Cell voltage balancer

Details Cell voltage balancer Cell voltage balancer

2000-09-26

2002-03-12

Toatley, Gregory (Department: 2838)

Electricity: battery or capacitor charging or discharging

Serially connected batteries or cells

C320S121000, C320S124000

Reexamination Certificate

active

06356055

ABSTRACT:

BACKGROUND OF THE INVENTION
This application incorporates by reference Taiwanese application Serial No. 89211927, Filed Jul. 11, 2000.
1. Field of the Invention
The invention relates in general to a cell voltage balancer, and more particularly to a cell voltage balancer which is capable of balancing the voltage of numerous cells which are connected in series.
2. Description of the Related Art
Unbalance charging usually occurs in cells connected in series. Cells connected in series have equal current flowing through but may not be equally charged due to various cell voltages and cell capacities of the cells. Consequently, some cells are overcharged while some are not fully charged.
The lifetime of overcharged cells could be shortened as a result of raised temperature. Irreversible chemical reaction may also occur, which reduces the performance of the cells and causes permanent damage of the cells.
Therefore, the application of a cell voltage balancer is important.
FIG. 1
, is a schematic diagram showing charging cells connected in series with the aid of a conventional cell voltage balancer. Four cells in charging are taken as an example. The conventional cell voltage balancer
100
includes resistances R
1
, R
2
, R
3
, R
4
, switches S
1
,S
2
,S
3
, and S
4
. Cells B
1
,B
2
,B
3
and B
4
are charged by charger
101
. Cell B
1
is connected in parallel with resistance R
1
and switch S
1
connected serially; cell B
2
is connected in parallel with resistance R
2
and switch S
2
connected serially; cell B
3
is connected in parallel with resistance R
3
and switch S
3
connected serially; and cell B
4
is connected in parallel with resistance R
4
and switch S
4
connected serially. The voltage V
1
, V
2
, V
3
, and V
4
of the respective cells B
1
, B
2
, B
3
and B
4
are inputted into the control signal generator
102
for controlling the switches S
1
, S
2
, S
3
and S
4
.
While in charging mode, The charger
101
can, for example, provide a constant current I to charge the cells. An average voltage V of the cells B
1
, B
2
, B
3
and B
4
is obtained after the voltages V
1
, V
2
, V
3
and V
4
are received by the control signal generator
102
. The voltages V
1
, V
2
V
3
and V
4
of each cell are then compared with the average voltage V. If the voltage of a cell is higher than the average voltage V, the corresponding switch is set on.
For example, while the voltage V
1
of the cell B
1
is higher than the average voltage V, and the voltages of the cells B
2
, B
3
and B
4
are all lower than the average voltage V, the control signal generator
102
sets the switch S
1
on and the switches S
2
, S
3
and S
4
off. Meanwhile, the current I will be separated in two directions. The first direction is still to charge cell B
1
. The second direction is through R
1
and S
1
. The charging current of B
1
is less then the others, so the voltage rise of B
1
will less then others.
When the charger
101
stops charging the cells B
1
, B
2
, B
3
and B
4
and these cells are not balanced in voltage, the control signal generator
102
can still receive voltages, V
1
, V
2
, V
3
and V
4
and set the corresponding switch on or off in order to balance the voltage of the cells.
However, the conventional method and mechanism stated above has the following disadvantages:
1. the resistance R
1
consumes a part of the electrical energy and therefore the efficiency of the charger is reduced and the energy dissipated in resistance R
1
is wasted; and
2. the heat produced by the resistance R
1
raises the surrounding temperature, which reduces the performance of the cells.
To reduce the disadvantage of energy consumption of the cell voltage balancer as shown in
FIG. 1
, the discharging resistance is replaced by a transformer.
FIG. 2A
is a circuit block diagram showing another conventional cell voltage balancer applicable to charging serially connected cells. And
FIG. 2B
illustrates the operation of the circuit as shown in FIG.
2
A. The transformer
202
includes a primary winding
204
and a secondary winding
206
and the transformer
208
includes a primary winding
210
and a secondary winding
212
. The primary windings
204
,
206
are serially connected with the switches S
1
, S
2
, respectively. The secondary windings
206
,
212
are serially connected with the diodes D
1
, D
2
, respectively. The transformers
202
,
208
, switches S
1
, S
2
, and diodes D
1
, D
2
together form a conventional cell voltage balancer
214
.
Cell B
1
and the serially connected primary winding
204
and switch S
1
are connected in parallel. Cell B
2
and the serially connected primary winding
210
and switch S
2
are connected in parallel. The secondary winding
206
and the diode D
1
connect to the secondary winding
212
and the diode D
2
in parallel. The turns of the secondary windings
206
,
212
are at least two times of that of the primary windings
204
,
210
. The following example is taken for illustration, assuming that the of the secondary windings
206
,
212
is two times of the primary windings
204
,
210
and the voltage of cell B
1
is larger than that of cell B
2
.
As shown in
FIG. 2B
, when the voltage of cell B
1
is larger than the average voltage of cell B
1
and cell B
2
, the switch S
1
is turned on and cell B
1
discharges. Meanwhile, current I
1
flows through the primary winding
204
. Because the voltage across the primary winding
204
is equal to the voltage of cell B
1
, the voltage across the secondary winding
206
is two times of that of the primary winding
204
; that is two times of the voltage of the cell B
1
. Consequently, the voltage of the node N
1
is larger than the sum of the voltage of cell B
1
and the voltage of cell B
2
. So, the induced current
12
of the secondary winding
206
flows through diode D
1
and cell B
1
and flows toward cell B
2
to charge cell B
2
. Consequently, the voltage of cell B
1
is decreased to effectively avoid overcharge of cell B
1
. In addition, the energy released by cell B
1
transfers to cell B
2
.
Similarly, when the charger
200
stops charging cell B
1
and cell B
2
, the voltage unbalance of cells B
1
and B
2
may occurs and the cell voltage balancer
214
could still function to balance the voltage of the cells.
FIG. 3
shows the conventional cell voltage balancer of
FIG. 2A
which charges n serially connected cells. In order to recover energy to the n serially connected cells, the voltage of the secondary winding must be n times of that of the primary winding. Thus, the turns of the secondary windings
304
1
,
304
2
. . . ,
304
n
of the transformers
302
1
,
302
2
. . . ,
302
n
should be more than n times of that of the primary windings
306
1
,
306
2
. . .
306
n
. Although the conventional cell voltage balancer is capable of recovering the energy of high voltage cells to others, the design of the cell voltage balancer therefor becomes much more complex and the size of the transformer is thus enlarged due to the various turns of the transformer while various number of cells are used.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a cell voltage balancer which effective recovers energy with the aid of a transformer. The transformer of the invention is provided in the form of a module so that the turns of the transformer will not be a function of the number of series connected cells. The turns of each transform is fixed so that the circuit design is simplified and the size of the transformer is reduced.
It is another object of the invention to provide a cell voltage balancer for balancing the voltage of the first cell and the voltage of the second cell. The cell voltage balancer includes the first input terminal, the second input terminal and the third input terminal. The cell voltage balancer includes a transformer, the first switch, the second switch, the first diode and the second diode. The transformer includes a primary winding and a secondary winding. The first switch and the primary winding are serially connected between the first input terminal and the se

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