fit of sampled values against a predetermined model.
18. The method of claim 17, wherein the predetermined model is an
RC curve.
19. The method of claim 16, further comprising: identifying a
change in magnitude of the Y capacitance; and transmitting a
notification in response thereto.
20. The method of claim 11, further comprising: periodically
closing the first switch and the second switch simultaneously.
21. The method of claim 20, wherein the step of periodically
closing the first switch and the second switch simultaneously
comprises closing the first switch and the second switch
simultaneously with a duty cycle of at least 20%.
22. The method of claim 11, in which the battery system further
comprises a powered load isolation switch, and the method further
comprises: conducting a safety pre-test of the first switchable
resistance and the second switchable resistance by opening the
powered load isolation switch, closing the first switch and the
second switch, and measuring current through the first resistance
and the second resistance; and determining that a failure condition
is present if a difference between the current through the first
resistance and the current through the second resistance exceeds a
threshold amount.
23. The method of claim 11, further comprising transmitting
information characterizing the isolation leakage current to a
network-connected server for centralized monitoring of a plurality
of deployed battery systems.
Description
RELATED APPLICATION AND CLAIM OF PRIORITY
[0001] This patent application claims priority to U.S. Provisional
Patent Application No. 62/891,226, titled LOCATION-DETERMINANT
FAULT MONITORING FOR BATTERY MANAGEMENT SYSTEM, which was filed on
Aug. 23, 2019.
TECHNICAL FIELD
[0002] The present disclosure relates in general to the electronics
field, and, in particular, to electronic hardware and software
(i.e., computer-implemented instructions) for high voltage battery
system management.
BACKGROUND
[0003] As battery cell technology and manufacturing capacity
improves, electric battery cells are used in an increasingly wide
variety of applications providing electrical power to powered
objects. For example, high-power yet cost-effective battery packs
may be critical to the commercial viability of electric cars and
other motive applications that may have traditionally been powered
by non-electric means. Battery systems are also increasingly used
for energy storage in solar panel applications, as well as a wide
variety of other industrial and consumer applications.
[0004] There are a number of design challenges in engineering
systems utilizing battery packs, particularly for large format
battery packs having large cell counts, with high energy density.
Such battery modules may develop high voltage levels, with capacity
for very high energy discharge rates, such that maintenance of
electrical isolation from, e.g., surrounding systems and people,
may be an important consideration for safety and reliability. For
example, in a passenger vehicle, an electric motive power system is
typically electrically isolated from the vehicle chassis. Vehicles
must typically meet minimum isolation standards for shock hazard.
Systems enabling accurate and early detection and characterization
of fault conditions may be valuable contributors to overall system
safety and reliability.
SUMMARY
[0005] In accordance with some embodiments, a battery management
system for monitoring the operation of a battery pack is provided.
The system may further include the cell stack and/or an
electrically-powered object, such as an electric vehicle. The
battery pack may include a cell stack comprised of a plurality of
battery cells electrically connected in series. The system further
contains a first electronically-controlled, switchable resistance
which includes a first switch, electrically connected between a
positive end of the cell stack and ground; and a second
electronically-controlled, switchable resistance which includes a
second switch, electrically connected between a negative end of the
cell stack and ground. The switchable resistances may include a
ladder of resistance with some number n bulk resistors. Each
switchable resistance will preferably exceed an isolation
resistance specification against which compliance is to be tested.
More preferably, each ladder of resistance will have an aggregate
bulk resistance greater than n/(n-1) times an isolation resistance
specification against which compliance is tested. The first and
second resistances may each further include a measured resistance
portion having a known resistance, such that current through them
may be accurately determined by measuring voltage across them using
an analog-to-digital converter.
[0006] In operation, a first current monitor may be configured to
measure a first current I1, through the first
electronically-controlled switchable resistance, when the first
switch is closed and the second switch is open; and a second
current monitor is configured to measure a second current I2,
through the second electronically-controlled switchable resistance,
when the second switch is closed and the first switch is open. The
location of an isolation fault or leakage current within the cell
stack is then determined based on a comparison of I1 and I2. In
some embodiments, an isolation leakage current may be calculated by
a microprocessor controlling the first and second switches, such as
by determining a ratio of I1/(I1+I2) or of I2/(I1+I2). In some
embodiments, the cell stack may include multiple,
physically-separate, electrically-connected cell substacks, such as
separate battery packs installed in varying locations within a
vehicle; in such embodiments, the isolation fault location may be
indicative of which substack or pack is experiencing isolation
leakage current. A magnitude of isolation leakage current may also
be determined, based at least in part on a comparison of a voltage
V1 measured across at least a portion of the first
electronically-controlled switchable resistance, and a voltage V2
measured across at least a portion of the second
electronically-controlled switchable resistance.
[0007] Methods for monitoring the operation of a battery system are
also provided. In some embodiments, steps include measuring a first
current through the first switchable resistance when the first
switch is closed and the second switch is open; measuring a second
current through the second switchable resistance when the second
switch is closed and the first switch is open; and determining
location of an isolation leakage current within the cell stack
based at least in part on the first current measurement and the
second current measurement. The step of measuring a first current
through the first switchable resistance may include measuring
voltage across a known resistor component within the first
resistance; and the step of measuring a second current through the
second switchable resistance may include measuring voltage across a
known resistor component within the second resistance. Embodiments
may also include the step of determining a magnitude of leakage
current based at least in part on the first current measurement and
the second current measurement; and/or the step of initiating one
of a plurality of responses to the isolation leakage current (such
as transmitting a fault condition notification) based at least in
part upon characterization of an isolation fault condition by
mapping the isolation fault location and isolation fault
magnitude.
[0008] In some embodiments, a battery system may include a Y
capacitance between a chassis ground and a motive ground, e.g. in
an electrically-powered vehicle. Such embodiments may include
sampling the first current multiple times following closure of the
first switch while the first current settles towards a steady
state; and/or sampling the second current multiple times following
closure of the second switch while the second current settles
towards a steady state. Preferably, such sampling may be performed
at least from a time of switch closure until the measured current
has fallen by at least 63% from a maximum current value. A
magnitude of the Y capacitance may then be evaluated based at least
in part on data recorded while sampling the first and/or second
current, such as by performing an exponential curve fit of sampled
values against a predetermined model, such as an RC curve model.
Unexpected changes in the magnitude of a measured Y capacitance may
be identified, and a notification may be transmitted in response
thereto.
[0009] In some embodiments, the first switch and the second switch
may be closed simultaneously, such as to partially discharge a Y
capacitance and reduce associated energy. The simultaneous switch
closing may be controlled via hardware and/or software. In some
embodiments, the first and second switches may be simultaneously
closed on a period basis, such as on a duty cycle of at least 20%
or a duty cycle of 25%.
[0010] In some embodiments, the battery system may further include
a powered load isolation switch. A safety pre-test of the first
switchable resistance and the second switchable resistance may be
performed by opening the powered load isolation switch, closing the
first switch and the second switch, and measuring current through
the first resistance and the second resistance. If the currents
through the first and second resistances differ by an amount
exceeding a threshold amount, it may be determined that a failure
condition is present.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated into this
specification, illustrate one or more exemplary embodiments of the
inventions disclosed herein and, together with the detailed
description, serve to explain the principles and exemplary
implementations of these inventions. One of skill in the art will
understand that the drawings are illustrative only, and that what
is depicted therein may be adapted based on the text of the
specification and the spirit and scope of the teachings herein.
[0012] In the drawings, in which non-limiting examples are
illustrated, where like reference numerals refer to like reference
in the specification:
[0013] FIG. 1 illustrates a circuit schematic diagram for
location-determinant fault monitoring (LDFM) by a battery
management system (BMS).
[0014] FIG. 2A is a schematic block diagram of a vehicle
electromotive and control system.
[0015] FIG. 2B is a schematic block diagram of a BMS.
[0016] FIG. 2C is a schematic block diagram of a high voltage power
system.
[0017] FIG. 3 is a process for location-determinant fault
monitoring (LDFM).
[0018] FIG. 4 is a plot of isolation fault location relative to
measured parameters.
[0019] FIG. 5 is a plot of isolation fault location versus measured
isolation resistance, for fault characterization.
[0020] FIG. 6 is a plot of LDFM monitor current settlement over
time in the presence of Y capacitance.
[0021] FIG. 7 is a block diagram of a network-connected BMS
monitoring system.
DETAILED DESCRIPTION
[0022] In the field of battery systems, particularly for battery
and energy diagnosis, management and monitoring, there were
problems associated with certain conventional systems and methods
for achieving shock safety, particularly with high voltage
applications.
[0023] Conventional isolation specifications provide some reduction
in the risk of shock but could be unreliable in certain conditions.
For instance, in some areas of the United States, during cold
winter months, with salt on the roads, exposure of a
battery-powered vehicle to copious amounts of salt and slush or
water may create additional mechanisms for electrical conduction
and may serve to significantly reduce isolation resistance, e.g.,
to the range of 10 k.OMEGA.. In such conditions, contact by a human
with a portion of the vehicle battery or electromotive system under
power may discharge sufficient current through the human to cause
harmful electric shock, thereby increasing risk and reducing
safety. Conventional solutions to this cold weather problem
included isolation monitoring, which simplistically shuts down the
system if isolation fails.
[0024] Related to these systems, the International Organization for
Standardization (ISO) promulgated a standard, i.e., ISO
6469-3:2018, titled "ELECTRICALLY PROPELLED ROAD VEHICLES--SAFETY
SPECIFICATIONS--PART 3: ELECTRICAL SAFETY", which includes two
requirements, (1) a minimum isolation resistance per volt in a
power system (e.g., 500.OMEGA.), and (2) a shock hazard may have a
total available energy of no more than 200 mJ.
[0025] With regard to (1), in some high voltage (e.g., 400V) packs
for electric vehicles, a large number of cells are provided in
series to achieve high voltage levels, with substantial discharge
current capabilities. To reduce the risk of shock in some vehicular
applications, battery system designs typically isolate the high
voltage pack from the chassis of the vehicle. Isolation resistance
(i.e. the effective resistance between the high voltage battery
pack and the vehicle chassis) is preferably designed to allow a
human to touch the chassis and one lead on the battery system
without harmful shock. In some systems, ensuring isolation
resistance of 1 M.OMEGA. or more provides sufficient protection
against harmful electrical shock when a human engages in a single
point of electrical contact with the battery system. To maintain
safe conditions, it may be valuable to actively monitor and
evaluate isolation resistance so that appropriate warnings or
mitigating actions may be taken in the event that conditions cause
reduced isolation resistance. However, conventional systems may be
less effective in monitoring isolation resistance, particularly
with regard to characterizing the location of an isolation
fault.
[0026] Regarding requirement (2), a battery pack or other high
voltage component, such as an inverter, may include a Y-capacitor
connected between the motive ground and chassis ground for purposes
of high frequency EMI reduction. Especially with high frequency
switching (e.g., a motor inverter, switching hundreds of volts),
EMI can wreak havoc with associated systems. Some systems utilize
relatively high value Y capacitors to achieve greater EMI reduction
and mitigate interference with other components. However, increased
Y capacitance may create a shock hazard. That is, with a high value
Y capacitor, if a human makes contact across a chassis ground to a
top of a cell stack (or any component electrically connected
therewith), a resulting capacitor discharge (e.g., on the order of
1/2 EV{circumflex over ( )}2=1/210 .mu.F(400V) {circumflex over (
)}2=800 mJ in some embodiments) to the human can be lethal or
significantly harmful. Some systems were not configured to detect
or reduce the risk of such harmful capacitor discharge.
[0027] It should be understood that the concepts described herein
are not limited to the particular methodology, protocols, etc.,
listed and as such may vary. The terminology used herein is for the
purpose of describing particular embodiments only and is not
intended to limit the scope of the invention, which is defined
solely by the claims.
[0028] As used herein and in the claims, the singular forms include
the plural reference and vice versa unless the context clearly
indicates otherwise. Other than in the operating examples, or where
otherwise indicated, all numbers expressing quantities used herein
should be understood as modified in all instances by the term
"about."
[0029] All publications (including published patents) identified
are expressly incorporated herein by reference for the purpose of
describing and disclosing, for example, the methodologies described
in such publications that might be used in connection with the
invention. These publications are provided solely for their
disclosure prior to the filing date of the present application.
Nothing in this regard should be construed as an admission that the
inventors are not entitled to antedate such disclosure by virtue of
prior invention or for any other reason. All statements as to the
date or representation as to the contents of these documents is
based on the information available to the applicants and does not
constitute any admission as to the correctness of the dates or
contents of these documents.
[0030] Unless defined otherwise, all technical and scientific terms
used herein have the same meaning as those commonly understood to
one of ordinary skill in the art to which this invention pertains.
Although any known methods, devices, and materials may be used in
the practice or testing of the invention, the methods, devices, and
materials in this regard are described herein.
Some Selected Definitions
[0031] Unless stated otherwise, or implicit from context, the
following terms and phrases include the meanings provided below.
Unless explicitly stated otherwise, or apparent from context, the
terms and phrases below do not exclude the meaning that the term or
phrase has acquired in the art to which the term or phrase
pertains. The definitions are provided to aid in describing
particular embodiments of the aspects described herein, and are not
intended to limit the claimed invention, because the scope of the
invention is limited only by the claims. Further, unless otherwise
required by context, singular terms shall include pluralities and
plural terms shall include the singular.
[0032] As used herein the term "comprising" or "comprises" is used
in reference to compositions, methods, and respective component(s)
thereof, that are essential to the invention, yet open to the
inclusion of unspecified elements, whether essential or not.
[0033] As used herein the term "consisting essentially of" refers
to those elements required for a given embodiment. The term permits
the presence of additional elements that do not materially affect
the basic and novel or functional characteristic(s) of that
embodiment of the invention.
[0034] The term "consisting of" refers to compositions, methods,
and respective components thereof as described herein, which are
exclusive of any element not recited in that description of the
embodiment.
[0035] Other than in the operating examples, or where otherwise
indicated, all numbers expressing quantities used herein should be
understood as modified in all instances by the term "about." The
term "about" when used in connection with percentages may
mean.+-.1%.
[0036] In embodiments of the disclosure, terms such as "about,"
"approximately," and "substantially" may include traditional
rounding according to significant figures of the numerical
value.
[0037] The singular terms "a," "an," and "the" include plural
referents unless context clearly indicates otherwise. Similarly,
the word "or" is intended to include "and" unless the context
clearly indicates otherwise. Thus, for example, references to "the
method" includes one or more methods, and/or steps of the type
described herein and/or which will become apparent to those persons
skilled in the art upon reading this disclosure and so forth.
[0038] Although methods and materials similar or equivalent to
those described herein may be used in the practice or testing of
this disclosure, suitable methods and materials are described
below. The term "comprises" means "includes." The abbreviation,
"e.g.", is derived from the Latin exempli gratia, and is used
herein to indicate a non-limiting example. Thus, the abbreviation
"e.g." is synonymous with the term "for example."
[0039] To the extent not already indicated, it will be understood
by those of ordinary skill in the art that any one of the various
embodiments herein described and illustrated may be further
modified to incorporate features shown in any of the other
embodiments disclosed herein.
[0040] The following examples illustrate some embodiments and
aspects of the invention. It will be apparent to those skilled in
the relevant art that various modifications, additions,
substitutions, and the like may be performed without altering the
spirit or scope of the invention, and such modifications and
variations are encompassed within the scope of the invention as
defined in the claims which follow. The following examples do not
in any way limit the invention.
[0041] Location-Determinant Fault Monitoring
[0042] Location-determinant fault monitoring (LDFM) for a battery
management system (BMS) may overcome some or all of the
aforementioned problems encountered with other BMSs for high
voltage packs. The LDFM-BMS may be configured to connect with a
battery pack and validate safety of a power architecture.
[0043] For instance, in some embodiments, the LDFM-BMS includes a
system for determining at least one location in a cell stack (e.g.,
a 92S, 160S, 192S, etc.) of a high voltage pack corresponding with
a source of current leakage, and the extent of such leakage.
[0044] FIG. 1 illustrates a diagram of an example of a battery
system 10 providing LDFM while powering a powered object, such as
an electric vehicle. Cell stack 100 includes a stack of battery
cells electrically connected in series. Common embodiments for
electromotive applications may include, for example, series
arrangements of 96, 160 or 192 cells. While the cell-stack may be
referred to herein throughout as being connected in series, it is
contemplated and understood that in some embodiments, such series
arrangements may include series-parallel cell configurations,
wherein fault locations may be determined at least relative to a
particular parallel set of cells.
[0045] Cell stack 100 creates a voltage potential between top
output 101A and bottom output 101B, which may be hundreds of volts
in common applications. Some BMS may monitor for isolation faults
by measuring current at, e.g., output 101A when the load connected
to cell stack 100 is switched off. If such leakage current exceeds
a threshold level, the BMS may generate a fault warning and take
appropriate action. The magnitude of such leakage current may be
indicative of the severity of the isolation fault.
[0046] However, such monitoring fails to provide significant
information concerning the location of a fault. Without such
location information, repair of a battery system showing isolation
fault may be difficult and/or impracticable. Moreover, in many
battery system applications, cells within cell stack 100 may be
split up amongst multiple discrete, interconnected battery packs.
Multiple interconnected battery packs may be utilized to, for
example, provide greater flexibility in physical configuration
(e.g. in an automotive application subject to weight distribution
and physical volume constraints), or divide up the total battery
weight into multiple units to provide easier handling of heavy
battery modules. With fault location information, a faulty battery
pack may be identified and swapped out, or faulty pack
interconnects may be identified and repaired, potentially improving
field repairability and/or reducing cost of system repair.
[0047] Several mechanisms and techniques may be utilized to detect,
locate and characterize isolation faults in the power system 10 of
FIG. 1. FIG. 2A illustrates an embodiment of an overall vehicle
system in which power system 10 may be implemented. Referring first
to FIG. 2A for system context, power system 10 includes high
density battery system 100, and integrated battery management
system (BMS) 202. BMS 202 is typically powered by a vehicle 12 volt
line and grounded by chassis ground, enabling communication via a
common vehicle digital communications bus. BMS 202 may include a
number of interconnections within power system 10, including a
number of temperature sensors, voltage sensors and current sensors
distributed throughout battery stack 100 and power system 10,
monitoring operating conditions associated with various portions of
the system. A number of sensors and analog-to-digital converters
may be implemented (whether on board BMS 202 or distributed within
battery pack 100 and connected digitally to BMS 202). BMS 202 then
communications with vehicle management unit (VMU) 230 via a digital
communications bus 210. In vehicle applications, digital
communications bus 210 is commonly implemented using the CANBUS
standard. VMU 230 in turn transmits control signals to vehicle
drive inverters 240, which are driven by current from power system
10, and which inverters in turn supply power to electric motors or
other loads within the system. While VMU 230 may be referred to as
a vehicle management unit, it is contemplated and understood that
in non-vehicular applications (such as stationary energy storage or
other industrial applications), VMU 230 may instead be another
system controller, external to power system 10, involved in control
of an electrical load to be powered by power system 10.
[0048] FIG. 2B is a simplified block diagram of portions of BMS
202. BMS 202 includes a microprocessor 260, digital memory 261
(storing, amongst other things, software instructions utilized to
cause microprocessor 260 to perform various calculations,
operations and other functions described elsewhere herein) and
multiple analog-to-digital converters (ADCs) 262. Optionally, a
communications interface such as modem 263 may be provided in order
to enable digital communications with remotely located systems; in
some embodiments, modem 263 will include, for example, an ethernet
interface, wireless ethernet interface and/or cellular data modem
to enable communications between BMS 202 and cloud-based or
network-connected systems, such as for automatic updates, remote or
centralized battery system performance monitoring, and/or fault
alerting. In other embodiments, a network communications interface
may be provided externally to BMS 202 (such as via another system
connected to communications bus 210).
[0049] Various levels of integration may be provided between BMS
202 and a battery pack. In some embodiments, one or more components
of BMS 202 may be implemented within the battery pack itself. In
other embodiments, BMS 202 may be completely physically separate
from, but electrically connected with, the battery pack. In some
embodiments, LDFM assembly 118 may be physically housed within a
separate BMS 202, while in other embodiments, LDFM assembly 118 may
be physically housed within a battery pack (e.g. battery system 10
of FIG. 1) and electrically interconnected with a separately-housed
BMS. In yet other embodiments, some or all components of BMS 202
may be physically integrated with a battery pack. FIG. 2C
illustrates one exemplary relationship between BMS 202 and a
battery system 10, in which BMS 202 monitors output terminals 102A
and 102B of battery system 10, while providing control lines 270
(e.g. to operate switches and measure voltages within battery
system 10). Embodiments illustrated herein are intended to be
illustrative, with the understanding that other variations and
permutations may be readily implemented.
[0050] Unlike conventional BMS's, which monitor overall battery
pack voltage and output current, in some embodiments, the
LDFM-enabled BMS 202 may be configured to connect/disconnect a high
side and/or a low side of cell stack 100 to a chassis ground
through a switchable ladder of resistance, as shown in FIG. 1. In
particular, battery system 10 may include a LDFM assembly 118. LDFM
assembly 118 includes a resistor 110 and electronically-controlled
switch 111 connected in series between positive terminal 101A of
cell stack 100, and chassis ground 112. LDFM assembly 118 further
includes resistor 114 and electronically-controlled switch 115
connected in series between negative terminal 101B of cell stack
100 and chassis ground 112.
[0051] In some embodiments (which may be appropriate for many
electric vehicle applications), switches 111 and 115 may be
1000V-rated switches. Resistors 110 and 114 are preferably resistor
packs formed from multiple resistors, such that failure of any one
resistor will not create an undesirably high discharge rate when
its associated switch is closed. The resistance of resistors 110
and 114 should be greater than the isolation resistance
specification for which compliance is to be tested, but low enough
that current flows through them can be accurately measured by a
cost-effective ADC implemented within BMS 202 or elsewhere. Thus,
for a resistor pack comprised of n bulk resistors seeking to test
against a X M.OMEGA. minimum isolation resistance, the aggregate
bulk resistance of the resistor pack will preferably exceed n/(n-1)
times X, such that any one resistor within the resistor pack may
fail while still ensuring that the resistor pack resistance exceeds
the desired isolation resistance specification.
[0052] As a practical example, in an electric vehicle with a 400V
battery system and a 1 M.OMEGA. minimum isolation resistance
specification, resistors 110 and 114 may each comprise a 5.1
M.OMEGA. resistor pack formed from bulk resistance (e.g. five
resistor bodies having 1 M.OMEGA. resistance each) and a 100
k.OMEGA. measured resistance. In such an embodiment, if one of the
bulk resistors fails, the resulting interconnection between cell
stack 100 and chassis ground 112 will still have a resistance of at
least 4 M.OMEGA., which may remain well above the minimum isolation
resistance specified by applicable standards (e.g. 1 M.OMEGA. for
ISO 6469-3:2018).
[0053] An analog-to-digital converter (e.g. ADCs 262 integrated
within BMS 202, or ADCs otherwise connected to BMS 202) may be
applied across the 100 k.OMEGA. measured resistor portion of
resistors 110 and 114, to enable BMS 202 to determine current flow
through resistors 110 and 114, upon closure of switches 111 and
115, respectively. A position of an isolation fault may be
determined by measuring current flow through resistor 110 when
switch 111 is closed and switch 115 is open, and by measuring the
current flow through resistor 114 when switch 111 is open and
switch 115 is closed.
[0054] In particular, FIG. 3 illustrates a process for operating
LDFM assembly 118 (e.g. by a microcontroller within BMS 202) to
evaluate the location and magnitude of an isolation fault. In step
S300, microprocessor 260 operates to temporarily close switch 111.
In step S310, microprocessor 260 measures current through resistor
110, e.g. by measuring voltage across a calibrated or measured
resistance portion of resistor 110 using an ADC 262 within BMS 202.
In step S320, switch 111 is opened and switch 115 is closed. In
step S330, current through resistor 114 is measured similarly (e.g.
by measuring voltage across a calibrated or measured resistance
portion of resistor 114 using an ADC 262 within BMS 202).
[0055] In step S340, BMS 202 identifies location of an isolation
fault based on a comparison of currents measured in steps S310 and
S330. For example, upon closing bottom (negative) switch 115 in
step S330: if natural leakage is on the negative side 101B of
battery pack 100, no change is observed (i.e. no current flows
through resistor 114); on the other hand, if leakage is present on
the positive side 101A, then a current can be measured through
resistor 114. Upon closing of top (positive) switch 111 in step
S310: if natural leakage is on the negative side 101B, a current
can be measured through resistor 110; and, if leakage is on the
positive side 101A, then no change is observed (i.e. no current
flows through resistor 110). If the isolation fault is in the exact
middle of cell stack 100 (for instance, between a pair of different
battery sub-packs, or if there is a bad cell in the middle of cell
stack 100 (e.g., a tin whisker creating a path to ground)), then
equal current can be measured through resistor 110 in step S310
(when switch 111 is closed) and through resistor 114 in step S330
(when switch 115 is closed). As such, by operating BMS 202 to
alternatively close switches 111 and 115, the ratio of measured
currents can be used to identify the relative location within cell
stack 100 at which an unintended leakage current (i.e. isolation
fault) is located.
[0056] FIG. 4 illustrates one exemplary mapping of measured
voltages to fault location. Horizontal axis 400 comprises the ratio
of current measured in step S310 (IA to the sum of currents
measured in step S310 and S330 (I.sub.1+I.sub.2) (both normalized
based on the magnitude of current measured in step S310). (Provided
that the measured resistances in resistor packs 110 and 114 are the
same, the voltages measured across those resistors may be used
directly in calculating the ratio.) Vertical axis 410 is the
position (electrically) of the isolation fault as between cell
stack top 101A (positive side) and cell stack bottom 101B (negative
side). If the ratio is zero (i.e. no current/voltage is measured in
step S310), then the isolation leakage current is determined to be
located at cell stack positive side 101A. If the ratio is 1 (i.e.
no current/voltage is measured in step S330), then the isolation
leakage current is determined to be located at cell stack negative
side 101B. Ratios between 0 and 1 indicate the relative position of
isolation leakage current as between the positive and negative
sides of the cell stack. For example, if the ratio is 0.5 (i.e.
currents measured in steps S310 and S330 are exactly equal) then
the isolation leakage current is determined to be located half way
between positive side 101A and negative side 101B. While the
formula I.sub.1/(I.sub.1+I.sub.2) may be used in some embodiments,
it is contemplated and understood that other formulas may be
likewise used, based on the above-described principals. For
example, another embodiment may use the formula
I.sub.2/(I.sub.2+I.sub.1), with a zero result indicative of fault
location at the negative terminal 101B, a one result indicative of
fault location at the positive terminal 101A, and results in
between being proportional to the distance of the leakage current
location between negative terminal 101B and positive terminal
101A.
[0057] Additionally, the absolute magnitudes of currents through
resistors 110 and 114 measured in steps S310 and S330 may be used
by BMS 202 to evaluate the magnitude of the isolation fault (step
S350). In particular, in some embodiments, the LDFM-enabled BMS 202
may be configured to control switches 111 and 115. When the
switches 111 and 115 are open, zero voltage is measured across
measurement resistors 110 and 114, respectively as it is an open
circuit. Upon closing top switch 111, an electrical path with a
resistance of 5.1 M.OMEGA. is inserted between battery positive
101A and chassis ground 112. The 5.1 M.OMEGA. is a fixed resistance
defined by resistor 110. If no leakage or parasitic resistance is
present, no current will flow and zero voltage is measured across
resistor 110. However, if a leakage (or parasitic) resistance X is
present between any point besides node 101A and chassis ground, a
small current will flow, resulting in a measurable, non-zero
voltage across resistor 110. The magnitude of this current is equal
to I=(Vbattery-Vposition_of_leakage)/(5.1 MOhm+X). In a similar
fashion, control switch 115 can be closed to measure the current
flow through resistor 114. If a leakage (or parasitic) resistance X
is present between any point besides node 101B and chassis ground,
a small current will flow. The magnitude of this current is equal
to I=(Vposition_of_leakage)/(5.1 MOhm+X). By solving for X and
Vposition_of_leakage with these two equations, both a magnitude and
electrical position of the new leakage resistance can be
determined.
[0058] In this way, LDFM assembly 118 may be utilized by BMS 202,
or an accessory system to which BMS reports, to characterize an
isolation fault, such as a characterization of whether remedial
action is appropriate (such as communication a fault alert and/or
automatically depowering a system). In some embodiments, BMS 202
may determine both the magnitude and location of an isolation
fault, and portray the two measurements as X and Y locations on a
fault characterization graph. Isolation faults may be plotted
accordingly, with plot regions utilized to classify types of
faults. FIG. 5 illustrates an exemplary plot 500 of fault location
within cell stack 100 on the Y axis 501, and isolation resistance
magnitude on the X axis 502. Faults within region 510 correspond to
acceptable modes of operation and faults within region 520
correspond to unsafe operating conditions. In other embodiments,
additional or alternative types of fault characterizations may be
applied as fault characterizations, for example: acceptable
isolation resistance, acceptable but concerning isolation
resistance (potentially triggering a warning or notification for
further monitoring or investigation), and/or one or more categories
of unacceptable isolation resistance associated with different
responsive functions (e.g. notifications, and/or alteration of
system operation potentially including electromotive system
shutdown). In some embodiments, additional or alternative fault
measurements may be utilized to characterize a fault; in some such
circumstances, fault characterizations may effectively be mapped in
a 3D or hyperdimensional space.
[0059] In practice, in some embodiments, it has been found that the
LDFM-enabled BMS configuration shown in FIGS. 1 and 2 may be used
to measure parasitic resistance up to 50 M.OMEGA.s with reasonable
accuracy and using standard commercial components. In some
embodiments, locations of a parasitic resistance up to 50 M.OMEGA.s
could be determined within a cell or two within a stack of over one
hundred cells. As parasitic resistance and capacitance varies, the
precision with which the LDFM-BMS may locate an isolation fault may
vary as well. For example, for a relatively small, i.e., 1-2 mJ,
shock hazard system, accuracy may be relatively poor, i.e., on the
order of +/-1 to 5 cells in a 192S cell stack. Whereas, with a
relatively large shock hazard (i.e. increased Y capacitance),
precision is relatively accurate, potentially enabling reliable
identification of a specific cell at which an isolation fault
occurs.
[0060] In some embodiments, as mentioned above, for some
applications such as certain vehicular applications, cell stack 100
may be comprised of multiple separate battery modules (i.e. cell
substacks), potentially located in distinct physical locations
within a vehicle. For example, a vehicular battery pack may be
comprised of multiple interconnected 24S packs, potentially
provided improved part modularity and/or improved flexibility for
physical positioning within a vehicle. Use of LDFM embodiments
described herein may be particularly helpful in embodiments with
multiple separate battery packs, as an engineer can more readily
ascertain which of a plurality of packs is leaking.
[0061] The battery pack formed from cell stack 100 may be connected
to external systems at output terminals 102A and 102B, via a
switching network 120, to avoid unintended discharge and improve
safety. Switching network 120 may include a first branch comprising
a series combination of resistor 121 and switch 122, and a second
branch comprising switch 123, the second branch being in parallel
with the first branch. During application of power from cell stack
100 to a load connected at terminal 102A, switch 122 may be closed
first, with resistor 121 providing a current-limiting function in
the event that an improper load is connected. Upon determining that
the load is appropriate (e.g. by measuring current through resistor
121), switch 123 may be closed to provide the full output capacity
of cell stack 100.
[0062] A LDFM-BMS may also be utilized to improve upon problems
associated with maximum shock hazard energy (such as the second
portion of the aforementioned ISO shock standard, imposing a 200 mJ
maximum shock hazard). In some embodiments, the LDFM-BMS may be
configured to detect and characterize a Y capacitance, by
monitoring current flow through a resistor over time.
[0063] As described above, battery pack embodiments often include a
Y capacitor, placed between chassis ground and motive ground (such
as EMI Filter 130 in the embodiment of FIG. 1). Such a capacitor
may be beneficial in reducing EMI interference. EMI interference
can be particularly problematic in electromotive vehicle
applications, e.g. with motor inductors performing high-frequency
switching of hundreds of volts. In particular, electric racing
vehicles are often equipped with particularly high-value Y
capacitors.
[0064] While Y capacitors may be useful in reducing EMI
interference, such implementations also may create an additional
shock hazard. For example, when the Y capacitor is energized, if a
human touches across chassis ground and the top of a cell stack,
the energy of the Y capacitor may be fully discharged into the
human's body despite high levels of isolation resistance. Depending
on the capacitance, the amount of energy discharged could be
harmful or even fatal. Thus, detecting and characterizing Y
capacitance may be highly beneficial for safety and/or other
purposes.
[0065] Thus, in some embodiments, LDFM assembly 118 is utilized to
detect and characterize Y capacitance in a power system. In
particular, when switch 111 is closed (such as in step S300),
rather than simply measuring steady-state current through (or
voltage across) resistor 110 (e.g. in step S310) (or through
resistor 114 in step S330 after switch 115 is closed in step S320),
the current through (i.e. voltage across) resistors 110 and 114 is
sampled over time (e.g. in steps S310 and S330). The system's
Y-capacitance (such as caused at least in part by EMI filter 130
comprising Y capacitor 132 and resistance 134) provides a
capacitance in parallel with resistor 110 (when switch 111 is
closed) or resistor 114 (when switch 115 is closed).
Characterization of the current flow through a resistor in a
parallel RC circuit is well-known. Thus, the current/voltage curve
for resistors 110 and 114 can be sampled over time by BMS 202, with
the resulting sampled data then characterized according to known RC
circuit behavior to solve for the value of Y capacitance.
[0066] Thus, some embodiments are configured to: plot magnitude of
current through (or voltage across) resistor 110 or 114 over time,
which tends to start relatively high and decrease over time to
generate a settling curve (i.e. an RC curve) (as illustrated in
FIG. 6), which is a two-parameter model; and from that, calculate
leakage resistance and Y capacitance. In accordance with an
illustrative embodiment, exemplary parameters may be as follows: if
resistors 110 and 114 are 5 M.OMEGA. each and Y capacitance is 10 g
(a high value, but potentially valuable for certain applications),
then the RC curve has a time constant of about 50 seconds.
Approximately one time constant is a good measurement period to be
able to extrapolate the leakage resistance and Y capacitance, with
a least squares exponential curve fit. Since the Y capacitance is
unknown when initially sampling, waiting for the measurement
current to fall by 63% from its initial value can be used as the
determining factor as to when to stop sampling.
[0067] While Y-capacitance is typically caused by an inverter EMI
filter, in some circumstances, other system effects may cause or
contribute to Y capacitance. Thus, embodiments described herein may
further be utilized to monitor (potentially over time) for changes
in, or unexpected, Y capacitance. Warnings or other notifications
may then be transmitted (e.g. by BMS 202) upon identification of a
change over time in Y capacitance, exceeding a threshold level of
change. For instance, some battery packs may be configured with
numerous parallel plates, which are stacked relatively close to one
another for thermal performance. With such configurations, a
relatively high dielectric is provided for isolation, which
effectively forms a capacitor. In such cases, parasitic Y
capacitance, e.g., up to 10 nF in some circumstances, may be
observed. Such levels of parasitic Y capacitance may be considered
in the overall system design.
[0068] Furthermore, in some embodiments, components described
herein may be operated to not only measure, but also mitigate
overall shock potential. Specifically, when both switches 111 and
115 are closed, the Y capacitance charges up to a maximum of 1/2
the pack voltage rather than 1 times the pack voltage. By
periodically closing both switches, energy may be bled from the
Y-capacitance and potential discharge current from parasitic
capacitance may be reduced. In many embodiments, closing both
switches 110 and 114 with a 25% duty cycle will be sufficient to
provide mitigation of discharge current from parasitic capacitance.
If the Y capacitance is sustained at an average charge of 200V
instead of 400V, this effectively moves the Y capacitance to the
middle of the stack. Because energy stored in a capacitor is
proportional to the square of the capacitance charge voltage (i.e.
1/2 CV.sup.2), reducing the capacitance voltage level in half will
reduce the discharge energy by 75%. Considering the above-described
embodiment with a 10 .mu.F Y capacitor installed in a 400V battery
system potentially yielding an 800 mJ shock hazard, operating
switches 110 and 114 to maintain average capacitor charge of 200V
reduces the maximum shock hazard to 1/210 .mu.F(200V){circumflex
over ( )}2=200 mJ instead of 800 mJ. As such, even with a
relatively high capacitance for EMI reduction purposes, unlike
typical conventional systems, the shock hazard may still be
maintained at or near the ISO standard limit.
[0069] Switches may be controlled with software and/or hardware. In
the context of using switches to achieve shock safety, hardware
control is preferred to reduce likelihood of failure. For example,
in some embodiments, switches 111 and 115 are hardware controlled
with a fixed 20% duty cycle.
[0070] Still further, analogously to the detection of parasitic
capacitance, BMS 202 may be configured to detect inductance in the
system by monitoring current through resistors 110 and 114 over
time, using well-known inductance curves.
[0071] Many systems, such as electric vehicle systems, that involve
high voltage (sometimes on the order of 1000V), are equipped with
switches and pre-charge systems that control the connection of the
battery pack to the rest of the high voltage system. In such
embodiments, the LDFM-BMS may be configured into the battery pack
to permit testing of the system with open pre-charge switches so as
to measure the parasitic resistance and capacitance of the battery
pack as an isolated subunit within the high voltage system; and
subsequent closing of the pre-charge switches so as to connect to
the rest of the high voltage system and test the parasitic
resistance and capacitance of the high voltage system as a
whole.
[0072] The LDFM-BMS may be configured to conduct a safety pre-test
for failure of 5 M.OMEGA. test branches within LDFM assembly 118 as
a self-test, by closing both switches 111 and 115, and determining
whether the pack is isolated based on a comparison of a current
through both 5 M.OMEGA. resistors 111 and 115. That is, the system
signals proper functioning and isolation when the same current is
flowing through both 5 M.OMEGA. resistors (i.e., the difference
between currents measured through resistor 111 and resistor 115 is
below a predetermined threshold), and improper functioning or
failure when a different current is flowing through both 5 M.OMEGA.
resistors (i.e., the difference between currents is above the
predetermined threshold). Also, the system may be configured so
that a calculated isolation resistance below a predetermined
threshold stops use of the test circuit.
[0073] In some embodiments, the LDFM-BMS 202 may be configured for
connection to the internet, such as via modem 263 or via another
network communications interface provided by an external system
(e.g. another system connected with communications bus 210) for
communication of information from BMS 202. In such circumstances,
the LDFM-BMS may be configured to allow central monitoring for one
or more vehicles. FIG. 7 illustrates such an embodiment. A
plurality of BMS 202A to 202n, may communicate via data network 700
(preferably including the Internet) with a remote server 710.
Server 710 may aggregate fault information from BMS 202A-n. Server
710 may be configured to aggregate generate indicia of fault
reported from BMS 202A-n, e.g., by generating a report linking a
vehicle identification to an isolation fault, i.e., "Vehicle VIN
<17 alphanumeric values> has an isolation fault". As such,
the server 710 may be configured to determine, for example, if a
vehicle returning a fault is in a location subject to extreme
environmental conditions that might affect system performance (such
as cold weather and salt-covered roads), potentially observed to be
impacting multiple vehicles in the region, or if the vehicle is
reporting an anomalous fault that appears to be linked to system
malfunction.
[0074] The above disclosures and descriptions are exemplary in
nature, and not intended to limit the scope of the invention. Any
person skilled in the art given the present disclosures could
design variations and additional embodiments of the same invention
based on these disclosures, which are all covered by this
application for letters patent.
[0075] Although some of various drawings illustrate a number of
logical stages in a particular order, stages which are not order
dependent may be reordered and other stages may be combined or
broken out. Alternative orderings and groupings, whether described
above or not, may be appropriate or obvious to those of ordinary
skill in the art of computer science. Moreover, it should be
recognized that the stages could be implemented in hardware,
firmware, software or any combination thereof.
[0076] The foregoing description, for purpose of explanation, has
been described with reference to specific embodiments. However, the
illustrative discussions above are not intended to be exhaustive or
to be limiting to the precise forms disclosed. Many modifications
and variations are possible in view of the above teachings. The
embodiments were chosen and described in order to best explain the
principles of the aspects and its practical applications, to
thereby enable others skilled in the art to best utilize the
aspects and various embodiments with various modifications as are
suited to the particular use contemplated.
* * * * *