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| United States Patent |
6,175,216
|
|
Stuck Andersen
, et al.
|
January 16, 2001
|
Method and apparatus for charging a rechargeable battery with monitoring of
battery temperature rate of change
Abstract
A battery charger monitors an open-circuit voltage across the battery and
the rate of change of temperature of the battery to determine a time to
terminate the process of charging the battery. Charging proceeds in four
stages. In the first stage the open-circuit voltage of the battery is
monitored until such voltage crosses a threshold value. In the second
stage, the rate of change of battery temperature is monitored to determine
a reference value, for example, a minimum of the monitored rate. In the
third stage, the rate of change of battery temperature is again monitored
to identify a time when such rate exceeds the reference value by a
predetermined amount. In the fourth stage, power supplied to charge the
battery is limited immediately after stage three or a predetermined time
after stage three for example by reducing the charging current to a
trickle-charge level or by reducing the voltage by about 100 mV. The
predetermined time may be a function of the elapsed charging time, for
example a predetermined percentage of about 25%.
| Inventors:
|
Stuck Andersen; Kim Arthur (Niv.ang., DK);
Rasmussen; Kim (Ballerup, DK);
.O slashed.stergaard; Kim (Vanl.o slashed.se, DK)
|
| Assignee:
|
Chartec Laboratories A/S (DK)
|
| Appl. No.:
|
047200 |
| Filed:
|
March 24, 1998 |
Foreign Application Priority Data
| Intern'l Class: |
H02J 007/04 |
| Field of Search: |
320/150,151,156
|
References Cited
U.S. Patent Documents
| 3852652 | Dec., 1974 | Jasinski.
| |
| 4503378 | Mar., 1985 | Jones et al.
| |
| 4554500 | Nov., 1985 | Sokira.
| |
| 4670703 | Jun., 1987 | Williams.
| |
| 4746854 | May., 1988 | Baker et al.
| |
| 5122722 | Jun., 1992 | Goedken et al.
| |
| 5200686 | Apr., 1993 | Lee.
| |
| 5329219 | Jul., 1994 | Garrett.
| |
| 5331268 | Jul., 1994 | Patino et al.
| |
| 5332957 | Jul., 1994 | Lee.
| |
| 5365160 | Nov., 1994 | Leppo et al.
| |
| 5519303 | May., 1996 | Goedken et al.
| |
| 5550453 | Aug., 1996 | Bohne et al.
| |
| 5563494 | Oct., 1996 | Cuesta et al. | 320/150.
|
| 5583871 | Dec., 1996 | Simmonds | 320/151.
|
| 5621302 | Apr., 1997 | Shinohara.
| |
| 5652500 | Jul., 1997 | Kadouchi et al. | 320/150.
|
| 5721481 | Feb., 1998 | Narita et al. | 320/150.
|
| 5864220 | Jan., 1999 | Reipur et al. | 320/134.
|
| 5867006 | Feb., 1999 | Dias et al. | 320/106.
|
| 5867008 | Feb., 1999 | Du et al. | 320/136.
|
| 5955869 | Sep., 1999 | Rathmann | 320/132.
|
| 5969625 | Oct., 1999 | Russo | 320/116.
|
| 5973479 | Oct., 1999 | Pomo et al. | 320/150.
|
| 5986435 | Nov., 1999 | Koenck | 320/136.
|
| Foreign Patent Documents |
| 0 593770 A1 | Jul., 1992 | EP.
| |
| 2253312 | Sep., 1992 | GB.
| |
| WO 95/09471 | Apr., 1995 | WO.
| |
| WO 95/20247 | Jul., 1995 | WO.
| |
Other References
Freeman, D. "Battery management tackles alternative battery technologies in
advanced portable systems", Wescon '94. Western electronic show and
convention, Anaheim, Sep. 27-29, 1994, Sep. 27, 1994, Institute of
electrical and Electronics Engineers, pp. 303-308.
Gross, S., "Charge control of nickel batteries", Conversion Technologies
(continued), Electrochemical Conversion, Boston, Aug. 4-9, 1991, vol. 3,
Aug. 4, 1991, Institute of electrical and electronics engineers, pp.
341-346.
|
Primary Examiner: Riley; Shawn
Assistant Examiner: Tibbits; Pia
Attorney, Agent or Firm: Lechter; Michael A., Bachand; William R.
Squire, Sanders & Dempsey L.L.P.
Claims
What is claimed is:
1. A method for charging a rechargeable battery, the method comprising:
providing a supplied power to charge the battery;
measuring battery temperature to provide a first plurality of values of
rate of change of battery temperature and to provide further values of
rate of change of battery temperature;
determining a reference value in response to the first plurality of values;
comparing the further values with the reference value; and
limiting the supplied power in response to a result of comparing.
2. The method of claim 1 wherein the supplied power is limited in response
to comparing a particular further value and the reference value, wherein
the particular further value differs from the reference value by at least
a predetermined amount.
3. The method of claim 1 wherein the reference value is determined in
further response to a minimum of the first plurality of values.
4. The method of claim 1 wherein:
a. the step of providing a supplied power comprises providing a charging
current to the battery; and
b. the step of limiting the supplied power comprises reducing the charging
current.
5. The method of claim 4 wherein the step of limiting the supplied power
further comprises:
determining a time period duration; and
after lapse of the time period, reducing the charging current to no more
than a trickle-charge current.
6. The method of claim 5 wherein the step of determining a time period
duration comprises:
determining a first charging duration during which the charging current had
been supplied to the battery; and
determining the time period duration in response to the first charging
duration.
7. The method of claim 5 wherein the step of determining the time period
duration comprises:
determining a first charging duration as including time up until the step
of limiting the supplied power is initiated; and
setting the time period duration to a duration in the range of 5% to 50% of
the first charging duration.
8. The method of claim 5 wherein the step of determining a time period
duration comprises setting the time period duration to a constant.
9. The method of claim 1 wherein the step of limiting the supplied power
comprises reducing a voltage across the battery.
10. The method of claim 9 wherein the voltage across the battery is reduced
at least 100 mV.
11. The method of claim 1 wherein the step of providing a supplied power is
performed for a predetermined period prior to performing the step of
measuring.
12. The method of claim 1 wherein:
a. the method further comprises measuring a battery voltage across the
battery; and
b. performance of the step of measuring battery temperature is deferred
until the measured battery voltage crosses a threshold value.
13. The method of claim 1 wherein:
a. the method further comprises the steps of reducing a charging current
supplied to the battery; and measuring a battery voltage across the
battery when the charging current is reduced; and
b. performance of the step of comparing the further values with the
reference value is deferred until the measured battery voltage crosses a
threshold value.
14. The method of claim 1 further comprises providing no more than a
trickle-charge current to the battery after performing the step of
limiting the supplied power.
15. An apparatus for charging a rechargeable battery, the apparatus
comprising:
a. a power supply that provides a supplied power to charge the battery; and
b. a circuit that:
provides indicia of a temperature of the battery;
measures rate of change of battery temperature in response to the indicia
to provide a first plurality of values of rate of change of battery
temperature and to provide further values of rate of change of battery
temperature;
determines a reference value in response to the first plurality of values;
compares the further values with the reference value; and
provides a signal to the power supply in response to a result of comparing,
wherein the power supply, in response to the signal, limits the supplied
power.
16. The apparatus of claim 15 wherein the circuit comprises:
a. a signal conditioning circuit, responsive to the indicia of the
temperature of the battery, for providing an analog signal responsive to
the temperature of the battery;
b. an analog to digital converter responsive to the analog signal for
providing a digital signal; and
c. control logic responsive to the digital signal for providing the signal
to the power supply.
17. The apparatus of claim 16 wherein the analog to digital converter and
the control logic are integral to an integrated circuit.
18. The apparatus of claim 15 wherein the circuit comprises:
a. a measurement circuit that measures rate of change of battery
temperature; and
b. a control circuit that determines the reference value, compares the
further values, and provides the signal to the power supply.
19. The apparatus of claim 18 wherein the measurement circuit comprises an
analog to digital converter.
20. The apparatus of claim 15 wherein the signal is provided in response to
comparing a particular further value and the reference value, wherein the
particular further value differs from the reference value by at least a
predetermined amount.
21. The apparatus of claim 15 wherein the reference value is determined in
response to a minimum of the first plurality of values.
22. The apparatus of claim 15 wherein the power supply, in response to the
signal, limits the supplied power by reducing a charging current supplied
to the battery.
23. The apparatus of claim 15 wherein the signal is provided to the power
supply so that a charging current supplied by the power supply to the
battery for charging the battery is reduced to no more than a
trickle-charge current after lapse of a time period duration beginning
from an identified point in time, the circuit determining the time period
duration and the identified point is time according to a method
comprising:
comparing the further values with the reference value to identify the point
in time;
determining a first charging duration during which the charging current had
been supplied to the battery; and
determining the time period duration in response to the first charging
duration.
24. The apparatus of claim 15 wherein the power supply, in response to the
signal, limits the supplied power by reducing a voltage across the
battery.
25. The apparatus of claim 15 wherein measurements for providing the first
plurality of values are made after a predetermined period of time during
which supplied power is being provided to charge the battery.
26. The apparatus of claim 15 wherein the circuit further measures a
voltage across the battery and determines the reference value in further
response to the first plurality of values being measured after the voltage
across the battery crosses a threshold value.
27. The apparatus of claim 15 wherein:
a. the circuit comprises a device for reducing current supplied to the
battery; and
b. the circuit measures the voltage across the battery when current
supplied to the battery is reduced.
28. A method for charging a rechargeable battery, the method comprising:
supplying a charging current to the battery;
determining a plurality of values of rate of change of temperature of the
battery during at least part of the process of charging the battery;
determining and storing a reference value based on the plurality of values;
after the step of determining the plurality, determining further values of
rate of change of temperature of the battery;
comparing further values of rate of change of temperature of the battery to
the reference value to provide a comparison; and
controlling termination of the charging process based on the comparison.
29. A method for charging a rechargeable battery, the method comprising:
supplying a charging current to the battery;
determining a plurality of values of rate of change of temperature of the
battery during at least part of the process of charging the battery;
determining and storing a reference value based on a minimum of the
plurality of values;
after the step of determining the plurality, determining further values of
rate of change of temperature of the battery;
comparing further values of rate of change of temperature of the battery to
the reference value to provide a comparison; and
controlling termination of the charging process based on the comparison.
30. A method for charging a rechargeable battery, the method comprising:
supplying a charging current to the battery;
determining a plurality of values of rate of change of temperature of the
battery during at least part of the process of charging the battery;
determining and storing a reference value based on a sum of a minimum of
the plurality of values and a constant;
after the step of determining the plurality, determining further values of
rate of change of temperature of the battery;
comparing further values of rate of change of temperature of the battery to
the reference value to provide a comparison; and
controlling termination of the charging process based on the comparison.
Description
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for charging a
rechargeable battery. More particularly, the present invention is directed
to the control of the termination of the charging process.
BACKGROUND OF THE INVENTION
Generally, when charging a rechargeable battery or a secondary battery,
including for example NiCd (Nickel-Cadmium) or NiMH
(Nickel-Metal-Hydride), it is known to have a rapid charging process
wherein a relatively high constant current is applied to the battery until
a certain event occurs. A typical method of detecting this event is to
measure the increase in battery temperature as a function of time in order
to detect when the battery temperature rate of change (dT/dt or
delta_T/delta_t) reaches a predetermined high limit, see for example U.S.
Pat. No. 3,852,652 to Jasinski, U.S. Pat. No. 5,329,219 to Garrett, and
U.S. Pat. No. 5,550,453 to Bohne et. al.
A common drawback of the above mentioned known charging processes is the
use of a constant predetermined reference value to be reached for the
measured battery temperature rate of change when terminating the charging
process. Use of a predetermined reference value which is constant
throughout the battery life sometimes results in undercharge of the
battery (leading to a poor battery capacity) or overcharge of the battery
(leading to a decreased battery lifetime). Therefore, the need exists for
a battery charging method and apparatus that avoid undercharge and
overcharge of the battery.
Another drawback of known charging processes in which a characteristic of
the battery is monitored for the detecting of an event that indicates the
termination of a rapid charging stage, is the appearance of peak values of
the characteristic of the battery at the initial stage of charging. To
avoid a premature termination of the charging process due to a rise in
such a characteristic, the need exists for a battery charging method and
apparatus that avoids the detection of the event during the initial
charging stage and yet allows the detection of a fully charged battery in
order to avoid overcharging of a battery which has already been fully
charged.
SUMMARY OF THE INVENTION
Accordingly, a method in one embodiment of the present invention for
charging a rechargeable battery includes the steps of: providing a
supplied power to charge the battery; measuring a first characteristic of
the battery to provide a first value; measuring a second characteristic of
the battery to provide a second value after the first value has crossed a
first threshold; and limiting the supplied power after the second value
has crossed a second threshold. In alternate methods, the first and second
characteristics are each selected from the group consisting of a battery
voltage, a charging current, a battery temperature, a rate of change of
battery voltage, a rate of change of charging current, and a rate of
change of battery temperature and are not the same characteristic. For
example the first characteristic may be a battery voltage and the second
characteristic may be a rate of change of battery temperature that crosses
a threshold based on a minimum of rate of change of battery temperature
measured after the battery voltage has crossed a voltage threshold. By
limiting supplied power in response to the second value that is measured
after the first value had crossed a threshold, premature termination of
the charging process is avoided. In a variation of such an alternate
method, supplied power is limited in further response to a reference value
determined in response to measurements of the second characteristic. Such
a reference value accurately accounts for battery technology, battery use,
and battery degradation to avoid undercharging the battery and avoid
overcharging the battery.
A method in another embodiment of the present invention for charging a
rechargeable battery includes the steps of: supplying a charging current
to the battery; determining a first plurality of values of rate of change
of battery temperature during charging; determining a reference value
based on the first plurality of values; determining further values of rate
of change of battery temperature; comparing the reference value and these
further values; and controlling termination of charging based on the
comparison. In an alternate method, the reference value is based on a
minimum of the first plurality of values. In another alternate method, the
reference value is based on a sum of a minimum of the first plurality of
values and a constant.
A method in yet another embodiment of the present invention for charging a
rechargeable battery includes the steps of: providing a supplied power to
charge the battery, measuring a rate of change of temperature of the
battery to provide a first plurality of values and a second value,
determining a reference value in response to the first plurality of
values, and limiting the supplied power in response to the reference value
and the second value. Such a reference value accounts for battery
technology, battery use, and battery degradation to avoid undercharging
the battery and avoid overcharging the battery.
An apparatus in one embodiment of the present invention for controlling
power supplied for charging a rechargeable battery cell includes a circuit
that measures a first characteristic of the cell (for example cell
voltage), measures a second characteristic of the cell (for example rate
of change of cell temperature), and provides a signal for controlling
power supplied in response to the second characteristic being measured
after the first characteristic crosses a threshold. Operation of the
apparatus accounts for battery technology, battery use, and battery
degradation to avoid undercharging the cell and avoid overcharging the
cell.
An apparatus for charging a rechargeable battery in a second embodiment of
the present invention includes the apparatus discussed above for
controlling supplied power, and includes a power supply. The power supply,
in response to the signal, limits the supplied power. By limiting supplied
power in response to the second value that is measured after the first
value had crossed a threshold, premature termination of the charging
process is avoided. In a variation of this second embodiment, supplied
power is limited in further response to a reference value determined in
response to the plurality of second values. Such a reference value
accurately accounts for battery technology, battery use, and battery
degradation to avoid undercharging the battery and avoid overcharging the
battery.
An alternate apparatus for charging a rechargeable battery includes: a
power supply that provides a supplied power to charge the battery and a
circuit that: measures rate of change of a temperature of the battery,
determines a minimum of the measured rate of change, determines a present
measured rate of change, provides a comparison of the minimum rate of
change and the present rate of change, and provides a signal to the power
supply in response to the comparison. The power supply, in response to the
signal, limits the supplied power.
Yet another alternate apparatus for charging a rechargeable battery
includes a power supply that provides a supplied power to charge the
battery and a circuit that: measures rate of change of temperature of the
battery to provide a first plurality of values and to provide further
values of the measured rate of change of the battery temperature,
determines a reference value in response to the first plurality of values,
compares the further provided values with the reference value, and
provides a signal to the power supply in response to this comparison. The
power supply limits the supplied power in response to the signal.
Practice of the methods and operation of the apparatus of the present
invention reduce or eliminate drawbacks of the prior art.
BRIEF DESCRIPTION OF THE DRAWING
The structure and operation of exemplary embodiments of the invention,
together with further objects and advantages thereof, may best be
appreciated by reference to the following detailed description taken in
conjunction with the accompanying drawing, in which:
FIG. 1 is a functional block diagram of a battery charging apparatus
according to an embodiment of the present invention;
FIG. 2 is a flow chart of a method of charging a rechargeable battery in
one embodiment of the invention;
FIG. 3 is a graph of a charging process according to the method of FIG. 2
as applied to an initially fully discharged battery;
FIG. 4 is a graph of a charging process according to the method of FIG. 2
as applied to a battery having a higher initial temperature than in FIG.
3;
FIG. 5 is a graph of a charging process according to the method of FIG. 2
as applied to a battery having about 90% of its final charge capacity at
the beginning of the process; and
FIG. 6 is a graph of a charging process according to the method of FIG. 2
as applied to a battery having a lower initial battery temperature and as
performed at a higher ambient charging temperature than in FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A functional block diagram of a battery charger according to the present
invention is illustrated in FIG. 1. FIG. 1 shows battery pack 10 which is
to be charged by battery charger apparatus 20. Battery pack 10 comprises a
number of series connected individual cells 11, battery temperature
sensing thermistor 12 (NTC thermistor), battery type resistor 13, and
battery pack output terminals 14, 15, 16, and 17. Battery output voltage
is provided across terminals 14 and 17. The charging current supplied to
the battery is sensed by sense resistor 23 connected to battery terminal
17 and ground. Thermistor 12 has current supplied through pull-up resistor
21, senses battery temperature, and provides a related output at terminal
16. Type resistor 13 has current supplied through pull-up resistor 22 and
provides a battery type related voltage at terminal 15.
In a variation of battery charger 20 and methods of charging rechargeable
batteries according to the present invention, type resistor 13 and
thermistor 12 are omitted from battery pack 10. An alternate thermistor is
located to sense battery temperature when the battery is being charged.
And, battery type is presumed or is identified by operator input, by a
conventional circuit, or by a conventional mechanical arrangement.
Battery charger 20 includes power supply 24, a charge controller 25, and
signal conditioning circuit 26. Power supply 24, preferably a switch mode
power supply, has power input 27 which is supplied with a DC voltage,
preferably in the range of 12-15 Volts DC. Power supply 24 provides
supplied power to charge the battery via output terminal 28, connected to
terminal 14 through switch 29. Supplied power is controlled via control
output 30 of charge controller 25. When power supply 24 is a switch mode
power supply, control output 30 is preferably a pulse width modulated
(PWM) signal that may be fed to a filter for converting the PWM signal to
a variable analog voltage. The analog voltage may then be used for the
control of power supply 24. When using a PWM signal at control output 30,
charge controller 25 controls the supplied power to battery 10 via
terminal 28 by controlling the duration of on- and off-periods of the PWM
signal.
Signal conditioning circuit 26 converts voltage input signals representing
the battery terminal voltage, the battery type, the battery temperature,
and the charging current, to voltage output signals being suitable as
input signals for analog to digital (A/D) converter inputs 32 of charge
controller 25. Preferably, current sense resistor 23 has a very low value
which may be about 0.1 ohm and conditioning circuit 26 may then include an
operational amplifier to provide a suitable output. The supply voltage for
charge controller 25 is preferably about 5 volts. Since the battery
terminal voltage may exceed 5 volts, conditioning circuit 26 may also
include a voltage divider for providing a suitable output signal for the
battery terminal voltage.
Charge controller 25 preferably includes switch control output 31 for
operating switch 29 on and off. Switch 29 may be turned off at short time
intervals during the charging process to measure an open-circuit voltage
of the battery, thereby decreasing the effect of the voltage drop from the
internal loss resistance when measuring the battery terminal voltage.
Charge controller 25, may include a processor, for example a COP 8ACC
marketed by National Semiconductor, programmed to implement battery
charging in accordance with the present invention. Charge controller 25
controls the power delivered from power supply 24 to battery 10 based upon
the input signals from conditioning circuit 26. These input signals
represent characteristics of the battery including battery type, battery
terminal voltage, battery temperature, and battery charging current.
Battery charger apparatus 20 in operation performs one or more methods of
charging a rechargeable battery according to the present invention. Such
methods are described below with reference to FIGS. 2 through 4.
FIG. 2 presents a method for charging a rechargeable battery according to
one embodiment of the present invention. Such a method begins at step 40.
At step 40, battery pack 10 is connected to charger 20 and charge
controller 25 is initialized.
During initialization, charge controller 25 reads the battery type and
battery voltage and uses these values for obtaining predetermined charging
parameters stored in charge controller 25. Such parameters may represent a
maximum charging current (Imax), an initial value for an end time period
(End_Time), a maximum change in the rate of change of battery temperature
(dT/dt_add), an initial value for the determined minimum value of the
battery temperature rate of change (Min_dT/dt), an initial voltage limit
(VoltLimit), and an initial time period (TimeLimit).
End_Time defines a safety time at which the charging process will be
stopped unless a new value of End_Time is determined and stored during the
charging process.
The value dT/dt_add defines a maximum allowed change in the rate of change
of battery temperature compared to Min_dT/dt.
Min_dT/dt is a variable determined during the charging process. The initial
value of Min_dT/dt is preferably set to a large value.
VoltLimit defines a minimum limit that the measured battery voltage should
reach before updating the predetermined initial value of Min_dT/dt. The
value of VoltLimit will typically be 1.4 Volts per battery cell.
TimeLimit defines a time period that expires before updating the
predetermined initial value of Min_dT/dt. The value of TimeLimit will
typically be 5 minutes for NiCd or NiMH batteries.
Exemplary initial values of End_Time, Min_dT/dt, and dT/dt_add are given
below with the description of FIGS. 3 through 5.
After initialization at step 40 the charging process is begun at process
step 41. The charging process is controlled based on measured values of
the open-circuit battery voltage (Vopen), the charging current (Ichar),
the battery temperature (Tbat), and the elapsed time since charging began
(Time). From values of Tbat, values of the battery temperature rate of
change (dT/dt) are calculated. During the first stage of the charging
process, it is preferred to increase the charging current Ichar until the
predetermined Imax has been reached. The magnitude of Imax is
predetermined at a value that will quickly charge the battery as opposed
to a trickle-charge amount. When Imax has been reached, a second charging
stage is entered, in which the output of the power supply is preferably
controlled so as to charge at a constant charging current, i.e. the output
of the power supply is controlled so that Ichar is close to Imax. In a
preferred embodiment, the value of Imax is chosen to be within the range
of 0.5 Amp through 1.5 Amp for NiCd and NiMH batteries.
If the measured battery voltage reaches VoltLimit before TimeLimit has
expired, the predetermined initial value of Min_dT/dt will be updated and
updating will continue from the point in time when VoltLimit had been
reached. Otherwise, updating of the predetermined initial value of
Min_dT/dt will begin when the TimeLimit period has expired.
At decision step 42, it is determined whether the charging time has reached
the stored value of TimeLimit and dT/dt is smaller than the stored value
of Min_dT/dt. If so, then at process step 43, the stored value of
Min_dT/dt is updated (replaced) with the measured value of dT/dt and the
process continues with decision step 44. If the requirements at step 42
are not fulfilled, the process continues directly with decision step 44.
At decision step 44, it is determined whether the measured open-circuit
battery voltage (Vopen) has reached the stored value VoltLimit. If not,
the process continues with decision step 49.
At decision step 49, it is determined whether the charging time has reached
the stored value of End_Time. If not, the process returns to step 41 for
further charging. If End_Time has been reached, then the normal charging
process is stopped and the charging current is reduced at step 50 to a
trickle-charge current (for example a low, maintenance current) to
maintain the charged status of the battery.
At step 50, the trickle-charge current is preferably set in the range of
0.05 C through 0.1 C, where 1 C is equivalent to a charging current in
Amps that would theoretically fully charge the battery in one hour.
Here it should be noticed that passing directly from step 44 to step 49 and
then to step 50 is not the route of a normal charging process. However,
the battery to be charged might be a defective battery or there might be a
faulty connection to the battery terminals, leading to the result that the
measured battery voltage did not reach the stored VoltLimit value within
the initial safety value of End_Time. Thus, the charger will automatically
terminate the charging process at expiration of the initial End_Time
period.
If the measured battery voltage has reached VoltLimit at step 44, the
process passes on to decision step 45. At step 45, it is determined
whether the measured value of dT/dt is smaller than the stored value of
Min _dT/dt. If so, then at process step 46, the stored value of Min _dT/dt
is updated (replaced) with the measured value of dT/dt and the process
continues with decision step 47. If the requirement at step 45 is not
fulfilled, then the process continues directly with decision step 47.
At decision step 47, it is determined whether the measured value of dT/dt
is larger than the sum of the presently stored minimum value of the change
in dT/dt and the predetermined maximum allowed value of the change in
dT/dt. That is, whether the measured value of dT/dt has reached the sum
Min_dT/dt plus dT/dt_add. If not, the charging process has not yet reached
the normal stage of termination and the process continues with decision
step 49, described above. If so, the process continues with process step
48.
At step 48, a third stage of the charging process is entered where the
remaining part of the charging process is controlled so that the measured
battery voltage does not exceed a maximum allowed battery voltage
(MaxVolt). The value of MaxVolt is not a predetermined value, but is a
function of the measured battery voltage Vopen.sub.dT/dt at the point in
time where dT/dt has reached the sum Min_dT/dt plus dT/dt_add. In a
preferred embodiment, MaxVolt is defined as f1(Vopen), where f1(Vopen) is
defined as (k1* Vopen.sub.dT/dt -k2). The constant k1 may be set to 1 and
the constant k2 may be set in the range of 0 through 100 mV per cell,
preferably about 40 mV, per battery cell. Thus, for a 4 cell battery the
value of MaxVolt may preferably be set to Vopen.sub.dT/dt -160 mV.
For NiMH batteries it is preferred to have such a reduction in the charging
voltage in order to avoid overheating of the battery, since such
overheating might damage the battery and decrease the battery lifetime.
The constant k2 may be set in the range of 0 to 50 mV. The constant k2 may
be set to zero for NiCd batteries. However, it is preferred to set k2 to
about 50 mV for NiCd batteries.
At process step 48, the stored initial value of End_Time is updated
(replaced) with a new End_Time value. The new End_Time value is determined
as a function of the total charging time Time.sub.dT/dt up to the point in
time where dT/dt has reached the sum of Min_dT/dt plus dT/dt_add. In a
preferred embodiment, the new End_Time value is defined as f2(Time) where
f2(Time) is defined as (k3*Time.sub.dT/dt +k4). The constant k3 may be set
to about 1.25 and the constant k4 may be set to zero. In a variation, k3
can be set in the range of 1 through 2 and k4 can be set to represent a
fixed time period in the range of 0 through 20 minutes.
At process step 48, it is determined how the charging process is to be
terminated, i.e. a final charging period and the maximum allowed battery
voltage are determined. Here it should be mentioned that in a preferred
embodiment the measured battery temperature is compensated for variations
related to the present rate of change in the battery temperature. Using a
maximum allowed battery voltage results in a decrease in the charging
current during the final charging period.
The termination of the charging process is illustrated by the loop
comprising process step 51 and decision step 52. After the determination
of MaxVolt and the new End_Time value, the charging process is continued
as described above until the total charging time reaches the stored value
of End_Time in step 52.
At step 52, when the total charging time (Time) reaches the stored value of
End_Time, the charging process is stopped and the charging current is
reduced as already described in step 50 above.
The predetermined charging parameters, including variables, constants, and
functions, for the above described preferred embodiment of a method of
charging a rechargeable battery are shown Table I.
TABLE I
Variables:
Voltage or Vopen Open-circuit battery voltage
MaxVolt Maximum allowed voltage across the battery
Ichar Charging current
Time Time elapsed since charging began
End_Time The time when normal charging is to be stopped
(initialized with a safe value and set to an
optimized value calculated during the
charging process)
Tbat Battery temperature
dT/dt Rate of battery temperature change
Min_dT/dt Minimum value of dT/dt during the charging process
Constants:
Imax Maximum allowed charging current
End_Time Predetermined initial charging time safety value
Initial
Min_dT/dt Predetermined initial minimum value of dT/dt
Initial
dT/dt_add The maximum allowed dT/dt is Min_dT/dt +
dT/dt_add
TimeLimit Min_dT/dt is updated when Time reaches TimeLimit
Initial (typically about 5 minutes)
VoltLimit Min_dT/dt is updated when Vopen reaches VoltLimit
Initial (typically 1.4V for single cell batteries)
Functions:
F1(Vopen) Example: k1 * Vopen - k2 (typically k1 = 1 and
k2 = 40 mV for single cell batteries)
F2(Time) Example: k3 * Time + k4 (typically k3 = 1.25 and
k4 = 0)
FIG. 3 shows a charging process controlled as described above with
reference to the flow diagram of FIG. 2 and as applied to charge a fully
discharged 1600 mAh NiMH battery with 6 cells. In FIG. 3, the battery is
fully discharged before the charging process is begun, and the battery is
charged at room temperature with an initial battery temperature about
23.degree. C. In FIG. 3 the thick solid line waveform represents the
measured open-circuit battery voltage, the dashed line waveform represents
the measured charging current and the thin solid line waveform represents
the measured battery temperature.
For the process shown in FIG. 3, predetermined charging parameters are set
as follows. Imax is set to 900 mA. The initial End_Time value is set to
160 minutes. The initial value of Min_dT/dt is set to a high value of
10.degree. C./minute, thereby disabling the effect of the rate of
temperature change during the upstart of the charging process. The value
of dT/dt_add is set to 0.5.degree. C./minute. The value of TimeLimit is
set to 5 minutes. And, the value of VoltLimit is set to 8.25 volts. The
function f1 for MaxVolt is set to Vopen.sub.dT/dt -240 mV, and the
function f2 for End_Time is set to 1.25*Time.sub.dT/dt.
Here it should be mentioned that the optimal value of dT/dt_add will vary
as a function of battery capacity and the maximum charging current. Thus,
the value of dT/dt_add should be larger both for a smaller nominal battery
capacity and for a higher charging current. In order to measure a relative
change in dT/dt of 0.5.degree. C./minute, it is necessary to measure
changes in the battery temperature at a relatively high resolution. In a
preferred embodiment the temperature is measured using a 10 bit A/D
converter resulting in a resolution in change of temperature of about
0.1.degree. C.
It is preferred when comparing Vopen to VoltLimit that VoltLimit be
compensated for battery temperature at the time Vopen is measured. Such
compensation might be 20 mV/.degree. C. added to VoltLimit for
temperatures below 35.degree. C.
The first stage of charging in FIG. 3 is rather short and the charging
current reaches Imax within a short time period. During the second stage
of charging the current is controlled to approximate Imax. During the
third stage of charging, equivalent to the final charging period, the
charging current is decreased.
During the second stage of charging, the value of TimeLimit (5 minutes) is
smaller than the time at which the compensated voltage Vopen reaches
VoltLimit (about 35 minutes). Thus, after TimeLimit has been reached, new
values of Min_dT/dt are stored according to steps 42 and 43.
In a preferred embodiment, none of the new values of Min_dT/dt are used for
the control of termination of the charging process before VoltLimit has
been reached according to step 44. During the charging process dT/dt is
measured at regular intervals, but the value of Min_dT/dt is not updated
before Time equals (or exceeds) TimeLimit. Further, when Voltage (Vopen)
reaches (or exceeds) VoltLimit, the measured values of dT/dt are used for
determining termination. These processes can be seen in steps 42 through
45 of FIG. 2.
The battery voltage does not reach the compensated value of VoltLimit until
Time is about 35 minutes. At Time equal about 35 minutes, battery
temperature is about 27.5.degree. C., and the corresponding voltage
compensation is about +150 mV. When Vopen reaches VoltLimit, VoltLimit has
a compensated value of about 8.4 volts (8.25+150 mV).
FIG. 4 shows a charging process controlled as described above with
reference to the flow diagram of FIG. 2 and as applied to charge a 1600
mAh NiMH battery with 6 cells. In FIG. 4, the battery is charged at the
same ambient temperature of about 23.degree. C. as the battery of FIG. 3,
but the battery of FIG. 4 has been stored at a higher temperature before
being charged, resulting in an initial battery temperature of about
27.degree. C.
In FIG. 4, the predetermined charging parameters are the same as for the
charging process of FIG. 3.
Because the battery in FIG. 4 has a higher initial battery temperature, the
temperature rate of change dT/dt will be smaller for the charging curves
of FIG. 4 than for the curves of FIG. 3. To avoid overcharging the
battery, the dT/dt termination value needs to be smaller for the charging
process of FIG. 4 than the termination value used for the charging process
of FIG. 3. By using an updated value of Min_dT/dt that is smaller for the
warm battery of FIG. 4 than for the colder battery of FIG. 3, the
resulting maximum allowed value of dT/dt (that is the sum Min_dT/dt plus
dT/dt_add) will be smaller in the charging process of FIG. 4 than in the
charging process of FIG. 3.
For batteries having lower initial temperatures than the battery of FIG. 3,
yet being charged at the same ambient temperature, higher values of dT/dt
will be observed during the initial stage of charging, which values might
reach the desired termination value of dT/dt. To avoid premature
termination of the charging process, the charging process should be
controlled so as to avoid termination based on a high value of dT/dt
during the initial stage of charging. Such control might be accomplished
in a simple way by having a TimeLimit set at a high value, for example 15
minutes. However, setting TimeLimit to a high value might cause
overcharging of the battery when an almost fully charged battery is being
charged.
According to a variation of a charging method of the present invention,
overcharging an almost fully charged battery is avoided. Use of the
parameter VoltLimit to determine when values of dT/dt should be used to
control termination of the charging process avoids overcharging of almost
fully charged batteries. When charging a battery that is already almost
fully charged, the battery voltage will reach a high value such as
VoltLimit within a relatively short time period, whereas when charging a
battery that is almost fully discharged, the battery voltage will increase
much more slowly.
In FIG. 4, the battery voltage does not reach the compensated value of
VoltLimit until Time is about 22 minutes. At Time equal about 22 minutes,
battery temperature is about 30.degree. C., and the corresponding voltage
compensation is about +100 mV. When Vopen reaches VoltLimit, VoltLimit has
a compensated value of about 8.35 volts (8.25+100 mV).
FIG. 5 shows a charging process controlled as described above with
reference to the flow diagram of FIG. 2 and as applied to charge a 1600
mAh NiMH battery with 6 cells. The initial battery temperature for the
charging process of FIG. 5 is the same as the initial battery temperature
for the charging process of FIG. 3. However, in the process of FIG. 5, the
battery is holding 90% of its capacity at the beginning of the charging
process. In FIG. 5, the battery voltage reaches VoltLimit within 4 minutes
from beginning the charging process compared to 35 minutes for the fully
discharged battery of FIG. 3.
The battery voltage does not reach the compensated value of VoltLimit until
Time is about 4 minutes. At Time equal about 4 minutes, battery
temperature is about 23.5.degree. C., and the corresponding voltage
compensation is about +230 mV. When Vopen reaches VoltLimit, VoltLimit has
a compensated value of about 8.48 volts (8.25+230 mV).
FIG. 6 shows a charging process controlled as described above with
reference to the flow diagram of FIG. 2 and as applied to charge a 1600
mAh NiMH battery with 6 cells. In the process of FIG. 6, the battery is
holding 50% of its capacity at the beginning of the charging process.
However, the initial battery temperature is very low, about -7.degree. C.,
and the ambient charging temperature is high, about 35.degree. C.
In FIG. 6, the predetermined charging parameters are the same as for the
charging process of FIG. 3.
In the charging process of FIG. 6, the battery is charged with a
trickle-charge current (for example, a low, maintenance current) until the
battery temperature reaches about 5.degree. C., from which point in time a
normal charging process is begun. In FIG. 6, the normal charging process
is initiated when Time is about 9 minutes. In the process of FIG. 6 the
initial value for TimeLimit is set to 5 minutes, as in FIG. 3. Therefore,
the value of Min_dT/dt is not updated before Time reaches about 14 minutes
(9+5).
The battery voltage does not reach the compensated value of VoltLimit until
Time is about 27 minutes. Due to the low battery temperature, the measured
values of Vopen should be required to reach a relatively high value before
reaching the compensated VoltLimit value. At 25.degree. C., the
corresponding voltage compensation is about +200 mV so VoltLimit has a
compensated value of about 8.45 volts (8.25+200 mV). When Time is about 27
minutes, the battery temperature has increased to about 25.degree. C. and
Vopen reaches about 8.45 volts.
For the process of FIG. 6 the value of Min _dT/dt is not updated before
Time equals 14 minutes (according to steps 42 and 43 of FIG. 2). However,
after Vopen has reached the compensated VoltLimit, values of dT/dt are
used not only to update (replace) the value of Min_dT/dt, but also for
determining the termination process (according to steps 44 through 47 of
FIG. 2). The value of dT/dt decreases through the charging process until
the almost fully charged state is reached. During the charging process,
the value of Min_dT/dt is also being decreased until this almost fully
charged state is reached. Overcharging the battery is avoided by
determining the termination process without requiring the value of dT/dt
to reach a fixed value. Rather, the termination process is determined when
the value of dT/dt reaches a reference value, Min_dT/dt+dT/dt_add, which
is updated (decreased) during the charging process.
The present invention provides a method where termination of the charging
process can be controlled in an optimum way by using a reference value
(Min_dT/dt) which is determined during the charging process based on
determined values of the rate of change of battery temperature, whereby
variation in the reference value between performances of the charging
process, for example as a consequence of battery life, has the effect of
varying the termination of the charging process to avoid overcharging the
battery.
In another embodiment of the present invention, a method for charging a
rechargeable battery includes:
connecting an electrical power source to the terminals of the battery and
supplying a charging current to the battery,
determining values of the rate of change of battery temperature during at
least part of the process of charging the battery,
determining and storing a reference value based on the obtained values of
the rate of change of battery temperature,
comparing values of the rate of change of battery temperature with the
stored reference value or a function thereof, and
controlling termination of the charging process based on said comparison.
When monitoring battery temperature during a charging process the
temperature will increase when the battery approaches the fully charged
state. It is preferred to have termination of the charging process based
on a threshold value. To allow this threshold value to be adaptive, it is
preferred to determine the threshold value based on a plurality of values
of the battery temperature rate of change.
For example, termination of the charging process may be initiated when a
determined value of the rate of change of battery temperature exceeds a
calculated threshold. The threshold value may be calculated by adding a
predetermined maximum allowed change in the rate of change of battery
temperature (dT/dt_add) to a determined reference value (Min_dT/dt). In
order to avoid overcharging of the battery it is preferred that the
determined reference value represents a minimum of the obtained values of
the rate of change of battery temperature and it is important to use the
smallest possible value for the predetermined maximum allowed change. Such
a predetermined maximum allowed change may be in the range of 0.25 through
2.degree. C./minute and preferably about 0.5.degree. C./minute. However,
the optimal value will depend on battery capacity and battery technology.
To control termination of the charging process, the power supplied to the
battery needs to be limited (reduced). Here it is preferred that
termination of the charging process includes reducing the charging
current. This reduction may be abrupt by turning the power supply off.
The termination of the charging process may alternatively include a final
charging period, during which period the battery may be charged with a
reduced current until the charging process is finally stopped. Here, the
duration of the final charging period may be determined as a function of
the total charging time passed at the point in time at which termination
of the charging process is initiated. As an example, the length of the
final charging period may be in the range of 5% through 50%, preferably
about 25%, of the total charging time elapsed up to the point in time at
which termination of the charging process is initiated.
The final charging period need not be a function of the charging time but
may have a predetermined duration.
To limit power supplied to the battery during the final charging period,
the charging process may be controlled so as to reduce battery terminal
voltage by at least a predetermined amount during initiation of the final
charging period for avoiding overcharging. As an example, the battery
terminal voltage may be reduced at least 100 mV, preferably at least 200
mV, at the initiation of the final charging period, with the battery
terminal voltage preferably not being increased during this final period.
The battery terminal voltage may also be reduced as a function of the
number of cells within the battery. Such a reduction may be in the range
of 10 mV through 100 mV per cell, preferably in the range of 20 mV through
70 mV per cell, and even more preferred about 40 mV per cell.
Alternatively, the charging power may be reduced by controlling the
charging process so that the battery terminal voltage is not allowed to
increase during the final charging period, for example by keeping the
voltage constant during this final charging period.
When charging a battery, the determined rate of change of battery
temperature during upstart of the charging process may vary as a function
of the initial battery temperature. Thus, for a battery which has been
stored at a low temperature but which is being charged at a higher
temperature, a high value of the rate of change of battery temperature can
be observed during upstart of the charging process until the battery has
reached the ambient temperature.
To avoid the influence of such an initial high rate of change of battery
temperature, it is preferred that the control of the termination of the
charging process be based on values of the rate of change of battery
temperature as determined after a predetermined time period has lapsed or
after the value of a characteristic start-up charging parameter measured
during an initial stage of the charging process has reached a
predetermined value. In a preferred embodiment the characteristic start-up
charging parameter is the battery terminal voltage, which may be measured
as an open-circuit voltage.
When the charging process has been stopped, the capacity of the battery may
decrease due to self-discharging. Such self-discharging may depend on
battery technology (or type) and on individual batteries of the same type.
If self-discharging might be a problem, it is preferred that the state of
charge of the battery be maintained after termination of the charging
process by a trickle-charge current (for example, a pulsating current or a
low, maintenance current).
In yet another embodiment of the present invention, a method of charging a
rechargeable battery includes:
connecting an electrical power source to the terminals of the battery and
supplying a charging current to the battery,
determining values of a first characteristic charging parameter during at
least part of the charging process, and
controlling termination of the charging process based on values of the
first parameter being determined after a point in time at which point in
time a second characteristic charging parameter measured during an initial
stage of the charging process has reached a predetermined value or
fulfills a predetermined criteria.
Here, the first characteristic charging parameter may be any characteristic
of the battery which is suitable for the control of the charging process
such as battery terminal voltage, charging current, battery temperature,
the rate of change of any of these parameters or any combination of these
parameters and/or their rate of change. Preferably, the first
characteristic charging parameter is the rate of change of battery
temperature calculated from measured values of the battery temperature.
Similarly, the second characteristic charging parameter may also be
selected from any of the above mentioned first characteristic charging
parameters with the exception that it should not be the same parameter as
the one chosen as the first characteristic parameter. However, it is
preferred that the second characteristic charging parameter is the battery
voltage.
In order to terminate the charging process it is preferred that the
obtained values of the first parameter be compared with a stored reference
value or a function thereof, and the termination of the charging process
be based on said comparison. Preferably, the termination of the charging
process is initiated when the measured values of the first charging
parameter reaches a threshold value being a function of the stored
reference value. Thus, for example, termination of the charging process
may be initiated when the obtained value of the first parameter exceeds
the stored reference value by a predetermined amount.
In a preferred embodiment the stored reference value is determined during
the charging process based on obtained values of the first parameter. The
reference value may be determined as a maximum of the obtained values, but
it is preferred that the reference value represents a minimum value of the
obtained first parameter values.
When terminating the charging process, it is preferred that the termination
includes a final charging period, and it is preferred that the length of
the final charging period is determined as a function of the total
charging time passed at the point in time at which termination of the
charging process is initiated. Furthermore, it is preferred that
termination of the charging process includes reducing the charging current
or charging with a reduced current during the final charging period.
In FIG. 1, charge controller 25 includes control logic (including memory
for storage of variables, constants, and programmed instructions), an
analog to digital converter, and input/output circuits. The control logic
includes a general purpose arithmetic logic unit (ALU) such as used in the
conventional micro controller. Cooperation of the control logic and
programmed instructions accomplishes the decision and processing steps
described with reference to FIG. 2, including such operations as
addressing, identifying, determining, comparing, detecting when a value
has crossed a threshold value, calculating, updating, determining elapsed
time, responding to inputs, and providing outputs. Cooperation of the
control logic and the A/D converter accomplishes steps involving input
values, including such operations as measuring, detecting, monitoring,
converting, comparing, obtaining, and sensing. Cooperation of the control
logic and the output circuit accomplishes controlling power supply 24 and
switch 29, including such operations as enabling the provision of supplied
power to charge the battery, enabling provision of a trickle-charge
current, and limiting supplied power. All such cooperation is accomplished
by conventional program development techniques in light of the disclosure
of the present invention.
The above description of battery charger 20 of FIG. 1 illustrates a
preferred implementation. In alternate implementations, the functions of
battery charger 20 are accomplished with analog circuitry, digital
circuitry, or any combination of analog and digital circuitry. As one
example, charge controller 25 and signal conditioning circuit 26 may be
integrated to form a single integrated circuit. As another example, the
functions of signal conditioning circuit 26 and the A/D converter portion
of charge controller 25 may be combined to form a measurement circuit that
provides a digital signal conveying battery temperature or may provide
rate of change of battery temperature. Such a measurement circuit may
cooperate with a processor that performs all remaining functions of charge
controller 25.
What has been described above illustrates how a reference value is compared
to the rate of change of battery temperature, which reference value is
determined and stored during the charging process. The obtained reference
value is used when determining a threshold value for control of
termination of the charging process, whereby this embodiment of the
present invention implements adaptive battery charging. Such adaptive
battery charging allows the present embodiments to account for changes or
differences in the threshold value of the battery temperature rate of
change due to aging or manufacturing tolerances. Furthermore, such
adaptive battery charging allows the present embodiments to account for
variations in the threshold value due to differences in ambient
temperatures.
The above described embodiments for a charging process also bring a
solution which accounts for differences in the initial battery
temperature, whereby a premature termination of the charging process due
to a high initial temperature rate of change is avoided.
The above described embodiments of the present invention apply to
recharging batteries such as NiMH (Nickel Metal Hydride) batteries and
other types of rechargeable batteries including, for example, NiCd (Nickel
Cadmium) and Lithium batteries.
The foregoing description of preferred exemplary embodiments of the
invention has been presented for the purposes of illustration and
description. It is not intended to be exhaustive or to limit the invention
to the precise form disclosed. Many modifications and variations are
possible in light of the description, as will be apparent to those skilled
in the art. All such modifications which retain the basic underlying
principles disclosed and claimed herein are within the scope of this
invention.
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