Electricity: battery or capacitor charging or discharging – Parallel connected batteries
Reexamination Certificate
2002-02-27
2004-01-20
Toatley, Jr., Gregory J. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Parallel connected batteries
C320S104000, C307S010100
Reexamination Certificate
active
06680600
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a power supply unit comprising a multiplicity of cells such as lithium secondary cells, nickel hydrogen cells, lead seal cells, electric double layer capacitors and fuel cells connected in series parallel, and a distributed power supply system and an electric vehicle including them.
In the case where a plurality of cells are connected in series, the variations of capacitance, initial voltage and temperature from one cell to another causes a different voltage for a different cell, thereby making it difficult for all the series-connected cells to share the voltage across the circuit uniformly.
Especially in the case where the lithium secondary cells or the electric double layer capacitors employing an organic solvent as an electrolytic solution are connected in series, voltage variations causes an overcharge or an overdischarge, often resulting in a rupture or a fire, or at least an overcharge or an overdischarge, which poses the problem of an extremely shortened service life of the cells.
In order to prevent the overcharge or overdischarge, the charge/discharge operation may be performed with a pre-set protective level. In charge mode, however, the charge operation stops when the voltage across a high-voltage cell has reached the protective level. As a result, the remaining low-voltage cells fail to be fully charged before the end of the charge operation.
In similar fashion, the discharge operation stops at the time point when the voltage across a low-voltage cell has reached a protective level. As a result, the remaining high-voltage cells cannot be fully discharged before the end of the discharge operation.
In the series connection of cells, therefore, the charge/discharge time becomes shorter than in the case where each cell is charged/discharged independently.
In a conventional battery charging apparatus intended to solve this problem, the charge current supplied through a bypass is changed by a current changing means progressively according as the voltage across the cells being charged approaches a set value thereby to set the cells into a uniform state. Examples are illustrated in U.S. Pat. No. 5,557,189 and a corresponding Japanese Patent No. JP-A-7-230829.
FIG. 12
is a diagram showing such a battery charging apparatus. In
FIG. 12
, reference numerals
1101
a
to
1101
c
designate cells, numerals
1102
a
to
1102
c
voltage detection means, numeral
1103
set voltage application means, numerals
1104
a
to
1104
c
comparison control means, and numerals
1105
a
to
1105
c
current changing means. The circuit for the cell
1101
a
is so configured that the voltage detection means
1102
a
, the comparison control means
1104
a
and the current changing means
1105
a
are connected in parallel to each other, and the set voltage application means
1103
applies a set voltage indicating the setting of a voltage value of the cell
1101
a.
The present voltage value of the cell
1101
a
is detected by the voltage detection means
1102
a
, and compared in the comparison control means
1104
a
with the set value of the voltage applied by the set voltage application means
1103
a
. According as the present cell voltage approaches the set voltage value, the charge current flowing in the current changing means is increased progressively. Specifically, the charge current to the cell
1101
a
is controlled progressively downward. In this way, an overcharge is prevented.
The fact about the cell
1101
a
described above equally applies to the cell
1101
b
and the cell
1101
c
. In other words, the voltage detection means
1102
b
, the comparison control means
1104
b
and the current changing means
1105
b
for the cell
1101
b
, and the voltage detection means
1102
c
, the comparison control means
1104
c
and the current changing means
1105
c
for the cell
1101
c
, work exactly the same manner as the corresponding means, respectively, of the cell
1101
a.
Another example of the prior art is disclosed in JP-A-2000-78768. This is intended to correct the variations caused at the time of charging the lithium ion secondary cell and to prevent the trouble such as overcharge for an improved service life. Specifically, a negative electrolytic solution circulation pump and a positive electrolytic solution circulation pump are used for correcting the variations of the charge/discharge operation. Still another example of the prior art is disclosed in JP-A-2000-511398. This is a system for equalizing the cells and is a combination of energy storage elements that can be switched. Specifically, the charge is shifted between batteries each including a plurality of cells connected in series. The charge is pulled out of a particular battery of a higher voltage and transferred to another battery of a lower voltage.
In the conventional battery charging apparatus, a cell voltage at the time of charging is compared with a set value, and with the approach of the cell voltage to the set voltage value, the charge current is progressively diverted to the current changing means in parallel to the cells thereby to assure uniform conditions of the cells.
According to the prior art, however, the amount of current that can be diverted is greatly limited by the heat generated in the current changing means. Thus, the effect of obviating the voltage variations among the cells is reduced. The current changing means having a large thermal capacitance through which a large current can flow, on the other hand, is large in size and the system becomes bulky. Also, an electrical circuit other than the cells is required and increases the cost. The method of circulating the electrolytic solution, on the other hand, requires a pump. Also, a battery equalizer including a switch circuit for moving the charge by switching and a control circuit for the switch circuit is required.
SUMMARY OF THE INVENTION
The present invention has been developed in view of the problems described above, and the object thereof is to provide an inexpensive, compact power supply unit which can correct the voltage variations among cells connected in series.
According to this invention, there is provided a power supply unit comprising a first cell group and a second cell group connected in parallel to the first cell group, in which the electrolytic solution of the second cell group can be electrolyzed or the generated gas can be recombined. A plurality of the parallel-connected pairs are connected in series to each other and also to a charger/discharger. The charger/discharger is adapted to charge the cells at appropriate timing to a voltage at which the electrolytic solution of the second cells is electrolyzed or a voltage at which the generated gas is recombined. As a result, a plurality of parallel-connected pairs including the cells of the first cell group and the second cell group are equalized at a voltage at which the electrolytic solution of the cells of the second cell group is electrolyzed or a voltage at which the generated gas is recombined.
In the parallel-connected pair of the first cell group and the second cell group according to the invention, the first cell group and the second cell group are connected in parallel through a current limiter. The current limiter limits the current flowing between the first cell group and the second cell group, and prevents the overcurrent of the first cell group or the second cell group, thereby making it possible to protect the power supply unit at the time of a fault.
A plurality of series circuits including the parallel-connected pairs of the first cell group and the second cell group are connected in parallel. As a result, the capacitance, the output and the service life of the power supply unit can be variably increased.
In this invention, the withstanding voltage of the cells of the first cell group is set to a level higher than the withstanding voltage of the cells of the second cell group. Specifically, the electrolytic solution of the cells of the second cell group is electrolyzed or the gas is generated and recombined withi
Emori Akihiko
Kinoshita Takuya
Miyazaki Hideki
Hitachi , Ltd.
Toatley , Jr. Gregory J.
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