Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2000-08-09
2001-04-24
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S164000
Reexamination Certificate
active
06222346
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a battery protection device for protecting a rechargeable battery, such as a lithium-ion rechargeable battery, from overcharging and overdischarging.
2. Description of the Related Art
Charging or discharging of rechargeable batteries beyond their suitable charge or discharge conditions may cause breakdown or deterioration of the battery. In particular, lithium-based rechargeable batteries tend to fail due to generation of gas and temperature increase when the electrolyte dissolves because of continuing overcharging. There is thus a need for a charge control, which makes sure that the rechargeable battery is not subjected to overcharging. It is therefore the normal practice to provide a battery protection device and to control charging and discharging of the rechargeable battery through the battery protection device, wherein the charge/discharge circuit is interrupted when overcharge or over-discharge is detected.
FIG. 6
shows the configuration of a protective device for a rechargeable battery as disclosed in Japanese Patent No. 2872365. A first switching element
31
and a second switching element
32
are connected in series in a charge/discharge circuit that connects a rechargeable battery
30
with input/output terminals
34
and
35
. A control means
33
switches the first and the second switching element
31
and
32
between a conducting state and an interrupting state. The control means
33
detects the voltage of the rechargeable battery
30
, in accordance with which it turns the first switching element
31
into the conducting state when the battery voltage is below a charge-prohibiting voltage, that is indicative of a voltage beyond which the battery will be overcharged, and turns the second switching element
32
into the conducting state when the battery voltage is above a discharge-prohibiting voltage, that is indicative of a voltage below which the battery will be over-discharged. In other words, when the rechargeable battery
30
complies with suitable charge/discharge conditions, the first and the second switching elements
31
and
32
are conducting, and the charge/discharge circuit for the rechargeable battery
30
establishes a conducting connection to the input/output terminals
34
and
35
.
When the battery voltage is above the charge-prohibiting voltage, the charge/discharge circuit is interrupted, because the control means
33
turns the first switching element
31
into the interrupting state, and the charging is stopped, so that the rechargeable battery
30
is prevented from becoming overcharged. Conversely, when the battery voltage is below the discharge-prohibiting voltage, the charge/discharge circuit is interrupted, because the control means
33
turns the second switching element
32
into the interrupting state, and the discharging is stopped, so that the rechargeable battery
30
is prevented from becoming over-discharged.
As shown in
FIG. 6
, the first and second switching elements
31
and
32
both have parasitic diodes
31
a
and
32
a
between source and drain. The first and second switching elements
31
and
32
are connected in a manner that the forward direction of the parasitic diode
31
a
of the first switching element
31
corresponds to the discharge direction of the rechargeable battery
30
, and the forward direction of the parasitic diode
32
a
of the second switching element
32
corresponds to the charge direction of the rechargeable battery
30
.
Thus, when the control means
33
turns the first switching element
31
into the interrupting state to prevent overcharging, a discharging circuit is formed through the parasitic diode
31
a,
so that the rechargeable battery
30
can be discharged through the parasitic diode
31
a,
and when discharging has caused the battery voltage to drop below a charge-permitting voltage that is lower than the charge-prohibiting voltage, the control means
33
turns the first switching element
31
into the conducting state, and returns to a discharge state, in which the parasitic diode
31
a
cannot be passed. Conversely, when the control means
33
turns the second switching element
32
into the interrupting state to prevent over-discharging, a charging circuit is formed through the parasitic diode
32
a,
so that the rechargeable battery
30
can be charged through the parasitic diode
32
a,
and when charging has caused the battery voltage to rise above a discharge-permitting voltage that is higher than the discharge-prohibiting voltage, the control means
33
turns the second switching element
32
into the conducting state, and returns to a charge state, in which the parasitic diode
32
a
cannot be passed.
By using the parasitic diodes
31
a
and
32
a
of the first and second switching elements
31
and
32
in this fashion, discharging is possible when charging is prohibited, and charging is possible when discharging is prohibited, so that the rechargeable battery
30
can still be used when the circuit is interrupted to prevent overcharging and over-discharging of the rechargeable battery
30
.
However, in this conventional configuration, there is the problem that during the time until a voltage is reached at which the charge-prohibiting state or the discharge-prohibiting state is cancelled, discharging or charging is carried out through the parasitic diodes
31
a,
32
a,
whereby, when a large current flows through the parasitic diodes
31
a,
32
a,
heat is generated in the first switching element
31
or the second switching element
32
, which may lead to their deterioration. That is, the power loss when a certain current flows through the parasitic diodes
31
a,
32
a
in forward direction is several times higher than the power loss when the same current flows between drain and source in the power MOSFETs used for the first and the second switching elements. Even though the heat generated by the current flowing between drain and source does not cause a failure of the power MOSFETs, the same current may lead to thermal destruction and failure of the power MOSFETs when flowing through the parasitic diodes.
In the case where a plurality of cells are connected in series to constitute the rechargeable battery
30
, it is not sufficient to detect only the battery voltage. That is, if there are variations in the battery capacity between various cells, those that have lower battery capacity may fall into a state of over-discharge. In an extreme case, some cells that have lower battery capacity may discharge until their battery capacity is zero, they will be charged by other cells, leading to rapid deterioration of the cells. Therefore, when the rechargeable battery is constituted by connecting a plurality of cells in series, control for preventing over-discharge must be effected in consideration of the overall balance of the battery capacity of various cells.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a battery protection device that allows discharging when charging is prohibited and charging when discharging is prohibited, while ensuring that no large discharge current or charge current flows through the parasitic diodes of the switching elements.
To attain this and other objects, a battery protection device according to a first aspect of the present invention includes:
a first switching means and a second switching means connected in series in a charge/discharge circuit connecting a positive electrode of a rechargeable battery to a positive input/output terminal and a negative electrode of the rechargeable battery to a negative input/output terminal; and
a control means for
detecting (i) a battery voltage between the positive electrode and the negative electrode of the rechargeable battery and (ii) a terminal voltage between the positive input/output terminal and the negative input/output terminal;
turning on the first switching means when the battery voltage is below a charge-prohibiting voltage at which charging the rechargeable
Jordan and Hamburg LLP
Matsushita Electric - Industrial Co., Ltd.
Toatley , Jr. Gregory J.
Tso Edward H.
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