Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: BACK-UP POWER SYSTEM
FIELD OF THE INVENTION
The present invention relates to the field of power systems. More
specifically, the
invention relates to a back-up power system, as well as to a method for
determining a
battery string back-up time that can be provided by a plurality of batteries,
at all times,
in a given application requiring a given current load.
BACIfGROUND OF THE INVENTION
Electric public utilities normally provide energy to telecommunication
networks.
Historically, the traditional wire line telephony networks have been required
to have
extremely high reliability levels (between 99.999% and 99.99999%) to handle
lifeline
services such as 911 and alarm systems, while the electric utilities only
offer a 99.9%
reliability level. It is therefore important and considered best practice for
telephone
companies to have 8 hours of standby energy to power their network equipment
in the
event of a power outage. More recent telecommunication technologies, such as
wireless
and broadband, are also moving towards a high level of network reliability.
Batteries for power sources are usually provided in banks or strings, for
example, a
string of 24 batteries is often used for back-up purposes in central offices
of
telecommunications providers and in remote locations of transmission stations.
These
backup battery power systems provide the energy to power equipment in the
event of
an electrical outage or failure. Therefore, maintaining the reliability of
battery power
systems, especially backup battery power systems, is extremely important.
Further, it is
important to be able to predict the level of power needed in case of power
outages or
failures and plan where extra.batteries may be needed.
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For the last century, operators and equipment suppliers have struggled to
offset the
costs and risks associated with battery reliability in the hostile remote
environment.
Much has been invested in lead-acid battery design effort, in charging system
design,
and in monitoring and prediction algorithm technology to overcome the problems
associated with lead-acid batteries. It is now recognized that lead-acid
batteries have
reached the maximum performance attainable by their electrochemical system,
and that
fundamental issues related to the unpredictability of their end-of life
coupled with their
short life under field conditions are not going to be resolved.
End-users, either in the telecommunication industry or in other industries
having
similar requirements, have a need to maintain reliability at required levels.
This need
cannot be filled when using lead-acid batteries because of the unpredictable
nature of
their electrochemical system. Its is almost impossible to accurately determine
their
State-Of Charge (SOC) or State-Of Health (SOH) over the life of lead-acid
batteries.
Typically, the only time at which end-users have an accurate measure of the
batteries'
SOC and SOH is during installation of brand new strings of batteries. As soon
as the
batteries have been exposed to field conditions, end-users cannot .
dynamically
determine the battery's SOC and SOH except by performing a deep battery
discharge,
which affect the SOC and SOH and requires every equipment site to be visited
by end-
users. Furthermore, the reserve time required at each equipment site
(typically 8 hours)
cannot be estimated or calculated due to the lead-acid unpredictability and
the fact that
electrical load cannot be monitored, calculated or integrated to the battery
system.
Traditional maintenance of lead-acid battery strings in the telecommunications
industry
has focused on a series of routines mandating periodic measurements of battery
parameters, such as cell voltage and specific gravity. It was thought that if
batteries
were physically maintained with proper water levels, visual inspections, and
correct
voltage and specific gravity readings, the batteries would provide the
necessary
capacity when needed. However, when forced on-line, batteries often failed or
produced far less than stated capacity even if they were properly maintained.
It is now
well-settled that these types of measurements are not accurate predictors of
battery
capacity.
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Various systems and methods have been devised to predict or monitor State-Of
Charge
of lead-acid batteries over their life span. For instance, US Pat No.
6,211,654 discloses
iterative calculations based on voltage readings at specific intervals to
estimate the
remaining back-up time or current discharge capability of a lead-acid battery.
The
method disclosed provides only a rough estimate of the back-up time and does
not take
into account temperature variables, specific loads of the equipment, and
battery age
and/or deterioration.
Lithium Polymer (LP) batteries on the other hand have relatively high density
energy
(high energy generation in a low volume package), relatively high safety
margins, and
produce energy from a highly predictable electrochemical system. Lithium
Polymer
batteries are equipped with on-board control and monitoring integrated
electronics able
to accurately measure each battery's SOH and SOC individually taking into
account
temperature variables.
More advanced systems and methods were devised for non-specific types of
batteries to
monitor a battery back-up power system. For instance, US Pat. No. 5,705,929
discloses
a method and apparatus for centrally monitoring the capacity of batteries in a
battery
string including electrical leads connected to each battery terminal of the
battery,string.
A capacity testing system a) switches between the electrical leads for
sequentially
selecting the leads associated with the terminals of each battery, b) measures
the
internal resistance of the battery associated with each selected pair of
electrical leads, c)
compares the internal resistance of each battery cell to an internal
resistance threshold,
and d) triggers an alarm when the internal resistance of a battery exceeds the
internal
resistance threshold. A central monitoring station monitors battery capacity
data and
alarm signals from various battery strings, schedules battery capacity
testing, transmits
control commands to each capacity testing system for i) scheduling testing,
ii)
initialising upload of capacity data, and iii) requesting status information,
provides
battery capacity data analysis, and uploads information to a network
management
computer. This system is an improvement over the previous manual testing
procedures
however it falls short in that it can only determine the apparent State of
Health of the
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battery power system as good or not good, detecting malfunctions of the
batteries
(alarms) and relaying the alarms to a central monitoring system. This system
is unable
to accurately predict battery back-up time based on real time data. When an
actual
power outage occurs, the end user is left hoping that the back-up system will
last.
Furthermore, when testing batteries to evaluate their capacity or state of
health, most
systems and apparatus known draw current from the batteries by placing a
resistive
load at the battery terminals for a short period of time. This leads to energy
waste as
the batteries must be recharged.
To fulfill the requirements of the telecommunications industry and other
critical
industries using battery packs as back-up power systems when electric public
utilities
fail, there is a need for a reliable monitoring system that accurately
predicts battery
back-up time based on real time data and on changing equipment load.
SUMMARY OF THE INVENTION
According to a broad aspect, the invention provides a back-up power system
having a
monitoring system for determining, and for allowing remote monitoring of, a
back-up
time that can be provided by a plurality of batteries, at all times, in a
given application
requiring a given current load. The back-up power system includes a plurality
of
batteries, each having an integrated circuit adapted to monitor individual
battery's state
of health. The back-up power system also includes a data management unit for
evaluating the back-up time available from the plurality of batteries. The
back-up time
is evaluated on the basis of a sum of individual battery available capacity, a
measured
ambient temperature and a continuously updated measured application current
load.
The value of available back-up time is accessible to a remote user via a
communication
link of the back-up power system.
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BRIEF DESCRIPTION OF THE DRA WINGS
A detailed description of examples of implementation of the present invention
is
provided hereinbelow, with reference to the following drawing, in which:
Figure 1 is a diagram of a back-up power system of telecommunication equipment
installed in a remote telecommunication outside plant.
In the drawing, embodiments of the invention are illustrated by way of
example. It is to
be expressly understood that the description and drawings are only for
purposes of
illustration and as an aid to understanding, and are not intended to be a
definition of the
limits of the invention.
DETAILED DESCRIPTION
Figure 1 is a diagram illustrating a string 10 of Lithium Polymer (LP)
batteries 10(1) to
10(n) installed as a back-up power source in a remote telecommunication
outside plant
typical to telecommunication networks. Figure 1 also depicts a monitoring
system 12
comprising a Data Management Unit 25 (DMU) and a Load Transmitter Unit 26
(LTU). The monitoring system 12 provides centralized monitoring of battery
capacity
for each battery 10(1) to 10(n), and of the total capacity of battery string
10. In such
applications, the system is totally automated and requires no manual
intervention after
being installed and initialized. The telecommunications industry generally
utilizes
strings of batteries, i.e., groups of batteries attached in parallel, series,
or both, to
supply DC power to telecommunications equipment. With the use of Lithium
polymer
batteries in a back-up telecommunications application, two to eight LP
batteries, rated
at 70Ah would be connected in parallel to form a battery string having a total
theoretical capacity of between 140 Ah to 560 Ah. The particular voltages and
amperages discussed are provided by way of example only, it being understood
that
depending upon the particular telecommunications or other applications, the
batteries or
battery strings may have different terminal voltages, different ratings,
smaller batteries
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may be connected in series in strings themselves connected in parallel,
different
number of batteries may be connected together, etc.
An example of implementation of a back-up power system, according to the
present
invention, is shown in the FIG. 1. The back-up power system, or source,
includes a
plurality of LP batteries 10(1) to 10(n), all of which are connected in
parallel
connection to provide power to a load 20 such as a telecommunication cabinet,
when
the public utility network fails. A rectifier 22 receives AC line power from
the public
utility network and provides power to load 20. Rectifier 22 is connected to
load 20 and
to battery string 10 and provides a rectified charging voltage to recharge the
string of
LP batteries 10(1) to 10(n) when required. A data management unit 25 having
memory
for storage of data and program algorithms for data processing is connected to
the
integrated control and diagnosis circuit of each battery 10(1) to 10(n) and to
a load
transmitter unit 26. Data management unit 25 further comprises an external
communication port 28 adapted to send and receive data to and from a remote
user 30
either through a local connection such as a portable computer using CAN, USB,
RS-
232, IrDA or TCP-IP protocols or through Internet or telephone linked to a
remote
network monitoring station using TCP-IP or modem protocols.
As previously mentioned each LP energy module 10(1) to 10(n) comprises an
integrated circuit which perform, at regular intervals, diagnosis evaluation
of the
module's individual critical parameters such as: its state of health (SOH);
its state of
charge (SOC); its initial capacity; its delivered capacity; and feeds this
data to data
management unit 25 along with its electronic signature which is stored into
the memory
of data management unit 25. The state of charge at the module level is defined
as:
SOC(%) _ (initial capacity - delivered capacity) / initial capacity
expressed as a percentage. The integrated circuit also generates an alarm
signal which
is relayed to data management unit 25 if it finds any deficiencies within its
module such
as damaged or defective cells.
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When performing its diagnosis, the integrated circuit of each module 10(1) to
10(n)
preferably measures the internal resistance of each individual electrochemical
cell of a
module during a small charge or discharge, one cell at a time, to establish
the state of
health of the module. The state of health of a module represents the
deterioration of the
module through chemical degradation and aging and is expressed as a percentage
of the
initial capacity of the module. The percentage value of the state of health of
the
module is applied to the initial capacity in the calculation of the module's
state of
charge. The state of health's value is also used to determine the selection of
an initial
capacity based on a corresponding discharge curve stored in the memory of the
integrated circuit, which is used to calculate the delivered capacity. The
delivered
capacity is calculated as the current delivered by the module over time, which
represents the area under the discharge curve. The selected initial capacity
and the
module's state of charge, adjusted with the state of health value, determine
the exact
available capacity of the module.
Although small, these repetitive discharges are routed into load 20 to avoid
wasting any
energy while performing diagnosis routine. This feature of the testing
procedure
enables the system to monitor the state of modules 10( 1 ) to 10(n) at any
time with
minimal energy wastage.
The determination of the available capacity of each module 10(1) to 10(n) may
also be
calculated at the level of the Data Management Unit 25. In this case,
discharge curves
corresponding to various states of health are stored in the memory of data
management
unit 25 instead of the memory of the integrated circuit and its selection
based on
measured internal resistance is performed by data management unit 25. Data
management Unit 25 receives from each modulel0(1) to 10(n), its state of
health value
and monitors the current delivered by each module over time to calculate the
delivered
capacity of each modulel0(1) to 10(n). Each module's available capacity is
then
calculated based on the selected initial capacity and the delivered capacity
for each LP
module 10(1) to 10(n).
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Data management unit 25 also monitors load 20 and the cabinet's ambient
temperature
through load transmitter unit 26. Specifically, a DC current transducer 34
arid a
thermocouple 35 are connected to load transmitter unit 26 which in turn
provides data
management unit 25 with ongoing readings of the current drawn by load 20 and
actual
temperature inside the telecommunication cabinet. The ongoing reading of the
current
drawn by load 20 is required to establish the exact power requirement of the
telecommunication equipment at any given time in order to effectively evaluate
the
exact back-up time available from battery string 10 relative to the load
current. Load
fluctuates according to demand and will affect the back-up time available from
battery
string 10. Furthermore, since telecommunication loads are likely to increase
due to
additions of regular and high speed lines, the power requirements are likely
to increase
which in effect will decrease the back-up time available from battery string
10. Data
monitoring unit 25 monitors load changes in the telecommunication equipment to
insure that when battery string 10 is no longer capable of providing an
effective eight
hours of back-up time, remote user 30 is made aware that additional modules
10(n) are
required to compensate for the load increases.
Each LP modules 10(1) to 10(n) comprises one or more heating element 14
required to
maintain or raise the electrochemistry of the LP module to an optimal
temperature for a
given condition (floating, charge and discharge). Heating elements 14 are
resistive
elements electrically driven, drawing their required current directly from
their
respective modules 10(1) to 10(n). The energy drawn by heating elements 14 is
therefore not available for as back-up energy and must be subtracted from the
individual module's initial capacity to obtain the exact total available
capacity of the
entire module string 10 to the load in case of power outage or failure.
Through
experiments, the capacity consumed by heating element 14 through a full
discharge has
been established as a function of the cabinet's ambient temperature. When the
ambient
temperature is low (e.g., -40°C and the like), the heating elements 14
will require more
energy then when ambient temperature is high (e.g., 30°C and the like)
and closer to
the optimal temperature for discharge condition which is around 60°C.
For example,
when the ambient temperature of the cabinet is -20°C, 9Ah will be
consumed by
heating element 14 over a full discharge at a load current of C/8. The entire
range of
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capacity consumed by heating element 14 for ambient temperature ranging from -
40°C
to 65°C and for various load current has been tabulated and stored into
the memory of
data management unit 25.
Data management unit's 25 primary function is to monitor the total available
capacity
of the entire module string 10, the state of health of each module 10(1) to
10(n), to
calculate the available back-up time of the power system and make this
information
available to remote user 30. When the critical parameters of each module 10(1)
to 10(n)
(state of health, available capacity, initial capacity, delivered capacity,
load current at
module level) and data from load transmitter unit 26 (ambient temperature and
current
load) have been received and stored into memory, data management unit 25
proceeds to
calculate the total available capacity of the entire module string 10. Data
management
unit first retrieves from memory the corresponding capacity (Ah) expected to
be
consumed by heating element 14 over a full discharge for the ambient
temperature and
instantaneous load current readings. Data management unit 25 then subtracts
the
capacity (Ah) expected to be consumed by heating element 14 from the initial
capacity
(Ah) of each module 10(1) to 10(n) and calculates the total available capacity
of the
module string 10 as the sum of the corrected available capacities of each
module 10(1)
to 10(n) : E(initial capacity -heater capacity - delivered capacity). The
total available
capacity of the power system expressed in C (Ah) is the total available energy
that can
be withdrawn from fully charged modules 10(1) to 10(n) for a specific set of
operating
conditions which include the instantaneous load current and ambient
temperature.
The back-up time available from module string 10 is the result of the total
available
capacity C (Ah) of the system divided by instantaneous load current expressed
in Amps
(A) and is expressed in hours. Best practice dictates that when modules 10(1)
to 10(n)
are in floating mode, the calculated back-up time remains at or above eight
hours. The
updated calculated back-up time value is available to remote user 30 in real
time.
In discharge mode, the integrated circuit of LP modules 10(1) to 10(n)
measures the
current perceived at the module level. Since LP modules 10(1) to 10(n) are
connected
in parallel, the current perceived by each module is a fraction of the current
supplied to
load 20 and measured by DC current transducer 34. The current load perceived
by each
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module is measured through a shunt resistance as is well known in the art and
continuously monitored. The current load perceived by each module expressed in
Amps (A) is transmitted to data management unit 25. Data management unit 25
can
calculate at any time the delivered capacity of each module 10(1) to 10(n).
Normally, the data management unit 25 is powered by the rectifier used to
charge
module string 10 in the exterior telecommunications cabinet. In the event of a
power
outage, LP modules string 10 immediately takes over the supply of DC current
to load
20 and to data management unit 25. During loss of primary power, data
management
unit 25 continues to monitor the battery capacity of each LP module 10(1) to
10(n) by
subtracting delivered capacity, and to calculate remaining back-up time based
on
instantaneous load current readings from load transmitter unit 26 and provides
remote
user 30 with updated remaining back-up time available from the entire module
string
10. The remaining back-up time is updated at short intervals such that the
remote user
30 is fully aware of the situation of the telecommunication cabinet in real
time.
All data information stored in memory are available for remote user 30 through
external communication port 28. Gathering data from modules 10(1) to 10(n) and
calculations are ordinarily performed using pre-programmed routine at regular
intervals. Intervals as well as calculations may be amended, updated or
changed by
remote user 30.
The critical issue of reliably predicting reserve time available at each
telecommunication equipment site when a power outage occurs are resolved by
using
LP modules having integrated electronics and a monitoring system that
accurately
monitors each battery's SOH and SOC and the equipment electrical load. The
result is a
back-up power system that can accurately and dynamically determine reliability
level
based on reserve time available at each telecommunication equipment site, and
that
monitors each individual module's State of Health and State Of Charge to
immediately
detect any problems with a module string:
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Although the present invention has been described in relation to particular
variations
thereof, other variation and modifications are contemplated and are within the
scope of
the present invention. Therefore the present invention is not to be limited by
the above
description but is defined by the appended claims.
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