U.S. patent application number 09/437529 was filed with the patent office on 2002-05-09 for self-tuning charge controller for nickel hydrogen batteries.
Invention is credited to RULISON, AARON J.
Application Number | 20020055035 09/437529 |
Document ID | / |
Family ID | 23736814 |
Filed Date | 2002-05-09 |
United States Patent
Application |
20020055035 |
Kind Code |
A1 |
RULISON, AARON J |
May 9, 2002 |
SELF-TUNING CHARGE CONTROLLER FOR NICKEL HYDROGEN BATTERIES
Abstract
A method of operating a rechargeable battery having a nickel
hydroxide positive electrode and an electrolyte comprises the steps
of charging the battery, after completion of a discharge cycle, at
a temperature T.sub.1 between approximately -10.degree. C. and
-30.degree. C. which is lower than a temperature T.sub.2, in the
range of approximately -15.degree. C. to +5.degree. C., at which
discharge begins, automatically determining the total charge to be
returned to the battery for establishing the ideal charge for the
battery leading into the next discharge cycle, and applying charge
energy to the battery having the magnitude as automatically
determined. After completion of the discharge cycle, the battery is
cooled to the temperature in the T.sub.1 range, then heated to
stabilize the temperature to that in the T.sub.1 range. Then the
battery is charged according to a nominal profile of charge power
as a function of time, the accumulated charge imparted to the
battery sensed as cooling proceeds, the nominal power profile
adjusted according to the accumulated charge obtained, and the
remainder of the nominal charge profile adjusted accordingly. More
specifically, the battery is heated to maintain a nominal
temperature profile as a function of time associated with an upward
transition in heater power for initializing the heating operation,
a plateau in heater power for maintaining the temperature setpoint
and a downward transition in heater power before the battery begins
to warm and the actual downward transition in heater power obtained
is sensed, and the nominal power profile is adjusted according to
the time of the actual downward transition sensed. This results in
decreasing the duration charge returned in the event the downward
transition operation sensed actually occurs prior to the nominal
transition time and results in increasing the charge returned in
the event the downward transition operation sensed actually occurs
after the nominal transition time operation.
Inventors: |
RULISON, AARON J;
(SUNNYVALE, CA) |
Correspondence
Address: |
J E KOSINSKI
LORAL SPACE & COMMUNICATIONS LTD
655 DEEP VALLEY DRIVE
SUITE 303
ROLLING HILLS ESTATES
CA
90274
|
Family ID: |
23736814 |
Appl. No.: |
09/437529 |
Filed: |
November 10, 1999 |
Current U.S.
Class: |
429/50 ; 320/134;
320/150; 429/62 |
Current CPC
Class: |
H01M 10/44 20130101;
Y02E 60/10 20130101; H01M 10/345 20130101 |
Class at
Publication: |
429/50 ; 429/62;
320/134; 320/150 |
International
Class: |
H01M 010/44; H01M
010/50; H01M 010/46; H02J 007/04 |
Claims
What is claimed is:
1. A method of operating a rechargeable battery comprising a nickel
hydroxide positive electrode and an electrolyte comprising the
steps of: (a) after completion of a discharge cycle, charging the
battery at a temperature T.sub.1 between approximately -10.degree.
C. and -30.degree. C. which is lower than a temperature T.sub.2, in
the range of approximately -15.degree. C. to +5.degree. C., at
which discharge begins; (b) automatically determining the total
charge to be returned to the battery for establishing the ideal
charge and temperature for the battery leading into the next
discharge cycle; and (c) applying charge energy to the battery
having the magnitude determined in step (b).
2. A method of operating a rechargeable battery as set forth in
claim 1 wherein step (a) includes the steps of: (d) after
completion of the discharge cycle, cooling the battery to the
temperature in the T.sub.1 range; (e) heating the battery to
stabilize the temperature to that in the T.sub.1 range; (f)
charging the battery according to a nominal profile of charge power
as a function of time; and wherein steps (b) and (c) include the
steps of: (g) sensing the accumulated charge imparted to the
battery as step (f) proceeds; (h) adjusting the nominal power
profile of step (f) according to the accumulated charge in step
(g); and (i) completing the remainder of the nominal charge profile
adjusted according to step (h).
3. A method of operating a rechargeable battery as set forth in
claim 2 wherein step (e) includes the steps of: (j) heating the
battery according to a nominal temperature profile as a function of
time which includes an upward transition in heater power for
arresting the battery cool down; a plateau in heater power for
maintaining the battery temperature at the setpoint; and a downward
transition in heater power before the battery begins to warm and
(k) sensing the actual downward transition in heater power obtained
as a result of step (j); and (l) performing step (h) according to
the time of the actual downward transition sensed in step (k).
4. A method of operating a rechargeable battery as set forth in
claim 3 wherein step (h) results in decreasing the applied charge
power in step (f) in the event the downward transition operation
sensed in step (k) actually occurs prior to the nominal transition
time; and wherein step (h) results in increasing the applied change
power in step (f) in the event the downward transition operation
sensed in step (k) actually occurs after the nominal transition
time operation.
5. A rechargeable battery comprising: a nickel hydroxide positive
electrode; an electrolyte; and a negative electrode comprised of
hydrogen; means for charging the battery at a temperature T.sub.1
between approximately -10.degree. C. and -30.degree. C. which is
lower than a temperature T.sub.2, in the range of approximately
-10.degree. C. to +5.degree. C., at which discharge begins; and a
charge controller for automatically determining the total charge to
be returned to the battery for establishing the ideal charge and
temperature for the battery leading into the next discharge cycle
and for applying only the necessary magnitude of charge energy to
the battery to assure such ideal charge and temperature are
obtained.
6. A rechargeable battery as set forth in claim 5 including: a
source of refrigeration for cooling the battery to a temperature
T.sub.1 between approximately -10.degree. C. and -30.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a technique for
operation of a nickel-hydrogen battery, and more particularly, to
recharging a nickel-hydrogen battery at a temperature which is
substantially lower than the temperature at which discharge is
performed, automatically correcting errors in the total charge
returned to the battery during recharge to thereby establish the
ideal battery charge and temperature leading into the next
discharge cycle.
[0003] 2. Description of the Prior Art
[0004] Commonly assigned U.S. Pat. Nos. 5,395,706 entitled
"Satellite Battery Thermal/Capacity Design" and No. 5,429,888,
entitled "Battery Recharging Technique" both relate to recharging a
nickel-hydrogen battery at a temperature which is substantially
lower than the temperature at which discharge is performed. The
disclosures of these patents are incorporated into the instant
disclosure in their entirety by reference. According to these
patents, the charging operation uses preset high, medium, taper,
and pulse charge rates. The battery's temperature is made to follow
a prescribed temperature profile throughout. Beginning at the
transition from high to medium charging, heater power is required
to keep the batteries at their cold charge temperature setpoint,
which typically is -20.degree. C. In charging the battery, about
half way through taper charge, the heater power falls to zero. This
marks a transition in the battery charging thermodynamics and
roughly corresponds to the point at which the batteries reach a
substantial fraction of their total charge capacity. Note that
throughout the heater power transition, the battery's temperature
remains at the setpoint of 20.degree. C. From the heater power
transition onward, much of the charge power is dissipated as heat,
rather than absorbed as useful energy capacity. There is,
therefore, a slight rise in the battery temperature during the last
portion of taper charging. At the end of taper charging, the
temperature setpoint is changed to -15.degree. C. (typically) in
preparation for battery discharge. There is an associated rise in
heater power after the end of taper charge.
[0005] The prior art in the form of the U.S. Pat. Nos. 5,395,706
and 5,429,888 just described is based on a feed-forward controller
which does not use information about the battery's temperature,
voltage, or pressure during recharge. Instead, it simply applies
charge energy to the battery in proportion to the energy lost by
the battery during the most recent discharge.
[0006] The primary advantage of the present invention over the
prior art is that it eliminates the labor Intensive and risky
manual tuning process required to achieve optimal battery charge
and thermal behavior. The prior art cited above has no ability and
makes no attempt to automatically correct errors in the total
charge returned to a battery during recharge. Such errors in the
total charge arise inevitably from the power control electronics
surrounding the battery, including battery charge and discharge
current monitor calibration and measurement errors, and charge
controller errors, Errors also arise from uncertainties in the
theory of thermal dissipation and charge acceptance efficiency that
leads to the prescribed charge currents. For digital systems,
errors also arise from discretization of the total charge to be
returned. Furthermore, the battery temperature near the end of
recharge is very sensitive to these errors. As a result of this
sensitivity, with the prior art the particular charge control
parameters must be manually tuned in order to achieve adequate
performance for each spacecraft. This tuning process is labor
intensive and, for satellite applications, risky since the tuning
process can only be done once the satellite is experiencing actual
eclipses on orbit. Furthermore, in the prior art one manual tuning
procedure may not be adequate over the spacecraft's entire life due
to drifting electronic component performance.
[0007] It was with knowledge of the foregoing state of the
technology that the present invention has been conceived and is now
reduced to practice.
SUMMARY OF THE INVENTION
[0008] The present invention relates to a method of operating a
rechargeable battery having a nickel hydroxide positive electrode
and an electrolyte. This method comprises the steps of charging the
battery, after completion of a discharge cycle, at a temperature
T.sub.1 between approximately -10.degree. C. and -30.degree. C.
which is lower than a temperature T.sub.2, in the range of
approximately -10.degree. C. to +5.degree. C., at which discharge
begins, automatically determining the total charge to be returned
to the battery for establishing the ideal charge and temperature
for the battery leading into the next discharge cycle, and applying
charge energy to the battery having the magnitude as automatically
determined. After completion of the discharge cycle, the battery is
cooled to the temperature in the T.sub.1 range, then heated to
stabilize the temperature to that in the T.sub.1 range. Then the
battery is charged according to a nominal profile of charge power
as a function of time, the accumulated charge imparted to the
battery sensed as cooling proceeds, the nominal power profile
adjusted according to the accumulated charge obtained, and the
remainder of the nominal charge profile adjusted accordingly. More
specifically, the battery is heated according to a nominal profile
of heater power as a function of time which includes an upward
transition portion for initializing the heating operation, a
plateau portion for maintaining the heating operation, and a
downward transition portion for terminating the heating operation,
and the actual downward transition in heater power obtained is
sensed, and the nominal power profile is adjusted according to the
time of the actual downward transition sensed. This results in
decreasing the total charge energy returned to the battery in the
event the downward transition operation sensed actually occurs
prior to the nominal transition time and results in increasing the
total charge returned to the battery in the event the downward
transition operation sensed actually occurs after the nominal
transition time operation.
[0009] A primary feature, then, of the present invention is the
provision of an improved technique for operation of a
nickel-hydrogen battery.
[0010] Another feature of the present invention is the provision of
such a technique for recharging a nickel-hydrogen battery at a
temperature which is substantially lower than the temperature at
which discharge is performed and automatically correcting errors in
the total charge returned to the battery during recharge to thereby
establish the ideal battery charge and temperature leading into the
next discharge cycle.
[0011] A further feature of the present invention is the provision
of such a technique which incorporates use of temperature
controller information in a nickel hydrogen battery charge
controller.
[0012] Still another feature of the present invention is the
provision of such a technique which automatically tunes the total
charge returned to the battery thereby correcting all of the error
sources to which the battery is subjected, thereby establishing the
ideal battery charge and temperature leading into the next
discharge cycle.
[0013] Yet another feature of the present invention is the
provision of such a technique which renders the known labor
intensive and risky manual tuning process largely obsolete.
[0014] Yet another feature of the present invention is the
provision of such a technique which eliminates the possibility of
overcharging, and hence, overheating, nickel-hydrogen batteries
during recharge after discharge and which, in turn, may lengthen
the battery's operational lifetime.
[0015] Still another feature of the present invention is the
provision of such a technique which, for satellite applications,
serves to improve the satellite's overall lifetime and
reliability.
[0016] Still a further feature of the present invention is the
provision of such a technique which eliminates the possibility of
undercharging nickel-hydrogen batteries during recharge after
discharge which, in turn, protects against (1) inadequate battery
energy capacity in the next discharge period and (2) catastrophic
battery failure due to cell fusion and, for satellite applications,
protects against forced payload turnoffs, or load shedding, and
prevents service interruptions.
[0017] Yet a further feature of the present invention is the
provision of such a technique which greatly reduces the labor
required to tune charge control parameters, using the same
parameter set for all batteries of a given capacity and thermal
environment, for satellite applications, there being one parameter
set for all spacecraft in a given class and there being no need to
manually tune each spacecraft's battery software parameters after
launch, a risky and costly procedure, and there being no need to
retune the software parameters as the spacecraft ages.
[0018] Still another feature of the present invention is the
provision of such a technique which uses a temporal setpoint for a
temperature controller output transition and subsequent temporal
shift of charge currents to optimally adjust total charge returned
to the battery and the battery's temperature before the next
discharge period.
[0019] Other and further features, advantages, and benefits of the
invention will become apparent in the following description taken
in conjunction with the following drawings. It is to be understood
that the foregoing general description and the following detailed
description are exemplary and explanatory but are not to be
restrictive of the invention. The accompanying drawings which are
incorporated in and constitute a part of this invention, illustrate
one of the embodiments of the invention, and together with the
description, serve to explain the principles of the invention in
general terms. Like numerals refer to like parts throughout the
disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a diagrammatic representation of a known
nickel-hydrogen battery system of the type disclosed in U.S. Pat.
Nos. 5,395,706 and 5,429,888;
[0021] FIG. 2 is a graph depicting the operation of a cell of the
known battery system of FIG. 1, presenting various parameters over
the course of time;
[0022] FIG. 3 is a schematic diagram illustrating the operation of
a feed-forward charge controller used with the known battery system
of FIG. 1;
[0023] FIG. 4 is a graph depicting the operation of a cell of the
known battery system of FIG. 1 which undesirably leads to battery
overcharge;
[0024] FIG. 5 is a graph depicting the operation of a cell of an
improved battery system embodying the invention which desirably
avoids battery overcharge;
[0025] FIG. 6 is a graph depicting the operation of a cell of the
known battery system of FIG. 1 which undesirably leads to battery
undercharge;
[0026] FIG. 7 is a graph depicting the operation of a cell of an
improved battery system embodying the invention which desirably
avoids battery undercharge;
[0027] FIG. 8 is a schematic diagram illustrating the operation of
a feed-forward charge controller used with the improved battery
system of the invention; and
[0028] FIG. 9 is a flow chart which relates the procedure employed
by a battery system embodying the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] Turn now to the drawings and, initially, to FIG. 1 which
generally illustrates a nickel-hydrogen battery system 20 of the
type with which the invention is concerned. The battery system 20
includes a cell 22 provided with a pressure vessel 24 and an
electrode stack 26 within the pressure vessel. The electrode stack
26, in turn, includes a positive electrode 28, a negative electrode
30, and a porous separator 32 which also serves as an electrolyte
reservoir for storing excess electrolyte within the electrode stack
26. The porous separator 32 may be composed of zirconia, asbestos,
plastic, and the like. The positive electrode 28 includes
electrochemically active nickel hydroxide and electrically
conductive material having a resistivity less than approximately
0.1 ohm/cm. The negative electrode 30 is of a material which
catalyzes the oxidation and reduction of hydrogen gas and, in
typical fashion, the electrolyte is a solution of potassium
hydroxide (KOH).
[0030] The cell 22 also has electrical lead throughs 34, 36 through
which negative and positive electrical leads 38, 40 respectively
pass.
[0031] A suitable electrically energized heater jacket 42 overlies
the pressure vessel 24. By closure of a switch 44, heating elements
within the heater jacket 42 can be energized by a suitable source
46 of EMF for heating the cell 22.
[0032] The cell 22 is thermally connected to an optical space
radiator (OSR) 48 via a thermally conductive sleeve 50. The sleeve
50 is mounted on one side 52 of the OSR 48 and sidably envelops an
outer peripheral surface 54 of the cell 22. A side 60 of the OSR 48
opposite side 52 faces black space. The cell is continuously cooled
by the OSR and its temperature is determined by bucking the OSR
with the heater jacket 42.
[0033] On a spacecraft which is a preferred venue for the cell 22,
a solar array 62 is the primary power source indicated for
recharging the cell 22 and a typical load 60 is indicated for the
discharge cycle of the cell. While the present disclosure is
written in the context of spacecraft applications, other
applications of nickel hydrogen batteries would also benefit from
the invention.
[0034] FIG. 2 is a graph showing features of a well-tuned cold
charging recharge cycle, the basis of which is described in U.S.
Pat. Nos. 5,395,706 entitled "Satellite Battery Thermal/Capacity
Design" and No. 5,429,888 entitled "Battery Recharging Technique".
The charging includes preset high, medium, taper, and pulse charge
rates. The battery's temperature is made to follow a prescribed
temperature profile throughout. Beginning at the transition from
high to medium charging, heater power is required to keep the
batteries at their cold charge temperature setpoint, which
typically is -20.degree. C., It will be appreciated that heat from
batteries used in space applications is radiated to deep space.
About half way through taper charge, the heater power falls to
zero. This marks a transition in the battery charging
thermodynamics and roughly corresponds to the point at which the
batteries reach a substantial fraction of their total charge
capacity. Note that throughout the heater power transition, the
battery's temperature remains at the setpoint of -20.degree. C.
From the heater power transition onward, much of the charge power
is dissipated as heat, rather than absorbed as useful energy
capacity. There is, therefore, a slight rise in the battery's
temperature during the last portion of taper charging. At the end
of taper charging, the temperature setpoint is changed to
-15.degree. C. (typically) in preparation for battery discharge.
There is an associated rise in heater power after the end of taper
charge.
[0035] The prior art being described here in FIG. 2 is based on a
feed-forward charge controller. FIG. 3 illustrates the concept. The
feed-forward method does not use information about the battery's
temperature, voltage, or pressure during recharge. Instead, it
simply applies charge energy to the battery in proportion to the
energy lost (for example, 115%) by the battery during the most
recent discharge.
[0036] FIG. 4 shows how the prior art may lead to battery
overcharge. This occurs when the total charge energy applied to the
battery is excessive. Such excessive charge energy may be due to
errors that can arise from the power control electronics
surrounding the battery or from uncertainties in the theory that
leads to the prescribed charge energy. In FIG. 4, the heater power
transition occurs prior to its nominal time. Since there still is a
substantial amount of recharge power left in the charge cycle and
since, from the heater power transition onward, most of the charge
power is dissipated as heat, there is a highly undesirable
temperature rise prior to the next eclipse.
[0037] FIG. 5 illustrates an aspect of the present invention that
enables the charge controller to automatically correct and prevent
the incipient overcharge. At the transition from high to medium
charge power, the novel charge controller begins watching watches
for a downward transition in the heater power, which would indicate
the battery is nearly fully charged. When it detects a downward
transition that occurs prior to the nominal transition time, the
novel controller automatically reduces the charge power to
correspond to that which would have been in force had the
transition occurred at the nominal time. It then completes the
remainder of the nominal charge profile. In this manner, an amount
of charge energy, .DELTA.Q, is automatically subtracted from the
total charge returned to the battery, and the overcharge is
avoided.
[0038] FIG. 6 illustrates how the prior art may lead to battery
undercharge, in which the total charge energy applied to the
battery is insufficient, due to errors that can arise from the
power control electronics surrounding the battery or from
uncertainties in the theory that leads to the prescribed charge
energy. In FIG. 6, the heater power did not occur at the nominal
time. As a result, the battery is undercharged for the next eclipse
and unable to supply the spacecraft's power needs. Moreover, the
thermodynamics are such that at the pulse charge (the last step in
the charge cycle), the battery's temperature can be driven downward
to a temperature at which catastrophic battery failure can occur
within seconds of entering the eclipse.
[0039] FIG. 7 illustrates an aspect of the present invention that
enables the charge controller to automatically correct and prevent
the incipient undercharge. At the transition from high to medium
charge power, the novel charge controller begins watching for a
downward transition in the heater power, which would indicate the
battery is nearly fully charged. When it observes a lack in
downward transition prior to the nominal transition time, the novel
charge controller automatically maintains the charge power
corresponding to that which would have been in force had the
transition occurred at the nominal time until the downward heater
power transition occurs. It then completes the remainder of the
nominal charge profile. In this manner, an amount of charge energy
.DELTA.Q is automatically added to the total charge returned to the
battery, and the undercharge is avoided.
[0040] FIG. 8 illustrates the novel charge controller of the
invention. It relates how the invention builds on the feed-forward
design of the prior art by adding a feedback loop that uses
information about the time of downward transition of heater power
that occurs when the battery is nearing full charge. The .DELTA.Q
in FIG. 8 corresponds to the same symbol in FIGS. 5 and 7).
[0041] The flow chart presented in FIG. 9 relates the entire
procedure just described.
[0042] While preferred embodiments of the invention have been
disclosed in detail, it should be understood by those skilled in
the art that various other modifications may be made to the
illustrated embodiments without departing from the scope of the
invention as described in the specification and defined in the
appended claims.
* * * * *