Electricity: battery or capacitor charging or discharging – Battery or cell discharging – With charging
Reexamination Certificate
2001-01-11
2003-06-10
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Battery or cell discharging
With charging
C320S136000, C320S150000
Reexamination Certificate
active
06577105
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a protective circuit and a protective element incorporated therein for protecting a rechargeable battery such as a lithium ion rechargeable battery against overcharging.
Charging of rechargeable batteries beyond their suitable charge conditions leads to the generation of gas or heat due to decomposition of electrolyte and may cause breakdown or deterioration of the battery. Lithium-based rechargeable batteries are particularly susceptible to deterioration or damage when operated at a voltage exceeding a predetermined range. It is therefore the normal practice to provide a battery protection device for protecting the battery from improper use.
The battery protection devices include those which are mounted in the rechargeable battery itself as a PTC (Positive Temperature Coefficient) element or a current shutoff valve, and those configured as a circuit substrate on which is formed a battery protective circuit that shuts off the charging and discharging circuit of the rechargeable battery in response to an abnormal state, for instance. The above-mentioned PTC device is serially connected to the charging and discharging circuit of the rechargeable battery, and generates heat itself when subjected to an excessively large current, thereby preventing the further flow of the excessively large current through a rapid increase in resistance caused by the rise in temperature. With a relatively large rechargeable battery, this PTC device is provided inside the sealing assembly of the battery. The above-mentioned current shutoff valve is normally installed in the sealing assembly, and when gas is generated within the rechargeable battery, it is deformed by the rise in internal pressure and breaks when the internal pressure exceeds a permissible value, thereby releasing the gas and, through its breakage, shutting off the flow of current to the charging and discharging circuit. PCT elements and current shutoff valves are well known as mechanisms provided in relatively large, cylindrical, lithium ion rechargeable batteries.
The above-mentioned battery protective circuit is disclosed, for example, in Japanese Patent No. 2,872,365, and is configured as shown in FIG.
12
. The voltage of a rechargeable battery
30
is detected by a control means
33
, and when a voltage over a predetermined charge-permitting voltage is detected, a MOSFET
31
serially connected to a charging and discharging circuit is put in an OFF state, whereby the charging and discharging circuit is shut off, and charging current is impeded. When a voltage below a predetermined discharge-permitting voltage is detected, a MOSFET
32
serially connected to the charging and discharging circuit is put in an OFF state, whereby the charging and discharging circuit is shut off, and discharging current is impeded. This control allows the rechargeable battery
30
to be protected against damage or diminished performance due to overcharging or overdischarging.
However, when the battery protective circuit is not operating properly, and particularly when the anti-overcharging function is not operating, there is the danger that the electrolyte will decompose as the overcharging state progresses and that the rechargeable battery
30
will be ruptured by the gas generated as the temperature rises. In view of this, as shown in
FIG. 13
, a battery protective circuit configuration has been proposed in which there are provided a control means
34
for preventing overcharging, overdischarging, and over-current, and an overcharging control means
36
for halting the overcharging state if this control means
34
should malfunction.
With this configuration, the control means
34
prevents overcharging, overdischarging, and over-current, and when an operational aberration occurs, such as a breakdown or malfunction in the ability of this control means
34
to prevent overcharging, a voltage corresponding to an overcharging state is detected by the overcharging control means
36
, and the overcharging control means
36
puts the MOSFET
35
in an ON state. When this MOSFET
35
is turned ON, a resistor
18
generates heat which melts a heat-coupled temperature fuse
19
, shutting off the charging and discharging circuit of the rechargeable battery
30
.
It is thereby possible to avoid a prolonged overcharging state, which is the most detrimental state for the rechargeable battery
30
to be in. Since the structure for preventing overcharging is provided redundantly, and the generation of gas due to a prolonged overcharging state is prevented, it is also possible to eliminate the above-mentioned current shutoff valve that mechanically shuts off the power circuit.
Nevertheless, although an anti-overcharging circuit that is redundantly provided as in the above conventional structure was indeed effective at preventing battery breakdown due to a prolonged overcharging state, because the second control means, which was actuated when something was amiss with the first control means, actuated a non-resettable shutoff means, the operation thereof could not be tested, so it was impossible to ensure proper operation and obtain high reliability by testing individual operating states. The proper operation of an overcharging protective circuit is essential with rechargeable batteries of high energy density, such as a lithium ion rechargeable battery, and it is necessary to be able to ensure the reliability of individual batteries or battery packs by testing the operation of the protective circuit.
Meanwhile, a temperature fuse, heating means, and so forth are parts that cannot be incorporated into an integrated circuit, and therefore require their own installation space, and this hinders designing a compact battery pack using small rechargeable batteries, or designing a rechargeable battery with an attached protective circuit in which the protective circuit is integrated with the rechargeable battery.
Making rechargeable batteries smaller is very important in terms of making portable devices more compact, and even with this smaller size, the energy density per unit of volume still needs to be increased. An effective means for achieving this is to use an electrical rather than mechanical current shutoff valve to shut off the power circuit, and we are awaiting the development of a protective circuit that effectively protects a rechargeable battery from a prolonged overcharging state, with a smaller structure for the electrical shutoff of the power circuit than that in prior art, as well as a protective element used in this protective circuit.
It is an object of the present invention to provide a protective circuit with a compact structure that affords reliable protection of a rechargeable battery, and at the same time, a protective element that is compatible with this protective circuit.
DISCLOSURE OF THE INVENTION
To achieve the stated object, the present invention provides a battery protective circuit incorporated in a rechargeable battery, comprising:
a first switching means and a second switching means connected in series in a charge/discharge circuit of the rechargeable battery;
a main protective circuit for
detecting a battery voltage between positive and negative electrodes of the rechargeable battery;
turning on the first switching means when the battery voltage is below a charge-prohibiting voltage, above which charging of the rechargeable battery must be prohibited; and
turning off the first switching means when the battery voltage is detected to be higher than said charger-prohibiting voltage, and maintaining the OFF state of the first switching means until the detected voltage becomes below a charge-permitting voltage that is lower than said charge-prohibiting voltage; and
a sub-protective circuit for
detecting the battery voltage between the positive and negative electrodes of the rechargeable battery;
turning on the second switching means when the battery voltage is below a second charge-prohibiting voltage which is higher than the first charge-prohibiting voltage; and
turning off the second
Jordan and Hamburg LLP
Tso Edward H.
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