Method and apparatus for detecting a full-charge condition while charging a battery

A circuit for detecting and indicating a full-charge condition of a rechargeable battery undergoing recharging while coupled to the circuit, the circuit comprising:

• a reference voltage means coupled to the battery to provide a reference voltage (V1);
• first and second capacitors (C1,C2);
• means for charging the first capacitor while a voltage produced by the battery is rising, so as to provide an indication of a maximum battery voltage achieved while the battery is charging;
• means for charging the second capacitor while the battery voltage is falling below the indicated maximum battery voltage, thereby developing a voltage across the second capacitor responsive to a negative delta voltage (NDV) drop of the battery voltage, said negative delta voltage being indicative of a full-charge condition of the battery;
• and means for comparing a voltage developed across the second capacitor to the reference voltage so as to provide an output signal (Vo) that changes state to indicate the full-charge condition of the battery.

A method of detecting a full-charge condition of a rechargeable battery undergoing recharging, comprising the steps of:

• charging a first capacitor while a voltage produced by the battery is rising so as to develop a charge on the first capacitor indicative of a peak battery voltage achieved while the battery is charging;
• if the battery voltage falls below the peak battery voltage, discharging the first capacitor through a second capacitor thereby charging the second capacitor as long as the battery voltage is below the peak battery voltage;
• monitoring a voltage developing across the second capacitor while it is charging so as to monitor a negative delta voltage (NDV) amount by which the battery voltage has fallen below the peak battery voltage;
• and
• if the amount by which the battery voltage has fallen below the peak battery voltage exceeds a predetermined NDV value, providing an indication that a full-charge condition of the battery is detected.