1. Field of the Invention
The present invention relates to battery charging and, more specifically, to a battery charging circuit for adaptively controlling the full charge voltage of a rechargeable battery in response to variations of the surrounding temperature.
2. Description of the Related Art
A battery charging circuit senses the full charge state of a rechargeable battery to prevent the battery from being damaged due to overcharging. There are several known techniques for sensing the full charge state of a rechargeable battery. One such technique continuously monitors the charge voltage of the rechargeable battery during the process of charging the battery. This known technique employs a complex circuit including, for example, an analog-to-digital (A/D) converter and a voltage comparator to detect the full charge point. A second known technique monitors temperature variations of the rechargeable battery to determine whether the battery is fully charged or not. Finally, a third known technique, a combination of the above first and second techniques, monitors the voltage and temperature variations of the rechargeable battery to detect the full charge state of the rechargeable battery.
Similarly, there are several known techniques for preventing the overcharging of a rechargeable battery. One such technique cuts off the charge voltage supplied to the rechargeable battery by using a voltage comparator and a mechanical or electronic switching element. A second known technique reduces the charge current instantaneously or gradually by using the voltage comparator and the mechanical or electronic switching element. A third known technique reduces an average charge current by detecting the charge voltage of the rechargeable battery and varying a pulse width of the charge current. Finally, a fourth known technique controls the charge current supplied to the battery through the use of a timer.
It is to be appreciated that the known techniques for detecting the full charge state of a rechargeable battery are so complex that the battery charging circuit needs to include a separate microprocessor. Obviously this is an uneconomical solution. Further, it is to be appreciated that the known techniques for preventing the overcharging of a rechargeable battery are disadvantageous in that the battery is discharged naturally with the lapse of time, after the battery has been fully charged. Accordingly, it would be desirable and highly advantageous for the above known techniques to include additional elements for reducing the charge current step by step, and also for reducing the charge current even in a standby state.
Korean patent application No. 19908/1994, filed by the applicant of this invention, discloses a battery charging circuit for blocking the supply of the charge current in the case that the charge current or the charge voltage of the rechargeable battery exceeds a reference current or reference voltage, respectively. Accordingly, the circuit allows for the control of the pulse width and the amount of the charge current supplied to the battery.
It is well known that the full charge voltage of a rechargeable battery varies according to the surrounding temperature. That is, the full charge voltage decreases with increasing temperature. For example, a rechargeable battery may have a full charge voltage of 1.45V at a surrounding temperature of 25.degree. C., while having a full charge voltage of 1.4V at a surrounding temperature of 50.degree. C. Accordingly, it would be desirable and highly advantageous to have a battery charging circuit which can adaptively control the full charge voltage of a rechargeable battery according to variations of the surrounding temperature.