Method and apparatus for monitoring the self discharge of a secondary battery upon completion of a charge cycle

DETAILED DESCRIPTION According to the present invention, a battery charger monitors the terminal voltage of a secondary battery without appreciably depleting the energy stored therein.
A battery connected to the battery charger of the present invention becomes discharged primarily through self-discharge.
In one embodiment, a battery charger includes (a) a charger controller having a high side sense input terminal, a low side sense input terminal, and a switch control output terminal; and (b) a first switch connected across the low side sense input terminal and a ground node and controlled by the switch control output terminal of the charger controller.
A secondary battery is connected across the high side sense input terminal and the low side sense input terminal of the charger controller.
The first switch controls the flow of charge current through the battery.
In another embodiment, a current sense resistor is connected between the first switch and the ground node for measuring the charge current flowing through the battery.
Upon the completion of the charge cycle, the battery charger monitors the terminal voltage of the battery.
First, the charger controller creates a high impedance node at the battery low side.
Then, the charger controller regulates the battery high side voltage to a value below the maximum allowable terminal voltage for the battery.
In response, the battery low side voltage will decrease to a value below ground since the battery is in a fully charged state and is left floating.
The charger controller monitors the terminal voltage across the battery by measuring the battery low side voltage.
In the present embodiment, the charger controller creates the high impedance node at the low side of the battery by making the low side sense input terminal to be high impedance and applying a predefined voltage to the first switch to open the switch.
The high impedance node is created to float the battery and to significantly reduce the current that flows between the high side and the low side of the battery through the controller circuit