Battery protection circuit, method for protecting battery, power supply device and program

A reusable period of a battery can be maintained for a long time by recharging while avoiding abnormal heating of the battery due to over discharge. A battery protection circuit comprises a control part that cuts off a discharge switch which is a discharge path to a load for a battery when a voltage value of the battery is equal to or less than a first threshold value, or when the voltage value of the battery is less than the first threshold value, wherein the control part cuts off a power switch as a power supply path to the control part itself when the voltage value of the battery is equal to or less than a second threshold value, or when the voltage value of the battery is less than the second threshold value which is lower than the first threshold value.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to, claims priority from and incorporates by reference Japanese patent application number 2009-078346, filed on Mar. 27, 2009.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery protection circuit, a method for protecting a battery, a power supply device, and a program thereof.

2. Description of Related Art

A battery protection circuit that protects against degradation of a battery caused by overcharge and over discharge of a power supply device with a built-in rechargeable secondary battery, such as a battery pack described in Japanese patent number 3382002 (hereafter patent reference 1), is widely known.

For example, the battery protection circuit described in patent reference 1 stops a battery from discharging immediately after (500 milliseconds) an alarm when over discharge of the battery progresses. Similarly, when overcharge of the battery progresses, charging of the battery is stopped immediately after the alarm.

In the battery protection circuit described in patent reference 1, when the overcharge or over discharge progresses, a discharge path or a charge path is cut off in order to protect the battery. At this time, when it is in an over discharge condition, the battery recovers from the over discharge condition through charging of the battery by a user and becomes reusable. On the other hand, when the user leaves an over discharge condition of the battery and does not recharge, over discharge further progresses. When the over discharge progresses, eventually, the battery is degraded until it is in a nonreusable condition.

When the battery is, for example, a lithium-ion battery, due to the over discharge, cobalt of a positive electrode is eluted, and copper of a collector of a negative electrode is eluted. As a result, the lithium-ion battery does not function as a secondary battery. Furthermore, it is well known that the over discharge of the lithium-ion battery could cause abnormal heating of the battery.

In contrast, the condition in which a user does not recharge even though the battery is in the over discharge condition is not assumed in the battery protection circuit of patent reference 1. In other words, the battery and an over discharge detecting part (or overcharge detecting part) are constantly connected to each other in the battery protection circuit of patent reference 1. Under this structure, the battery keeps supplying power with respect to the over discharge detecting part (or overcharge detecting part) even though the over discharge progresses up to the point in which the battery is nonreusable.

Accordingly, it is effective to extend a period of time to fall in a condition in which abnormal heating due to over discharge occurs by eliminating all of the elements that progress the over discharge in the over discharge detecting part (or overcharge detecting part) with respect to the battery which is in the nonreusable condition due to the over discharge.

SUMMARY OF THE INVENTION

The present invention is provided against the background discussed above. An object of the present invention is to provide a battery protection circuit, a method for protecting a battery, a power supply device, and a program that can maintain a long reusable period by recharging while avoiding abnormal heating of the battery due to the over discharge.

The first aspect of the present invention is a battery protection circuit which comprises: a control part that cuts off a discharge path to a load for a battery when a voltage value of the battery is equal to or less than a first threshold value, or when the voltage value of the battery is less than the first threshold value, wherein the control part cuts off a power supply path to the control part itself when the voltage value of the battery is equal to or less than a second threshold value, or when the voltage value of the battery is less than the second threshold value which is lower than the first threshold value.

In the battery protection circuit according to the present invention, it is preferable that the control part cuts off a charge path to the battery when the control part cuts off the power supply path to the control part itself.

The second aspect of the invention is a method for protecting a battery which is executed by a control part that cuts off a discharge path to a load of a battery when a voltage value of the battery is equal to or less than a first threshold value, or when voltage value of the battery is the less than the first threshold value, wherein the control part executes a step to cut off a power supply path to the control part itself when the voltage value of the battery is equal to or less than a second threshold value, or when the voltage value of the battery is less than the second threshold value which is lower than the first threshold value.

In the method for protecting the battery according to the present invention, it is preferable that a charge path to the battery is also cut off as a cut off processing step when the control part cuts off the power supply path to the control part itself.

The third aspect of the invention is a power supply device which comprises the battery protection circuit according to the present invention and a battery which is a subject of protection by the battery protection circuit.

The fourth aspect of the invention is a program that realizes a function of the battery protection circuit according to the present invention in an information processing device by being installed in the information processing device.

According to the present invention, a reusable period of a battery can be maintained for a long time by recharging while avoiding abnormal heating of the battery due to over discharge.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[Regarding Structure of Power Supply Device1According to Embodiment of Present Invention]

As shown inFIG. 1, a power supply device1according to an embodiment of the present invention is configured with a battery protection circuit2, a battery3, a discharge switch4, a charge switch5, diodes6and7, a ground potential line8, terminals9and10, an alarm terminal11, and a load12or a charger13. The power supply device1is, for example, configured as a battery pack which includes the battery3.

A structure of the battery protection circuit2is explained later in detail by usingFIG. 2. The battery3is, for example, a lithium-ion battery. The battery3may be configured with a plurality of cells even through the battery3is shown in the figures as a single cell. The discharge switch4turns ON (connected)/OFF (disconnected) power supplied from the battery3to the load12. The charge switch5turns ON/OFF charging current supplied from the charger13to the battery3. The diode6prevents reverse current from flowing from the load12to the battery3. The diode7prevents reverse current from flowing from the battery3to the charger13. The ground potential line8provides “0” (V) internally to the power supply device1. The terminals9and10are connected to either the load12or the charger13.

The alarm terminal11outputs an alarm signal from the battery protection circuit2. For example, a managing device (not shown) of an administrator is connected to the alarm terminal11. Or, when the load12has an input terminal (not shown) that receives the alarm signal of the battery protection circuit2, the output of the alarm terminal11can be connected to the load12. Note that both the discharge switch4and the charge switch5are normally in the turned ON condition. Even though the discharge switch4and the charge switch5are in the ON condition, the reverse current of the discharging current and charging current are blocked by diodes6and7. The discharge switch4and the charge switch5are controlled to an OFF condition by the battery protection circuit2at the time of over discharge or overcharge.

The battery protection circuit2is configured with a voltage measuring part20, a control part21, a power switch22, and a resistor23as shown inFIG. 2. The voltage measuring part20measures a voltage value of the battery3and outputs a signal to the control part21. The control part21controls the turning ON/OFF of the charge switch5, the turning ON/OFF of the power switch22, and the turning ON/OFF of the discharge switch4depending on the voltage value output from the voltage measuring part20. The control part21also controls an alarm output depending on the voltage value output from the voltage measuring part20. The power switch22turns ON/OFF power supplied to the battery protection circuit2. The resistor23has an extremely large resistance value compared to a resistance value of the load12, and draws only a small part of current that is supplied to the load12by the battery3into the battery protection circuit2.

[Regarding Operation of Control Part21]

Operation of the control part21is explained with reference to the flow diagram inFIG. 3. Note that an explanation for charge control is omitted because it is the same as conventional technology, so that discharge control is primarily explained hereafter.

START: When the load12is connected to the power supply device1, power is supplied from the battery3to the load12. The control part21recognizes that the power supply to the load12from the battery3has started based on a change of a voltage value measured by the voltage measuring part20, and shifts to the processing of S1.

S1: The control part21determines whether or not the voltage value measured by the voltage measuring part20is equal to or less than a threshold value Th#1. The control part21shifts to the processing of S3when the voltage value measured by the voltage measuring part20is equal to or less than the threshold value Th#1(YES at S1). On the other hand, the control part21shifts to the processing of S2when the voltage value measured by the voltage measuring part20is more than the threshold value Th#1(NO at S1).

S2: The control part21maintains the discharge switch4ON and returns to the processing of S1.

S3: The control part21not only turns the discharge switch4OFF but also outputs an alarm, and then shifts to the processing of S4.

S4: The control part21determines whether or not the voltage value measured by the voltage measuring part20is equal to or less than the threshold value Th#2. The control part21shifts to the processing of S5when the voltage value measured by the voltage measuring part20is equal to or less than the threshold value Th#2(YES at S4). On the other hand, the control part21returns to the processing of S1when the voltage value measured by the voltage measuring part20is more than the threshold value Th#2(NO at S4).

S5: The control part21not only turns the charge switch5OFF, but also turns the power switch22OFF, and then ends the processing (END).

FIG. 4is an explanatory diagram of two threshold values, Th#1and Th#2, of the control part21. As shown inFIG. 4, two threshold values, Th#1and Th#2, are provided in the control part21with respect to a discharge curve of the battery3. For example, when the battery3is a lithium-ion battery, a maximum voltage value at the time of a full charge is approximately 4.1 V; the threshold value Th#1is set at around 2.1 V; and the threshold value Th#2is set at around 1.1 V.

As shown inFIG. 4, when the voltage value of the battery3is equal to or less than the threshold value Th#1, the discharge switch4is turned OFF, and a charge alarm is output to a user. At this time, because the battery protection circuit2is being operated, the battery3is chargeable by connecting the charger13to the terminals9and10. A period discussed above is referred to as a “charge alarm period.” In other words, just after completion of a full charge through the end point of the “charge alarm period” is referred to as a “chargeable period.”

During the “charge alarm period,” when a user does not charge, the voltage value of the battery3will be further decreased. When the voltage value of the battery3is equal to or less than the threshold value Th#2in due time, and when there is a possibility that the voltage value becomes to the extent that it might cause degradation of the battery3, the control part21turns the charge switch5and the power switch22OFF to cut the power supply to the control part21itself. Accordingly, charging to the battery3is no longer possible. However, a period until abnormal heating occurs due to completely discharging the battery3can be extended compared to the conventional technology (shown in dashed line form). The period from time in which the voltage value of the battery3is equal to or less than the threshold value Th#2through the time in which the battery3is completely discharged is referred to as a “complete discharge extending period.”

As discussed above, when the discharge condition of the battery3is lowered to the voltage value that could cause degradation of the battery3, the progress of degradation of the battery3can be slowed by cutting off all of the discharge paths of the battery3. And, when the period of time from when the voltage value of the battery3is equal to or less than the threshold value Th#2through when the battery3is completely discharged is short as the “conventional technology” shown with the dashed line inFIG. 4, a value of the threshold value TH#2has to be set as a higher value in view of safety. In other words, the threshold value Th#2is set as 1.1 V in the control part21; however, this value is required to be set as a higher value in the conventional technology.

In contrast, in the power supply device1, because the “complete discharge extending period” is longer than the conventional technology, the threshold value Th#2can be set as a lower value. As a result, the “charge alarm period” can be set longer so that the “chargeable period” can be set longer with respect to the battery3.

The embodiments of the present invention can be modified in various ways so long as such variations are not to be regarded as a departure from the sprit and scope of the invention. For example, it is explained that the alarm output is output only at the “charge alarm period” shown inFIG. 4and is stopped at the “complete discharge extending period.” This is because, at the “complete discharge extending period,” the power consumption is required to be none. Accordingly, when there are any alarm means in which power is not consumed, the alarm output can be continued since the voltage value of the battery3is equal to or less than the threshold value Th#1.

For example, as for the alarm means that does not consume power, an alarm output device with a display content retention type is contemplated. When an input signal is received for a short period of time, magnetic energy occurs on an electromagnet. As a result, the alarm output device mechanically switches the display content “from normal to alarm” by moving a piece of iron that is drawn by the magnetic energy as a trigger.

Moreover, at the processing of Si in the flow diagram inFIG. 3, it is possible to substitute the determination processing of “the voltage value is less than the threshold value Th#1? for “the voltage value is equal to or less than the threshold value Th#1?.” Similarly, at the processing of S4in the flow diagram inFIG. 3, it is possible to substitute the determination processing of “the voltage value is less than the threshold value Th#2?” for “the voltage value is equal to or less than the threshold value Th#2?”

The battery protection circuit, the method for protecting the battery, the power supply device, and the program being thus described, it will be apparent that the same may be varied in many ways. Such variations are not to be regarded as a departure from the sprit and scope of the invention, and all such modifications as would be apparent to one of ordinary skill in the art are intended to be included within the scope of the following claims.