U.S. patent application number 09/942231 was filed with the patent office on 2002-03-21 for method for monitoring and controlling the charging of gastight alkaline rechargeable batteries.
This patent application is currently assigned to NBT GmbH. Invention is credited to Kohler, Uwe.
Application Number | 20020033693 09/942231 |
Document ID | / |
Family ID | 7656287 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020033693 |
Kind Code |
A1 |
Kohler, Uwe |
March 21, 2002 |
METHOD FOR MONITORING AND CONTROLLING THE CHARGING OF GASTIGHT
ALKALINE RECHARGEABLE BATTERIES
Abstract
In a method for monitoring the charging of gastight alkaline
rechargeable batteries, the characteristics of the critical
charging voltage (U.sub.crit) on a rechargeable battery are
determined as a function of the charging current (I) for various
temperatures (T) and are linearized by means of a function in the
form A(I).times.T+B(I). The value pairs A and B are stored as
parameter arrays in a battery management system, and during
operation of a physically identical rechargeable battery which is
to be monitored, the associated critical charging voltage is
calculated by measuring the temperature and charging current and is
used to control the charging of the rechargeable battery. On
reaching the critical charging voltage (U.sub.crit) which
corresponds to the critical state of charge (LZ.sub.crit), the
value of the critical state of charge can be calculated by
measuring the current (I) and temperature (T).
Inventors: |
Kohler, Uwe; (Kassel,
DE) |
Correspondence
Address: |
IP Department
Schnader Harrison Segal & Lewis
36th Floor
1600 Market Street
Philadelphia
PA
19103
US
|
Assignee: |
NBT GmbH
|
Family ID: |
7656287 |
Appl. No.: |
09/942231 |
Filed: |
August 29, 2001 |
Current U.S.
Class: |
320/152 |
Current CPC
Class: |
G01R 31/367 20190101;
H02J 7/0091 20130101; H01M 10/443 20130101; H01M 10/34 20130101;
H01M 10/345 20130101; H02J 7/007194 20200101; Y02E 60/10
20130101 |
Class at
Publication: |
320/152 |
International
Class: |
H02J 007/04; H02J
007/16 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2000 |
DE |
DE 100 45 622.7 |
Claims
What is claimed is:
1. A method of controlling charging of a gastight alkaline
rechargeable battery comprising: determining characteristics of
critical charging voltage (U.sub.crit) of the rechargeable battery
as a function of charging current (I) at selected temperatues (T);
linearizing the critical charging voltage according to the follow
formula: U.sub.crit=A(I).times.T+B(I), wherein
A=.DELTA.U.sub.crit/T/V and B=U.sub.crit at 0.degree. C./V, and
wherein A and B are stored as parameter arrays in a battery
management system containing a substantially physically identical
rechargeable battery; calculating an associated critical charging
voltage in the substantially physically identical rechargeable
battery by measuring temperature and charging current; and
comparing associated critical charging voltage information with the
critical charging voltage of the rechargeable battery to control
the charging of the rechargeable battery.
2. The method as claimed in claim 1, wherein critical states of
charge (LZ.sub.crit), which correspond to the critical charging
voltage U.sub.crit, on the rechargeable battery are determined as a
function of charging current (I) and battery temperature (T), and
resulting data are stored as parameter arrays in the battery
management system, and wherein, during operation of the
substantially physically identical rechargeable battery, and on
reaching the critical charging voltage (U.sub.crit) which
corresponds to the critical state of charge (LZ.sub.crit), the
value of this critical state of charge (LZ.sub.crit) is determined,
from the measurement of the current (I) and temperature (T), by
comparison with the data stored in the parameter arrays.
3. The method as claimed in claim 2, wherein the rechargeable
battery is charged to the critical charging voltage (U.sub.crit) at
predetermined times.
4. The method as claimed in claim 2, wherein the rechargeable
battery experiences an increase in the state of charge by net
charging from any given state of charge, and wherein, on reaching
the critical state of charge (LZ.sub.crit), this state of charge
value is stored in the battery management system and, during
further operation of the rechargeable battery, this value is used
as a reset value for state of charge monitoring by current
integration with respect to time.
5. The method as claimed in claim 4, wherein the state of charge is
determined at fixed time intervals or after feeding in a specific
amount of charge.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a method of monitoring and
controlling the charging of gastight alkaline rechargeable
batteries and determining critical states of charge, particularly
to a method that can monitor charging based on voltage and
temperature.
BACKGROUND
[0002] Rechargeable alkaline battery systems are used in large
quantities for modern equipment applications. In addition to these
applications, they will also be increasingly used in the future in
vehicles, both as a propulsion battery in hybrid vehicles and as
batteries for vehicle power supply systems. High power output and
the capability of feeding electrical energy back at high power
effectively are essential characteristics of alkaline systems.
[0003] Of the alkaline secondary systems: nickel cadmium (NiCd),
nickel metal hydride (NiMH), nickel zinc (NiZn), and nickel iron
(NiFe), the nickel metal hydride system has been found to be the
system having the best characteristics. In comparison with other
alkaline secondary systems, it has a better charge capacity, longer
life and avoids the feared "memory effect". In addition, it does
not make use of toxic heavy metals.
[0004] The capabilities for rapid charging of alkaline secondary
systems extend down to the range of a few minutes. Rapid charging
is limited by critical cell voltages being exceeded, which are
governed by the decomposition voltage of the water and oxygen
gassing at the positive electrode, whose capacitance with respect
to the negative electrode is underdimensioned in a gastight
alkaline cell. Oxygen gassing at the positive electrode takes place
as a parasitic reaction when the positive electrode is approaching
the fully charged state, and is the reason why it is necessary to
limit the charging current.
[0005] The oxygen gassing reaction can result in pressure building
up in the cell which, in the worst case, leads to safety valves
operating and to charging gases and electrolyte escaping. Since
both can have a negative effect on the life expectancy of the
gastight cells, it is desirable to identify such critical states of
charge at an early stage, and to limit or cut off the charging
currents in good time.
[0006] However, identification of the critical states of charge is
problematic. A pressure measurement is regarded as being too
complex. Only the cell voltage and temperature are available as
variables which can be measured from outside the cell. Since oxygen
gassing reactions in gastight alkaline secondary systems are
accompanied by an exothermal oxygen dissipation reaction at the
negative, opposing electrode, the rate of temperature rise, which
is normally observed, is in general also a signal of the start of
gassing and, thus, that the pressure inside the cell is rising.
However, particularly with very high charging currents, the
temperature signal can be used only to a limited extent since the
high thermal capacity of aqueous battery systems leads to the
temperature rising only relatively slowly as a consequence of the
onset of overcharging.
[0007] Thus, it would be highly advantageous to provide a method
for monitoring the charging and determining critical states of
charge, in which only the voltage and temperature of the
rechargeable battery to be monitored are measured and are used for
assessment.
SUMMARY OF THE INVENTION
[0008] This invention relates to a method of controlling charging
of a gastight alkaline rechargeable battery including determining
characteristics of critical charging voltage (U.sub.crit) of the
rechargeable battery as a function of charging current (I) at
selected temperatues (T), linearizing the critical charging voltage
according to the follow formula: U.sub.crit=A(I).times.T+B(I),
wherein A=.DELTA.U.sub.crit/T/V and B=U.sub.crit at 0.degree. C./V,
and wherein A and B are stored as parameter arrays in a battery
management system containing a substantially physically identical
rechargeable battery, calculating an associated critical charging
voltage in the substantially physically identical rechargeable
battery by measuring temperature and charging current, and
comparing associated critical charging voltage information with the
critical charging voltage of the rechargeable battery to control
the charging of the rechargeable battery.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a graph showing the voltage profile U.sub.CELL,
pressure response P.sub.CELL and temperature profile T.sub.CELL of
an NiMH cell.
[0010] FIG. 2 is a graph of voltage maxima of the cell from FIG. 1
at selected temperatures.
[0011] FIG. 3 is a graph showing the relationship between critical
voltage and temperature for the cell of FIG. 1 at selected charging
currents.
[0012] FIG. 4 is a graph of parameter B which is a function of
critical voltage and voltage versus charging current.
[0013] FIG. 5 is a graph of parameter A which is a function of
charge in critical voltage and voltage versus charging current.
[0014] FIG. 6 is a three-dimensional pictogram showing the
relationship between critical state of charge, temperature and
charging current.
DETAILED DESCRIPTION
[0015] The method according to the invention will be explained in
more detail in the following text with reference to FIGS. 1 to
6.
[0016] FIG. 1 shows the voltage profile U.sub.CELL, the pressure
response P.sub.CELL and the temperature profile T.sub.CELL of an
NiMH cell with a rated capacity of 9 Ah, which is charged in steps
comprising 0.9 Ah charging pulses (10% of the capacity) at
20.degree. C. at a current level of 90 A (10 C rate). After
completion of each individual charging step, the cell remains at
rest for 30 minutes to allow the temperature to equalize with the
environment, and to bring the cell back to the rest potential,
temperature and pressure. If a 90% state of charge is exceeded in
the described example, a considerable pressure rise can be
observed, which is associated with the voltage signal U.sub.CELL
tending to the horizontal. On further charging, the voltage may
even decrease slightly, with a further pressure rise. This effect
is known as a "negative delta U-shift" (depolarization) and is now
widely used as a switch-off signal, although the charging currents
generally remain below the half-hour rated current (2 C rate).
[0017] Here and in the following text, the load currents on the
rechargeable battery are quoted in C, that is to say, a charging
current of 1 C corresponds to a rechargeable battery with a rated
capacity of 9 Ah being charged with a charging current of 9 A, and
charging at 10 C means a charging current of 90 A.
[0018] The value of the voltage maximum which correlates with the
pressure rise caused by oxygen gassing and the gas dissipation
mechanism at the negative electrode is referred to as the "critical
voltage magnitude" U.sub.crit.
[0019] If the measurement described in FIG. 1 is carried out for a
large number of different currents at different temperatures and
the voltage maxima observed in the process which are correlated
with the pressure rise are defined as critical voltage levels
associated with these parameters, then this results in the graph
shown in FIG. 2. As the charging current rises, the U.sub.crit
values are shifted toward higher voltage levels. Lower temperatures
likewise cause the critical values to be shifted toward higher
voltages.
[0020] The relationship, shown in FIG. 3, between the critical
voltage U.sub.crit and the temperature (using the charging currents
in C as a parameter) shows virtually linear profiles, which be be
described by a simple mathematical relationship in the form:
U.sub.crit=A(I)*T+B(I).
[0021] The linear relationship between U.sub.crit and the
temperature, which is shown in FIG. 3 for charging currents of 1,
2, 5 and 10 C, but which also applies to charging currents between
these values, means that, if the parameters A and B are known,
critical charging voltages can be calculated from the above
relationship. The critical charging voltage levels can be
calculated as reference values in the battery monitoring system,
and can be compared with the actual system voltage level. If the
actual charging voltage exceeds the critical level, measures are
taken to reduce the charging current. The only precondition for
this is that the parameter arrays for the value pairs A and B are
stored in the battery management system.
[0022] Both parameters A and B depend on the charging current (I).
The profile of A (rate of rise), wherein A=.DELTA.U.sub.crit/T/V,
is illustrated in the form of a graph and as a function of the
current level I in FIG. 5, and that of B, wherein B=U.sub.crit at
0.degree. C./V, is illustrated in the form of a graph in FIG. 4.
For practical use, both variables are stored in tabular form, as a
parameter table, in a battery management system.
[0023] The described relationship between the temperature, charging
current and critical voltage can, conversely, also be used to
determine the state of charge, since the critical voltage levels
are, of course, correlated with a specific critical state of charge
(LZ.sub.crit), as can also be seen in FIG. 1. This critical state
of charge LZ.sub.crit is also, once again, a function of the
parameters temperature (T) and charging current (I). FIG. 6 shows
the relationship between the critical state of charge
(LZ.sub.crit), the temperature and the charging current. On
reaching a critical charging voltage U.sub.crit which is determined
as explained above, the state of charge can be deduced from the I
and T values on which this graph is based, using the
three-dimensional relationship illustrated in FIG. 6. This can be
used, for example, to reset a state of charge detection system,
which is generally based on charge balancing (Ah counter).
[0024] However, the described method is dependent on the
criticality criterion. In monitored conditions, this may be done in
such a way that, for example, when a vehicle is first brought into
use, or at specific intervals defined on a time basis or after
specific amounts of charge have been fed in, the rechargeable
battery is in general regarded as being virtually discharged. Net
charging (positive charging factor) is carried out, by appropriate
charging control, until the U>U.sub.crit criterion occurs. The
state of charge is then determined from the temperature and
charging current which are associated with this U.sub.crit value,
based on the data in FIG. 6. If, for example, voltage criticality
occurs at a temperature of 0.degree. C. at a current of 10 C, then
an 80% state of charge can be deduced from this. At 20.degree. C.
at a current of 10 C, on the other hand, criticality means a state
of charge of virtually 100%.
[0025] The state of charge is stored in the battery management
system and used as a reset value for state of charge monitoring by
means of current integration with respect to time. Such state of
charge determination can be carried out a fixed time intervals or
after feeding in a specific amount of charge.
* * * * *