U.S. patent application number 09/794967 was filed with the patent office on 2001-09-06 for method and device for balancing charges of a plurality of series-connected battery cells.
Invention is credited to Dudley, Geoffrey John, Olsson, Dan Hakan Lennart.
Application Number | 20010019256 09/794967 |
Document ID | / |
Family ID | 8847608 |
Filed Date | 2001-09-06 |
United States Patent
Application |
20010019256 |
Kind Code |
A1 |
Olsson, Dan Hakan Lennart ;
et al. |
September 6, 2001 |
Method and device for balancing charges of a plurality of
series-connected battery cells
Abstract
A method and device for balancing the charges of a plurality of
series-connected battery cells, the device comprising a current
dissipative loop connected across the terminals of each battery
cell, the voltage across each battery cell being measured, the
average value of the measured voltages being determined, the
measured voltage being then compared to the average value, and the
measured voltage of each battery cell being adjusted to the average
value so as to equalize charges of all battery cells.
Inventors: |
Olsson, Dan Hakan Lennart;
(Voorhout, NL) ; Dudley, Geoffrey John; (De Kaag,
NL) |
Correspondence
Address: |
Barry L. Kelmachter
BACHMAN & LaPOINTE, P.C.
Suite 1201
900 Chapel Street
New Haven
CT
06510-2802
US
|
Family ID: |
8847608 |
Appl. No.: |
09/794967 |
Filed: |
February 27, 2001 |
Current U.S.
Class: |
320/118 |
Current CPC
Class: |
H02J 7/0016
20130101 |
Class at
Publication: |
320/118 |
International
Class: |
H02J 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2000 |
FR |
0002645 |
Claims
There is claimed:
1. A method for balancing the charges of a plurality of
series-connected battery cells (4), comprising the steps of
connecting a current dissipative loop across each battery cell,
measuring voltage across each battery cell, determining an average
voltage value of said measured voltages, comparing each of said
measured voltages to said average voltage value, and for each
battery cell, adjusting said measured voltage to said average
voltage value so as to equalize the charges of all battery
cells.
2. The method according to claim 1, wherein the dissipative loop
connected to each of said battery cells is closed if the voltage
measured across the battery cell is greater than said average
voltage value, and opened if the voltage measured across said
battery cell is lower than said average voltage value.
3. A device for balancing the respective charge currents of a
plurality of series-connected battery cells, comprising first means
for measuring individual voltages of said battery cells, second
means for determining an average voltage of all series-connected
battery cells, and a control unit for triggering an equalization of
said measured voltages with said determined average voltage.
4. The device according to claim 3, wherein each battery cell
comprises a current dissipative loop connected to said control
unit, said control unit comprising means for opening the
dissipative loop, if the individual voltage of the battery cell is
lower than said average voltage, and closing said dissipative loop
if the individual voltage of the battery cell is greater than said
average voltage.
5. The device according to claim 3, further comprising for each
battery cell, a comparator, a first resistor having a first
terminal at a first voltage connected to a positive terminal of
said comparator via a second resistor, and a second terminal at a
second voltage connected to a negative terminal of said comparator
via a third resistor, each of said positive and negative terminals
of said comparator being additionally connected to a negative and a
positive terminal of said cell respectively, via a fourth and a
fifth resistor, respectively, an output of said comparator being
connected to a dissipation resistor, a difference between said
first voltage and said second voltage corresponding to said average
voltage.
6. The device according to claim 5, wherein the dissipative loop of
each battery cell is controlled to be closed when the following
condition is fulfilled: V.sub.c++V.sub.r->V.sub.r++V.sub.c-,
V.sub.c- and V.sub.c+ being respective electric voltages of said
negative and positive cell terminals.
7. The device according to claim 3, further comprising, for each
battery cell a comparator having a positive and a negative input
terminal, each being connected to a first resistor and a second
resistor, the second resistors being connected to battery
terminals, respectively, the first resistors being connected
respectively to a positive and a negative terminal of said cell,
and an output of said comparator being connected to one of said
cell terminals via a dissipation resistor.
8. The device according to claim 7, wherein the second resistors
have a resistance which equals n times a resistance of said first
resistors, n being a number of battery cells connected in series,
said device further comprising, in parallel to each second resistor
a respective third resistor having a resistance n-1 times the
resistance of the first resistor, and switching means for
connecting either said second resistors, or said third resistors
across the battery terminals, respectively, according to whether
all battery cells are operating, or whether one of said battery
cells is faulty.
9. The device according to claim 7, wherein the second resistors
have a resistance equal to n-1 times the resistance of the first
resistors, n being the number of battery cells connected in series,
said device further comprising means for generating a voltage equal
to (n-1)/n times the voltage across the battery terminals, and
switching means for applying to the second resistors, either the
voltage across the battery terminals in case of failure of one cell
in the battery, or the voltage supplied from the voltage generation
means otherwise.
10. The device according to claim 4, further comprising for each
battery cell, at least one Zener diode which is series-connected
within said dissipative loop, so as to avoid discharging of said
cell when its voltage decreases below a given threshold.
11. The device according to claim 4, further comprising for each
battery cell, a transistor having a base terminal connected to the
output of the comparator, and an emitter and a collector terminal
connected respectively across the terminals of said cell, a
dissipating resistor being provided in series within the
dissipative loop comprising a collector-emitter junction of said
transistor T.
12. The device according to claim 11, further comprising for each
battery cell, at least one Zener diode connected in series between
the output of said comparator and the base terminal of said
transistor for increasing a threshold voltage at which the
collector-emitter junction of transistor is turned on, and
therefore, for avoiding discharging of said cell when the voltage
across its terminals decreases below a given threshold.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method and a device for
balancing the charges of a plurality of series-connected battery
cells.
[0003] 2. Description of the Prior Art
[0004] The devices known for balancing the charges of series
connected battery cells are based either on a comparison of the
individual voltage of each battery cell with an overcharge voltage,
either upon actuation of an overcurrent relay, connecting the
different battery cells together. These techniques are not adapted
for lithium battery cells because current or voltage overcharge can
cause the destruction of battery cells.
[0005] EP-0 767 524 (MOTOROLA) describes a method for balancing the
charge distributed among two series connected cells. This method
consists in comparing the currents through each of the cells and
discharging the cell whose charging voltage is the highest when the
difference between the detected currents is greater then a
predetermined value. One drawback of this device that, on the one
hand, currents can reach high values and damage the cells, and on
the other hand, that the arrangement required for carrying out this
comparison is complex.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to provide a method
and device for balancing the charges of series-connected battery
cells while avoiding the destruction of such battery cells due to a
voltage or current overcharge.
[0007] This object is achieved by a method wherein a current
dissipative loop is connected across each battery cell, the voltage
across each battery cell is measured, an average value of the
measured voltages is determined, the measured voltage is compared
to said average value, and for a given battery cell, the measured
voltage is adjusted to said average value so as to equalize charges
of all battery cells.
[0008] For this purpose, the dissipative loop is closed if the
voltage measured across the relevant battery cell is greater than
the average of the measured voltages, and said dissipative loop is
opened if the voltage measured across said battery cell is lower
than the average of the measured voltages.
[0009] The method according to this invention is carried out by a
device comprising means for measuring the individual voltage of
each battery cell, means for determining the average voltage of all
series-connected battery cells and control means for equalizing the
measured voltage with said determined average voltage.
[0010] Other features and advantages of the invention will become
apparent from the following description of preferred embodiments of
the present invention, provided by way of non-limiting examples, in
reference to the appended Figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 schematically shows a block diagram of a device for
carrying-out the method according to the present invention;
[0012] FIG. 2 schematically shows a first preferred embodiment of
the device shown in FIG. 1
[0013] FIG. 3 schematically shows a second preferred embodiment of
the device shown in FIG. 1;
[0014] FIG. 4 shows a modification of the measurement and control
circuit in one cell, according to the second more specific
embodiment shown in FIG. 3;
[0015] FIG. 5 shows another modification of the measurement and
control circuit in one cell, according to the second specific
embodiment shown in FIG. 3;
[0016] FIG. 6 shows still another modification of the cell
measurement and control circuit, according to the second specific
embodiment shown in FIG. 3.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] The device shown in FIG. 1 comprises a first means 2 for
measuring the individual voltage of each battery cell 4, a second
means 6 for determining the average voltage of all series-connected
battery cells 4, and a control unit 8 for the equalization of the
measured voltage with said determined average voltage.
[0018] Each battery cell 4 comprises a current dissipative loop 10,
connected to said control unit 8, which causes either the opening
of loop 10, if the individual voltage of battery cell 4 is lower
than the average voltage, or the closing of said loop if the
individual voltage of battery cell 4 is greater than the average
voltage.
[0019] In a preferred embodiment of the present invention shown in
FIG. 2, said dissipative loop 10 comprises a comparator 12 and a
resistor Rc 14, a first terminal 16 of which, of electric potential
V.sub.r+, is connected to the positive terminal 18 of comparator 12
through a first resistor R.sub.S, and a second terminal 20 of
which, of electric potential V.sub.r-, is connected to the negative
terminal 22 of said comparator 12 via a second resistor R.sub.S,
which is identical to the first one, each of said positive 18 and
negative 22 terminals of comparator 12 being additionally
connected, respectively, to the negative terminal 24 and to the
positive terminal 26 of battery cell 4 of electric potentials
V.sub.c- and V.sub.c+, respectively, via a third and a fourth
resistors R.sub.S, which are identical to the first one. The output
of comparator 12 is connected to a dissipating resistor Rb. The
dissipative loop 10 therefore comprises, in the drawing of FIG. 2,
a resistor Rb, the comparator 12, which controls the opening and
closing of the loop, and fourth resistor R.sub.S.
[0020] With such a device, the dissipative loop 10 is controlled so
as to be closed when the following condition is fulfilled:
V.sub.c+V.sub.r->V.sub.r++V.sub.c- (1)
[0021] In an embodiment which is not shown, the operation of the
device according to the present invention is improved by the
addition of a positive feedback resistor connecting the output of
the comparator 12 to the positive input 18 of said comparator. This
causes a hysteresis that prevents oscillations when the voltages of
battery cells are close to each other.
[0022] In another embodiment, the number of opening cycles of
dissipative loop 10 can be substantially reduced by a slight
modification in the input resistances R.sub.S of comparator 12 so
as that the dissipative loop is closed when the individual voltage
of a battery cell 4 exceeds the average voltage by a predefined
value. This allows for small variations in the voltages of battery
cell 4 without closing the dissipative loop 10 at the same
time.
[0023] According to another embodiment, the value of both summing
resistors at the input of comparator C can be slightly changed so
as to derive from the cell current only when their charge exceeds
the predetermined average value by a given value. This causes a
reduction in the number of switching cycles and allows small
variations in the cell voltages to be obtained without at the same
time deriving therefrom.
[0024] According to yet another embodiment shown in FIG. 3, the
resistor network Rc was eliminated from the circuit shown in FIG.
2, and resistors R.sub.S connecting the positive and negative
inputs of comparator 12, respectively, to voltages V.sub.r+ and
V.sub.r- are replaced by resistors of value n*R.sub.S, where n is
the number of battery cells 4, and which are related to the battery
voltages V.sub.B+ and V.sub.B-, respectively. Therefore, the
voltage comparison which is thus carried-out by comparator 12 is
equivalent to a voltage comparison performed by the corresponding
cell with the average cell voltage (battery voltage/n).
[0025] By means of this arrangement, the input voltages of the
comparator are always ranging between its supply voltages V.sub.CC
and V.sub.DE, whatever the imbalances between the respective
charges of cells 4. The battery power which is dissipated within
the resistor network Rc becomes zero. In addition, it is easier to
adapt the circuit to the cell number when one of them is detected
to be faulty and is disconnected. The latter feature is essential
for space applications, since there are normally other devices in
the battery which will permanently disconnect any cell detected as
faulty.
[0026] In the case of the circuit shown in FIG. 2, resistor Rc
corresponding to a faulty cell should be short-circuited, which
requires the addition of n switching circuits, where n is the
number of battery cells. On the contrary, in the circuit shown in
FIG. 3, only one switch is necessary for the entire battery. In
this respect, according to a first aspect shown in FIG. 4, each
cell is associated with two additional resistors of resistance
(n-1)*R.sub.S, which are thus dimensioned for a battery comprising
n-1 cells. Before cell failure, the first set of resistors
n*R.sub.S is connected in parallel with the connection terminals of
the battery. When detecting a cell failure, the first resistor set
is disconnected, whereas the second resistor set (n-1)*R.sub.S is
connected, by means of a switch 25.
[0027] In a specific embodiment of the present invention shown in
FIG. 4, the output of comparator 12 is connected to the base of a
pnp transistor T connected in parallel across the positive 26 and
negative 24 terminals of each cell 4, the collector of transistor T
being connected to terminal 24 of the cell through a resistor R1,
which is series-connected to a Zener diode Z1 or to a potential
barrier, so as to avoid cell discharging if its charge voltage has
decreased below a predetermined voltage threshold. In this circuit,
the dissipative loop comprises resistor R1, Zener diode Z1 and the
collector-emitter junction of transistor T. Of course, Zener diode
Z1 can be series-connected to other Zener diodes so that the
predetermined voltage threshold can be achieved.
[0028] Transistor T is useful in the case where the output current
of comparator 12 is insufficient for efficiently discharging the
corresponding cell 4 when its voltage is greater than the average
voltage of the battery cells.
[0029] According to another preferred embodiment of the present
invention, shown in FIG. 5, adjustable resistors R2 are
series-connected with resistors Rs for adjusting the resistance of
the latter. Moreover, in this embodiment, transistor T is
controlled by the output of comparator 12, which is connected to
two Zener diodes Z1, Z2, and a resistor R3, which are
series-connected, resistor R3 being connected to the base of
transistor T. In this circuit, the dissipative loop comprises the
collector-emitter junction of transistor T and a resistor R4, which
is connected between the emitter of transistor T and terminal 24 of
the corresponding cell 4. Zener diodes Z1 and Z2 allow the
threshold voltage at which the collector-emitter junction of
transistor T is turned on, and therefore below which cell 4 is not
discharged, to be increased.
[0030] According to a second aspect shown in FIG. 6, each circuit
in the cell is provided with a set of resistors
R.sub.S(n-1)*R.sub.S, which is adapted for a battery of n-1 cells.
Before cell failure, this resistor set is connected through a
switch 30 to the junction of two series-connected resistors Rd and
(n-1)*Rd that comprise a resistor divider bridge, connected to
terminals 28, 29 of the battery so as to achieve a voltage equal to
(n-1)/n of the voltage (V.sub.B+-VB.sub.-) provided by the battery.
After cell failure, switch 30 is toggled so as to apply the entire
voltage provided by the battery, across both resistors
(n-1)*Rd.
[0031] With respect to the first aspect shown in FIGS. 4 and 5,
this aspect uses more energy due to the presence of the divider
bridge, but requires less additional components.
[0032] Of course, any programmable device such as a computer or a
custom-made circuit can be used for controlling cell charging and
discharging according to the invention, while remaining within the
scope of this invention.
* * * * *