1. Field of the Invention
The present invention relates to a so-called battery protection circuit for protecting a secondary battery from overcharge and overdischarge.
2. Description of the Related Art
When a battery is brought in the overdischarge state, a battery performance thereof is deteriorated considerably. For this reason, a protection circuit is provided to prevent the battery from being deteriorated. In a battery pack in which a plurality of batteries are connected in series, batteries can be protected from the overcharge and the overdischarge by turning on and/or off a battery charger detecting the overcharge and the overdischarge and the load side of the batteries on the basis of a total voltage of the batteries connected in series.
A battery protection function is not provided on the battery side but is provided on the battery charger and the load side thereof. Also, the battery protection circuit additionally includes a circuit for turning on and/or off the power supply by detecting a temperature by a thermostat or the like incorporated within the battery or battery pack when an abnormal overcharge or overcurrent is caused in the charger or load side.
Further, as other battery protection circuit, there is known opened Japanese patent (laid-open patent publication No. 4-75430, laid-open on Mar. 10, 1992) proposed by ASAHI CHEMICAL INDUSTRY CO., LTD.
This previously-proposed battery protection circuit is such one that a rechargeable power apparatus having a rapid chargeable secondary battery includes an overdischarge preventing mechanism and an overcharge preventing function by constructing a discharging circuit and a charging circuit by using a device including therein a parasitic diode.
That is, upon discharging, when the voltage across the battery becomes less than a constant voltage, a MOS FET (metal oxide semiconductor field effect transistor) is changed from the conductive state to the interrupted state by the output voltage of the control means to thereby interrupt the battery discharging circuit. When the voltage across the battery is restored by the charging through the parasitic diode of the MOS FET, the battery is charged in a normal fashion (with low loss). Similarly, upon charging, when the voltage across the battery becomes higher than a certain voltage, the MOS FET is changed from the conductive state to the interrupted state by the output from the control means to thereby interrupt the battery charging circuit. When the voltage across the battery is recovered by the discharge through the parasitic diodes of the MOS FET, the normal discharge (with low loss) is carried out.
However, according to the above-mentioned method, when characteristics of batteries in the battery formed of a plurality of batteries are not equal, there is then the risk that only particular battery is overcharged or overdischarged. Particularly, in a battery whose efficiency is considerably deteriorated by the overdischarging and overcharging, the usable battery capacity is reduced or such battery cannot be utilized substantially.
To solve this problem, the assignee of the present application has previously proposed a method (Japanese patent application No. 3-213019 filed on Jul. 31, 1991 and now Japanese laid-open patent publication No. 5-49181, laid-open on Feb. 26, 1993).
According to this previously-proposed method, in the process in which secondary batteries connected in series are repeatedly charged and discharged, even when a battery capacity balance is lost by a difference of individual batteries, a charging and discharging circuit includes a battery capacity balance circuit having a function to recover such balance to thereby prevent the battery from being overcharged and overdischarged.
More specifically, the battery capacity balance circuit is fundamentally formed of a circuit for protecting the battery. In other words, this battery capacity balance circuit is comprised of circuit groups for detecting the overcharge, turning off the charging current, detecting the overflow, detecting the overdischarge, turning off the discharge current, and detecting the hysteresis and the overcurrent to protect the battery.
According to the conventional technique and the previously-proposed technique, even when the overdischarged state or overcharged state of the battery is detected, a current, for example, is flowed to the circuit groups after the overdischarging was detected. Hence, a consuming current is constantly flowed. Consequently, the battery remaining capacity maintaining period is reduced. For example, if the remaining capacity is 30 mAh and a circuit current is 20 .mu.A, then the battery remaining capacity amount is used up in 1500 hours (about two months).