Lithium-ion battery pack

A lithium-ion battery pack, despite having a simple circuit configuration, allows the charge amount of the individual battery cells provided therein to be made uniform quickly. For each battery cell, a cell balance circuit is provided that includes a resistor, a transistor, and a control circuit. The resistor and the transistor are connected in series, with their end terminals connected to the two terminals of the battery cell. The control circuit controls the state of the transistor so that the two terminals of the battery cell are short-circuited through the resistor when necessary. The control circuit includes an oscillation circuit for producing a sawtooth wave voltage that oscillates within a predetermined voltage range and outputting it after adding it to the voltage at the negative terminal of the battery cell, and a comparator for comparing the voltage at the positive terminal of the battery cell with the voltage output from the oscillation circuit and outputting, when the former exceeds the latter, a control voltage that makes the transistor conduct. When the cell voltage is within the range of the voltage produced by the oscillation circuit, the transistor is driven with a pulse-width-modulated driving voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a lithium-ion battery pack, and more 
particularly to a lithium-ion battery pack provided with a cell balance 
system for keeping uniform the amount of electric charge accumulated in 
individual battery cells connected in series. 
2. Description of the Prior Art 
A lithium-ion battery pack, which includes a plurality of lithium-ion 
battery cells connected in series, is rechargeable, meaning that a battery 
pack discharged by use can be recharged for further use. To achieve safe 
recharging, it is essential to prevent overcharging of the battery cells, 
and thus the charging of the battery cells is typically so controlled as 
to be stopped as soon as the voltage across any one of the battery cells 
reaches the maximum permissible voltage. 
However, since the individual battery cells discharge different amounts of 
electric charge in actual use, they often come to have different voltages 
from one another. In such a case, recharging tends to bring into a fully 
charged state only the battery cell that has discharged the least during 
use, leaving the other battery cells in an incompletely charged state. 
For this reason, a lithium-ion battery pack is usually provided with a cell 
balance system, which serves to prevent overcharging and simultaneously 
achieve uniform charging of all of its battery cells. FIG. 6 shows the 
circuit configuration of a conventional lithium-ion battery pack. 
In this circuit configuration, for each of a number n of battery cells 
LC(k) (k=1, . . . , n), a cell balance circuit CLB(k) is provided that is 
composed of a resistor Rb(k), a transistor Tb(k) acting as a switching 
device, and a control circuit CLC(k). For each battery cell LC(k), the 
resistor Rb(k) and the transistor Tb(k) are connected in series between 
the two terminals of the battery cell LC(k). The control circuit CLC(k) 
monitors the voltage across the battery cell LC(k) to control the 
conduction state of the transistor Tb(k) in accordance with that voltage. 
FIG. 7 shows the circuit configuration of the control circuit CLC(k). The 
control circuit CLC(k) is composed of a reference voltage generator VG(k), 
a comparator CMP(k), two input terminals Vp(k) and Vn(k) connected to the 
positive and negative terminals, respectively, of the battery cell LC(k), 
and an output terminal Vc(k) connected to the gate of the transistor 
Tb(k). The comparator CMP(k) has two input terminals of which one is 
connected to the input terminal Vp(k) and the other is connected through 
the reference voltage generator VG(k) to the input terminal Vn(k), and has 
an output terminal that is connected to the output terminal Vc(k). 
The reference voltage generator VG(k) receives, via the input terminal 
Vn(k), the voltage at the negative terminal of the battery cell LC(k), and 
outputs a reference voltage that is equal to a predetermined level 
V.sub.TH0 added to the received voltage. The comparator CMP(k) receives, 
via the input terminal Vp(k), the voltage at the positive terminal of the 
battery cell LC(k), and compares the received voltage with the voltage fed 
from the reference voltage generator VG(k). In accordance with which of 
these two voltages is higher, the comparator CMP(k) outputs a first or a 
second predetermined voltage in such a way as to make the transistor Tb(k) 
conduct when the voltage at the positive terminal of the battery cell 
LC(k) is higher than the reference voltage. When the transistor Tb(k) 
conducts, a bypass path is formed through the resistor Rb(k) in parallel 
with the battery cell LC(k). 
FIG. 8 shows the relationship between the voltage of the battery cell LC(k) 
and the operation of the transistor Tb(k) during charging. In FIG. 8, the 
graph at (a) shows the voltage (cell voltage) V.sub.cell between the two 
terminals of the battery cell LC(k), and the graph at (b) shows whether 
the transistor Tb(k) is in a conducting (on) state or in a non-conducting 
(off) state. In both graphs, the lapse of time after the start of charging 
is taken along the horizontal axis. 
As the result of the operation of the control circuit CLC(k) as described 
above, when the cell voltage V.sub.cell is lower than the predetermined 
level V.sub.TH0, the transistor Tb(k) is kept in the non-conducting state, 
with the result that a current flows through the battery cell LC(k) so as 
to charge it. By contrast, when the cell voltage V.sub.cell is equal to or 
higher than the predetermined level V.sub.TH0, the transistor Tb(k) is 
kept in the conducting state, with the result that the current flows 
mostly through the bypass path, greatly slowing down the progress of 
charging. 
Consequently, the battery cells that have a cell voltage lower than the 
predetermined level V.sub.TH0 are charged with priority. On the other 
hand, for the battery cells that have a cell voltage equal to or higher 
than the predetermined level V.sub.TH0, the increasing rate of their cell 
voltage is kept so low that the maximum permissible voltage V.sub.OCH that 
is set to prevent overcharging is reached slowly. In this way, the amount 
of electric charge accumulated in the individual battery cells is made 
substantially uniform. 
However, in this conventional cell balance system, the control circuit 
CLC(k) controls the operation of the transistor Tb(k) on the basis of only 
one predetermined level V.sub.TH0. As a result, the adjustment of the 
charge amount needs to be started as late as immediately before the 
completion of charging, and thus an unduly long time is required to make 
the charge amount of all of the battery cells uniform. Moreover, for the 
battery cells whose cell voltage has already reached the predetermined 
level V.sub.TH0, a current is kept flowing through the bypass path, and 
thus the resistor Rb(k) produces much heat. To prevent the resistor Rb(k) 
from being destroyed by heat, it is inevitable to give it a considerably 
high resistance to reduce the current that flows through the bypass path. 
This, however, makes even longer the time required to make the charge 
amount of the individual battery cells uniform. 
Japanese Laid-Open Patent Application No. H8-19188 proposes a charging 
apparatus for charging a set battery having a plurality of battery cells 
connected in series. In this charging apparatus, for each of the battery 
cells, a bypass path is provided that is composed of a resistor and a 
transistor connected in series, and in addition a control circuit is 
provided that controls the conduction state of each bypass path 
individually. During charging, the control circuit monitors the cell 
voltage of each battery cell and calculates the voltage difference 
.DELTA.Vmin between the cell voltage of each battery cell and the minimum 
cell voltage; in accordance with this voltage difference .DELTA.Vmin, the 
control circuit controls the conduction state of the bypass path by one of 
the following two methods. 
According to the first method, for the battery cells whose voltage 
difference .DELTA.Vmin is equal to or higher than a first predetermined 
level, the bypass path is made to conduct so that the difference in the 
charge amount among the battery cells will be reduced. Moreover, when the 
voltage difference .DELTA.Vmin of the battery cells for which the bypass 
path has been kept conducting to keep the charging rate low becomes equal 
to a second predetermined level that is lower than the first predetermined 
level, the bypass path for those battery cells is cut off so that charging 
will be continued while maintaining the reduced difference in the charge 
amount. 
According to the second method, the current that flows through the bypass 
path is adjusted in accordance with the voltage difference .DELTA.Vmin. 
Specifically, for each battery cell, the transistor is so controlled as to 
operate with a duty factor that varies in proportion to the voltage 
difference .DELTA.Vmin. The higher the duty factor with which the 
transistor of a bypass circuit operates, the higher the current that flows 
through the bypass circuit. Thus, the larger the amount of electric charge 
accumulated in a battery cell, the lower the charging rate at which the 
battery cell is charged. In this way, the difference in the charge amount 
among the battery cells is gradually reduced. 
On the other hand, Japanese Laid-Open Patent Application H9-28042 proposes 
a charge control apparatus for charging a set battery. In this charge 
control apparatus, too, for each of the battery cells, a bypass path is 
provided that is composed of a resistor and a transistor connected in 
series, and in addition a control circuit is provided that controls the 
conduction state of each bypass path individually. Before starting 
charging, the control circuit monitors the cell voltage of each battery 
cell and calculates the voltage difference .DELTA.Vmax between the cell 
voltage of each battery cell and the maximum cell voltage; in accordance 
with this voltage difference .DELTA.Vmax, the control circuit controls the 
conduction state of the bypass path by one of the following three methods. 
According to the first method, for the battery cells whose cell voltage has 
reached a predetermined level, the bypass path is made to conduct; in 
addition, the current that is allowed to flow through each bypass path is 
determined in accordance with the voltage difference .DELTA.Vmax so that 
the cell voltage of all of the battery cells will reach, substantially at 
the same time, the maximum permissible voltage that is set to prevent 
overcharging. According to the second method, for each battery cell, the 
level of the cell voltage that activates the transistor of the bypass path 
is determined in accordance with the voltage difference .DELTA.Vmax so 
that the cell voltage of all of the battery cells will reach, 
substantially at the same time, the maximum permissible voltage that is 
set to prevent overcharging. According to the third method, the variation 
of the cell voltage among all of the battery cells is calculated from the 
voltage difference .DELTA.Vmax of each battery cell so that the bypass 
path will be allowed to conduct only when the variation exceeds a 
predetermined range. 
The charging apparatus and the charge control apparatus described above are 
both capable of adjusting the current that flows through the bypass path, 
and are therefore capable of making the charge amount of all of the 
battery cells uniform with ease. It is possible even to apply the control 
methods used in those apparatuses to a cell balance system for a 
lithium-ion battery pack. 
However, in both of the apparatuses described above, the bypass path is 
controlled on the basis of the voltage difference .DELTA.Vmin between the 
cell voltage of each battery cell and the minimum cell voltage, or the 
voltage difference .DELTA.Vmax between the cell voltage of each battery 
cell and the maximum cell voltage. This requires, in addition to circuits 
for monitoring the cell voltage of the individual battery cells, circuits 
for performing comparison to find the minimum or maximum cell voltage, and 
also circuits, or a computing device such as a microcomputer, for 
calculating voltage differences. Thus, it is inevitable to use complicated 
and large-scale control circuits, in particular in a control module that 
is composed solely of analog circuits without using any logic circuit such 
as a microcomputer. Using such control circuits in a cell balance system 
for use in a battery pack ends in making the entire battery pack unduly 
large. 
A wide variety of lithium-ion battery packs are commercially available, 
ranging from small-scale ones that include as few as two battery cells to 
large-scale ones that include a hundred or more battery cells. The smaller 
the scale of a lithium-ion battery pack, the more seriously the 
compactness of the battery pack is spoilt by the introduction of a 
large-scale cell balance system. Since small-scale battery packs are used 
mainly in portable electronic appliances such as portable telephones, they 
are required to be as small and light as possible, and accordingly a cell 
balance system for use in such appliances is required to have as simple a 
circuit configuration as possible. For this reason, it is not necessarily 
advisable to apply the control methods used in the two apparatuses 
described above to a cell balance system for use in a lithium-ion battery 
pack. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a lithium-ion battery pack 
that allows the charge amount of the individual battery cells provided 
therein to be made uniform quickly under the control achieved by a simple 
circuit configuration. 
To achieve the above object, according to the present invention, a 
lithium-ion battery pack is provided with: a series of battery cells 
composed of a plurality of battery cells connected in series; a set of a 
resistor and a switching device connected in series and provided one set 
for each of the battery cells, the set of the resistor and the switching 
device having one end connected to the positive terminal of the battery 
cell and having the other end connected to the negative terminal of the 
battery cell so that, every time the switching device conducts, the 
positive and negative terminals of the battery cell are short-circuited 
with each other through the resistor; and a control circuit provided one 
for each of the battery cells. In this lithium-ion battery pack, while the 
series of battery cells is being charged, with a voltage applied between 
the two end terminals thereof, the control circuit monitors the voltage 
between the positive and negative terminals of the battery cell to control 
the switching device in such a way that, when the monitored voltage is 
lower than a first predetermined level that is fixed relative to a maximum 
permissible voltage for the battery cell, the switching device is 
continuously kept in a non-conducting state, that, when the monitored 
voltage is equal to or higher than the first predetermined level and lower 
than a second predetermined level that is fixed relative to the maximum 
permissible voltage for the battery cell, the switching device is 
intermittently brought into a conducting state, and that, when the 
monitored voltage is equal to or higher than the second predetermined 
level, the switching device is kept continuously in the conducting state. 
Thus, for each of the battery cells constituting the series of battery 
cells, a cell balance circuit is provided that is composed of a resistor, 
a switching device, and a control circuit. The cell balance circuit allows 
the charging rate to be adjusted for each battery cell. Here, in 
accordance with the cell voltage, the switching device is brought into one 
of the following three states: a continuous non-conducting state, an 
intermittent conducting state, and a continuous conducting state. The 
charging of the battery cell progresses at a high rate when the switching 
device is in the continuous non-conducting state, at a medium rate when 
the switching device is in the intermittent conducting state, and at a low 
rate when the switching device is in the continuous conducting state. 
As the charging progresses, when the cell voltage of a battery cell reaches 
the first predetermined level, the battery cell will thereafter be charged 
at a medium rate. As the charging further progresses, when the cell 
voltage of the battery cell reaches the second predetermined level, the 
battery cell will thereafter be charged at a low rate. Thus, even if the 
individual battery cells have discharged different amounts of electric 
charge during use, whereas the battery cells that are already in the late 
stages of charging are charged at a medium rate, the battery cells that 
are still in the early stages of charging are charged at a high rate, and 
accordingly the difference in the charge amount diminishes quickly. As a 
result, when the cell voltage of one battery cell reaches the second 
predetermined level, the difference in the charge amount among the battery 
cells is already slight, and therefore it is possible to make the charge 
amount of all of the battery cells uniform quickly. In addition, the 
switching device is brought into a conducting state only intermittently, 
and therefore the resistor produces less heat. This makes it possible to 
increase the current that flows through the resistor, and thereby make the 
charge amount of the individual battery cells uniform more quickly. 
Each control circuit controls the charging of the corresponding battery 
cell in accordance with the cell voltage of that battery cell alone, and 
therefore the charging of one battery cell is not affected by the cell 
voltage of the other battery cells. Accordingly, there is no need to 
provide a means to calculate the minimum or maximum cell voltage and to 
calculate the difference between the cell voltage of the individual 
battery cells and the minimum or maximum cell voltage. Moreover, the first 
and second predetermined levels are kept constant relative to the maximum 
permissible voltage, and therefore there is no need to provide a means to 
vary the first and second predetermined levels. Thus, charging can be 
controlled with a simple circuit configuration. 
The control circuit may be so configured that, when the cell voltage is 
equal to or higher than the first predetermined level and lower than the 
second predetermined level, the length of the period in which the 
switching device is held in the conducting state is varied in accordance 
with the difference between the cell voltage and the first predetermined 
level. This makes it possible, with the battery cells whose cell voltage 
has reached the first predetermined level, to adjust the charging rate in 
accordance with the already accumulated charge amount, and thereby make 
the charge amount of the individual battery cells uniform more quickly. 
Specifically, for example, the length of the period in which the switching 
device is held in the conducting state is varied substantially in 
proportion to the difference between the cell voltage and the first 
predetermined level. This makes it possible to gradually reduce the 
charging rate for the battery cells whose cell voltage has reached the 
first predetermined level, and thereby minimize the variation in the 
charge amount among all of the battery cells on completion of the 
charging. 
When the cell voltage is equal to or higher than the first predetermined 
level and lower than the second predetermined level, the length of the 
period in which the switching device is held in the conducting state can 
be varied substantially in proportion to the difference between the cell 
voltage and the first predetermined level by bringing the switching device 
into the conducting state at regular intervals while keeping constant the 
sum of the length of the period in which the switching device is held in 
the conducting state and the length of the period in which the switching 
device is held in the non-conducting state. 
Specifically, for example, the control circuit is composed of an 
oscillation circuit for producing a sawtooth wave voltage that oscillates 
between the first predetermined level and the second predetermined level 
at regular intervals and outputting it after adding it to the voltage at 
the negative terminal of the battery cell, and a comparator for comparing 
the voltage at the positive terminal of the battery cell with the voltage 
output from the oscillation circuit and outputting, when the voltage at 
the positive terminal of the battery cell is equal to or higher than the 
voltage output from the oscillation circuit, a first control voltage that 
causes the switching device to be brought into the conducting state and, 
when the voltage at the positive terminal of the battery cell is lower 
than the voltage output from the oscillation circuit, a second control 
voltage that causes the switching device to be brought into the 
non-conducting state. 
It is preferable that all of the control circuits provided one for each of 
the battery cells be formed within a single semiconductor chip, and that 
all of the sets of the resistor and the switching device provided one set 
for each of the battery cells be formed separately from that semiconductor 
chip. This makes it possible to separate the control circuit from the 
resistor and the switching device, through which a relatively high current 
flows, and thereby stabilize the operation of the control circuit with 
ease. Moreover, forming a number of control circuits on a single 
semiconductor chip makes it possible to use semiconductor chips of an 
identical design in various types of battery packs that include as many or 
fewer battery cells, and thus helps enhance manufacturing efficiency. 
It is also possible to additionally provide a first electrode terminal 
connected through a protection switching device to one of the two end 
terminals of the series of battery cells, a second electrode terminal 
connected to the other of the two end terminals of the series of battery 
cells, and a protection control circuit for monitoring the voltage between 
the positive and negative terminals of each of the battery cells so that, 
when at least one of the monitored voltages is out of a predetermined 
range, the protection switching device is brought into a cut-off state to 
inhibit the charging or discharging of the series of battery cells. In 
this case, it is preferable that the protection control circuit be formed 
within the semiconductor chip. This makes it possible to prevent 
overcharging and overdischarging of the battery cells. In addition, 
forming all the circuits related to the control of the charge amount of 
the battery cells within a single chip helps minimize the size of the 
battery pack.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, lithium-ion battery packs embodying the present invention will 
be described with reference to the accompanying drawings. FIG. 1 shows the 
circuit configuration of the lithium-ion battery pack 1 of a first 
embodiment of the invention. The lithium-ion battery pack 1 includes a 
number n of lithium-ion battery cells LC(1), . . . , LC(n) of an identical 
capacity connected in series. These battery cells LC(1), . . . , LC(n) 
constitute a series 10 of battery cells. 
In the following descriptions, for simplicity's sake, the battery cells 
LC(1), . . . , LC(n) will be referred to as the battery cell LC(k) (k=1, . 
. . , n). Moreover, as will be described in detail later, the lithium-ion 
battery pack 1 also includes the same number n of each of a few kinds of 
circuit element as the number n of the battery cells LC(k). Those circuit 
elements will be identified similarly with symbols followed by the 
subscript k in parentheses. 
The lithium-ion battery pack 1 further includes a positive terminal B+ and 
a negative terminal B-. The positive terminal B+ is connected, through a 
protection fuse 11 and two transistors 12 and 13, to one end terminal (the 
positive end terminal) of the series 10 of battery cells (i.e. the 
positive terminal of the first battery cell LC(1)). The negative terminal 
B- is connected directly to the other end terminal (the negative end 
terminal) of the series 10 of battery cells (i.e. the negative terminal of 
the nth battery cell LC(n)). The transistors 12 and 13 operate under the 
control of a protection control circuit 14. 
The protection control circuit 14 is formed on a single semiconductor chip 
CP1 having output terminals FE1 and FE2, an input terminal MO, and a 
terminal GND. The output terminals FE1 and FE2 are connected to the gate 
of the transistors 12 and 13, respectively. The input terminal MO is 
connected through a resistor 15 to the junction between the transistors 12 
and 13. The terminal GND is connected to the negative end terminal of the 
series 10 of battery cells. 
The protection control circuit 14 further has a number n of input terminals 
V(k). Each of the input terminals V(k) is connected through a resistor 
Rf(k) to the positive terminal of the corresponding battery cell LC(k), 
and is also connected through a capacitor Cf(k) to the negative end 
terminal of the series 10 of battery cells. Each pair of the resistor 
Rf(k) and the capacitor Cf(k) forms a low-pass filter. 
The transistor 12 serves to prevent overcharging of the battery cell LC(k), 
and is controlled by a control voltage that is fed from the protection 
control circuit 14 via the output terminal FE1. Normally, the protection 
control circuit 14 keeps the transistor 12 in a conducting state; during 
charging, when the cell voltage V.sub.cell of any of the battery cells 
LC(k) reaches the maximum permissible voltage V.sub.OCH, the protection 
control circuit 14 brings the transistor 12 into a cut-off state. The cell 
voltage V.sub.cell of the individual battery cells LC(k) is detected as 
the difference between appropriate ones among the voltages appearing at 
the input terminals V(k) and the voltage appearing at the terminal GND. 
The transistor 13 serves to prevent overdischarging of the battery cell 
LC(k) during discharging, i.e. during use, and also serves, when an unduly 
high current flows through the battery pack, to shut off the current 
temporarily. The transistor 13 is controlled by a control voltage that is 
fed from the protection control circuit 14 via the output terminal FE2. 
Normally, the protection control circuit 14 keeps the transistor 13 in a 
conducting state; during discharging, when the cell voltage V.sub.cell of 
any of the battery cells LC(k) drops below the minimum permissible voltage 
V.sub.MIN, the protection control circuit 14 brings the transistor 13 into 
a cut-off state. In addition, the protection control circuit 14 detects, 
by way of the resistor 15, the current that flows through the transistor 
13 so as to bring the transistor 13 into a cut-off state also when the 
detected current exceeds a predetermined level. 
Thus, the transistors 12 and 13 and the protection control circuit 14 
protect the battery cells LC(k) by keeping the cell voltage V.sub.cell of 
all of the battery cells LC(k) equal to or higher than the minimum 
permissible voltage V.sub.MIN and lower than the maximum permissible 
voltage V.sub.OCH ; in addition, when an unduly high current is likely to 
flow through the battery pack as a result of, for example, a short circuit 
between the positive and negative terminals B+ and B-, they prevent a 
hazard by shutting off the current. 
For each of the battery cells LC(k), a cell balance circuit CB(k) is 
provided that is composed of a resistor Rb(k), a transistor Tb(k) acting 
as a switching device, and a control circuit CC(k) for controlling the 
operation of the transistor Tb(k). These cell balance circuits CB(k) 
together constitute a cell balance system 20 that prevents overcharging 
and simultaneously makes uniform charging of all of the battery cells 
possible. The resistor Rb(k) and the transistor Tb(k) are connected in 
series with each other, and are together connected in parallel with the 
battery cell LC(k), with their end terminals connected to the positive and 
negative terminals of the battery cell LC(k). 
All of the control circuits CC(k) are formed on a single semiconductor chip 
CP2 having a terminal GND and a number n pairs of an input terminal Vp(k) 
and an output terminal Vc(k). The terminal GND is connected to the 
negative end terminal of the series 10 of battery cells (i.e. the negative 
terminal of the nth battery cell LC(n)). The input terminal Vp(k) is 
connected through a resistor Rf(k) to the positive terminal of the battery 
cell LC(k). The output terminal Vc(k) is connected to the gate of the 
transistor Tb(k). 
The difference between the voltages appearing at two adjacent input 
terminals Vp(k) and Vp(k+1) represents the cell voltage V.sub.cell of the 
battery cell LC(k), and is used by the control circuit CC(k). The 
difference between the voltage appearing at the input terminal Vp(n) and 
the voltage appearing at the terminal GND represents the cell voltage 
V.sub.cell of the nth battery cell LC(n), and is used by the control 
circuit CC(n). 
FIG. 2 shows the circuit configuration of the control circuit CC(k). The 
control circuit CC(k) is composed of an oscillation circuit OSC(k), a 
comparator CMP(k), the above-mentioned input and output terminals Vp(k) 
and Vc(k), and an input terminal Vn(k). Here, in reality, the terminal 
Vn(k) is the same as the terminal Vp(k+1). The output terminal Vc(k) is 
connected to the output terminal of the comparator CMP(k), and the input 
terminal Vp(k) is connected to one input terminal of the comparator 
CMP(k). The input terminal Vn(k) is connected to the input of the 
oscillation circuit OSC(k), and the output of the oscillation circuit 
OSC(k) is connected to the other input terminal of the comparator CMP(k). 
The oscillation circuit OSC(k) produces a sawtooth wave voltage that 
oscillates between a first predetermined level V.sub.TH1, and a second 
predetermined level V.sub.TH2 at regular intervals, and outputs the thus 
produced oscillating voltage as a reference voltage V.sub.R after adding 
it to the voltage V.sub.N received via the input terminal Vn(k) from the 
negative terminal of the battery cell LC(k). The first and second 
predetermined levels V.sub.TH1 and V.sub.TH2 are determined in 
consideration of the maximum and minimum permissible voltages V.sub.OCH 
and V.sub.MIN for the battery cell LC(k) so as to be constant relative to 
the maximum permissible voltage V.sub.OCH. FIG. 3 shows an example of the 
waveform of the reference voltage V.sub.R output from the oscillation 
circuit OSC(k). Note that, although the rising and falling inclinations of 
the voltage are illustrated as being equal to each other in this figure, 
they may be different from each other in practice. 
The comparator CMP(k) outputs at the output terminal Vc(k) a control 
voltage that has either of two different levels at a time, and thereby 
controls the conduction state of the transistor Tb(k). The comparator 
CMP(k) compares the voltage V.sub.P received via the input terminal Vp(k) 
from the positive terminal of the battery cell LC(k) with the reference 
voltage V.sub.R received from the oscillation circuit OSC(k), and changes 
the level of the control signal it outputs in accordance with the result 
of the comparison. Specifically, the comparator CMP(k) keeps the 
transistor Tb(k) in a conducting state when the voltage V.sub.P at the 
positive terminal of the battery cell LC(k) is equal to or higher than the 
reference voltage V.sub.R, and keeps it in a non-conducting state when the 
voltage V.sub.P at the positive terminal of the battery cell LC(k) is 
lower than the reference voltage V.sub.R. 
Accordingly, when the cell voltage V.sub.cell of the battery cell LC(k) is 
lower than the first predetermined level V.sub.TH1, the transistor Tb(k) 
is kept in the non-conducting state, and therefore no current flows 
through the resistor Rb(k). On the other hand, when the cell voltage 
V.sub.cell of the battery cell LC(k) is equal to or higher than the second 
predetermined level V.sub.TH2, the transistor Tb(k) is kept in the 
conducting state, and therefore a current flows through the resistor 
Rb(k). 
When the cell voltage V.sub.cell of the battery cell LC(k) is equal to or 
higher than the first predetermined level V.sub.TH1 and lower than the 
second predetermined level V.sub.TH2, the result of the comparison by the 
comparator CMP(k) is inverted at regular intervals in synchronism with the 
oscillation of the reference voltage V.sub.R. As a result, the transistor 
Tb(k) is brought into the conducting state intermittently, and therefore a 
current flows through the resistor Rb(k) intermittently. In addition, the 
length of the individual periods in which the transistor Tb(k) is held in 
the conducting state varies in proportion to the difference between the 
cell voltage V.sub.cell and the first predetermined level V.sub.TH1 ; 
specifically, the smaller the voltage difference, the shorter the 
conducting period of the transistor Tb(k), and the larger the voltage 
difference, the longer the conducting period of the transistor Tb(k). 
Thus, when the cell voltage V.sub.cell is equal to or higher than the first 
predetermined level V.sub.TH1 and lower than the second predetermined 
level V.sub.TH2, the control circuit CC(k) subjects the control voltage 
fed to the transistor Tb(k) to pulse-width modulation (PWM) in accordance 
with the difference between the cell voltage V.sub.cell and the first 
predetermined level V.sub.TH1. 
FIG. 4 shows the relationship between the cell voltage V.sub.cell of the 
battery cell LC(k) and the operation of the transistor Tb(k) as observed 
while the lithium-ion battery pack 1 is being charged. In FIG. 4, the 
graph at (a) shows the cell voltage V.sub.cell of the battery cell LC(k), 
and the graph at (b) shows whether the transistor Tb(k) is in a conducting 
(on) state or in a non-conducting (off) state. In both graphs, the lapse 
of time after the start of charging is taken along the horizontal axis. 
Here, note that, for simplicity's sake, the conduction state of the 
transistor Tb(k) is illustrated in a simplified manner for the period T 
during which the cell voltage V.sub.cell is in the range between the first 
and second predetermined levels V.sub.TH1 and V.sub.TH2. In reality, 
during this period T, the conduction state of the transistor Tb(k) changes 
at short intervals in synchronism with the oscillation cycle of the 
oscillation circuit OSC(k) in such a way that, as the cell voltage 
V.sub.cell becomes closer to the second predetermined level V.sub.TH2, the 
length of the conducting period increases and the length of the 
non-conducting period decreases within each cycle. 
When the cell voltage V.sub.cell is lower than the first predetermined 
level V.sub.TH1, no current flows through the resistor Rb(k), and a charge 
current flows through the battery cell LC(k) so that the battery cell 
LC(k) is charged at a high charging rate. When the cell voltage V.sub.cell 
reaches the first predetermined level V.sub.TH1, a current starts to flow 
intermittently through the resistor Rb(k) so that the charging rate of the 
battery cell LC(k) is gradually reduced. When the cell voltage V.sub.cell 
reaches the second predetermined level V.sub.TH2, the current starts to 
flow continuously through the resistor Rb(k) so that only a slight 
charging current flows through the battery cell LC(k) and thereby the 
charging rate thereof is reduced further. When the charging finally makes 
the cell voltage V.sub.cell equal to the predetermined maximum permissible 
voltage V.sub.OCH, the protection control circuit 14 shuts off the 
charging current, which is supplied through the transistor 12. 
In cases where different amounts of electric charge remain in the 
individual battery cells LC(k) because of uneven discharging during use, 
charging proceeds in the following manner. Of the number n of battery 
cells LC(k), let the battery cell that has the largest amount of electric 
charge remaining therein be called "cell A", and let the battery cell that 
has the smallest amount of electric charge remaining therein be called 
"cell B". After the start of charging, cell A and cell B are at first 
charged substantially at the same, high charging rate, and the cell 
voltage of cell A reaches the first predetermined level V.sub.TH1 earlier. 
After the cell voltage of cell A reaches the first predetermined level 
V.sub.TH1, the charging rate of cell A is gradually reduced, whereas cell 
B continues to be charged at the initial, high charging rate. Accordingly, 
when the cell voltage of cell B reaches the first predetermined level 
V.sub.TH1, the difference in the charge amount between cells A and B, 
which was relatively large at the start of charging, has already been 
reduced greatly. 
After the cell voltage of cell B reaches the first predetermined level 
V.sub.TH1, the charging rate of cell B is gradually reduced; at this time, 
the charging rate of cell A, which is already in a later stage of 
charging, has become even lower. Thus, also during the period T, the 
difference in the charge amount between cells A and B continues to become 
smaller. Accordingly, when the cell voltage of cell A reaches the second 
predetermined level V.sub.TH2, there remains almost no difference in the 
charge amount between cells A and B. 
The other battery cells, which had less electric charge than cell A and 
more electric charge than cell B remaining therein at the start of 
charging, are charged in the same way as cell B so as to eventually have 
the same charge amount as cell A. Accordingly, when the cell voltage of 
cell A reaches the second predetermined level V.sub.TH2, all of the 
battery cells LC(k) have almost the same charge amount. In this way, the 
charge amount of all of the battery cells LC(k) is made uniform quickly. 
In addition, once the cell voltage of cell A reaches the second 
predetermined level V.sub.TH2, it is not necessary to continue the 
charging for a long time. This helps reduce the time required for 
charging. Moreover, during the period T, a current flows through the 
resistor Rb(k) only intermittently, and, after the second predetermined 
level V.sub.TH2 has been reached, the length of the period in which a 
current flows continuously through the resistor Rb(k) is short. This helps 
reduce the heat produced by the resistor Rb(k). Accordingly, it is 
possible to increase the charging current without increasing the risk of 
destroying the resistor Rb(k) with heat, and thereby make the charge 
amount of all of the battery cells LC(k) uniform even more quickly. 
The first and second predetermined levels V.sub.TH1 and V.sub.TH2 may be 
determined arbitrarily; however, it is to be noted that these levels 
directly affect the time required for charging and the uniformity of the 
charge amount of the individual battery cells at the end of charging. For 
satisfactory results, it is preferable to set the first predetermined 
level V.sub.TH1 at about 50 to 75% of the maximum permissible voltage 
V.sub.OCH, and set the second predetermined level V.sub.TH2 at about 85 to 
95% of the maximum permissible voltage V.sub.OCH. 
The voltage produced by the oscillation circuit OSC(k) may have a waveform 
other than a sawtooth-shaped waveform; for example, it may have a waveform 
as obtained by subjecting a sine wave to full-wave rectification. In that 
case, during the period T in which the cell voltage V.sub.cell is in the 
range between the first and second predetermined levels V.sub.TH1 and 
V.sub.TH2, the length of the period in which the transistor Tb(k) is held 
in the conducting state does not vary in proportion to the difference 
between the cell voltage V.sub.cell and the first predetermined level 
V.sub.TH1 ; even then, the length of the conducting period of the 
transistor Tb(k) varies, in a way similar to the way it varies when the 
produced voltage has a sawtooth-shaped waveform, in accordance with the 
difference between the cell voltage V.sub.cell and the first predetermined 
level V.sub.TH1 in such a way that, as the difference becomes larger, the 
conducting period becomes longer. 
Although a relatively high current flows through the resistor Rb(k) and the 
transistor Tb(k), the protection control circuit 14 and the control 
circuit CC(k) are formed on a separate chip therefrom, and are therefore 
almost insusceptible to such a high current. Thus, the lithium-ion battery 
pack 1 offers high reliability. As the switching device for 
short-circuiting the two terminals of the battery cell LC(k) through the 
resistor Rb(k), it is possible to use any appropriate device such as an 
FET, MOSFET, or bipolar transistor as long as it can be switched between a 
conducting and a non-conducting state in accordance with the level of the 
control voltage fed thereto. 
Although as many control circuits CC(k) as the battery cells LC(k) are 
provided in the embodiment described above, it is also possible to form a 
number of control circuits on the semiconductor chip CP2 so that, in the 
course of the manufacture of a battery pack, as many control circuits as 
are necessary are actually connected to the battery cells. This makes it 
possible to use semiconductor chips of an identical design in various 
types of battery packs that include different numbers of battery cells, 
and thus helps enhance manufacturing efficiency. 
The series 10 of battery cells may include two or any larger number of 
battery cells LC(k) so as to offer voltages as desired in specific 
applications. Moreover, each battery cell LC(k) may be composed of two or 
more battery cells connected in parallel with each other in order to 
obtain a lithium-ion battery pack having a large capacity as a whole. 
FIG. 5 shows the circuit configuration of the lithium-ion battery pack 2 of 
a second embodiment of the invention. In contrast to the lithium-ion 
battery pack 1 of the first embodiment, where the protection control 
circuit 14 that controls the transistors 12 and 13 provided for protection 
against overcharging and overdischarging is formed on one semiconductor 
chip CP1 and the control circuits CC(k) constituting the cell balance 
system 20 are formed on another semiconductor chip CP2, in the lithium-ion 
battery pack 2 of the second embodiment, the protection control circuit 14 
and the control circuits CC(k) of the cell balance system 20 are formed on 
a single semiconductor chip CP3. In terms of the circuit configuration and 
the operation, there is no difference between the lithium-ion battery 
packs of the first and second embodiments. The resistors Rb(k) and the 
transistors Tb(k), which are also constituent elements of the cell balance 
system 20, are provided separately from the semiconductor chip CP3. 
Forming the protection control circuit 14 and the control circuits CC(k) of 
the cell balance system 20 on a single semiconductor chip CP3 in this way 
makes the wiring to the battery cells LC(k) easy, and thus helps simplify 
the manufacturing process of the battery pack In addition, it is possible 
to minimize the increase in the size of the battery pack that results from 
the introduction of a cell balance system 20 therein. 
As described above, according to the present invention, in a lithium-ion 
battery pack, it is possible to start the adjustment of the charging rate 
of the individual battery cells in the early stages of charging, and thus 
it is possible to make the charge amount of all of the battery cells 
uniform quickly. Moreover, since the adjustment of the charging rate is 
achieved by passing a current intermittently through a resistor, it is 
possible to increase the charging current without increasing the heat 
produced by the resistor, and thus it is possible to make the charge 
amount of all of the battery cells uniform more quickly. 
In addition, the charging of each battery cell is controlled on the basis 
of the cell voltage of that battery cell alone and by the use of a 
constant reference level, and therefore there is no need to provide a 
means to compare the cell voltages among the battery cells or a means to 
vary the reference level. This helps simplify the circuit configuration 
required to control charging, and thereby minimize the increase in the 
size and weight of the battery pack. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims, the invention may be 
practiced other than as specifically described.