Abstract: A battery charging device for providing a fast charging a rechargeable battery and controlling the charging process to prevent overcharging. The battery charging device detects a decrease in the voltage of the rechargeable battery as it is being charged and approaches a full charge, and modulates the switching of a switching power supply supplying current to the rechargeable battery in response thereto. The battery charging device has a first resistor in series with the rechargeable battery which is to be charged, a power supply which charges the rechargeable battery, a charging control unit which detects a decrease in the voltage across the rechargeable battery using capacitors and which outputs battery condition signals and battery error signals in response thereto, and a power supply control unit which accepts a mode change signal and a battery error signal from the charging control unit and outputs a control signals for controlling the power supply.

Abstract: Secondary batteries, also called rechargeable batteries, are subject to damage from overcharging during rapid charging. A fully-charged condition of a battery undergoing recharging, for example a NiCd or NiMH battery, is detected by a signature drop in battery voltage called a negative delta voltage (NDV). Low-cost analog circuits and methods are disclosed for detecting the NDV. Op amp circuits are arranged to charge one capacitor while the battery voltage is rising to determine a peak voltage, and charge another capacitor while the battery voltage is falling below the peak voltage. The second capacitor voltage is compared to a reference voltage to determine when the battery voltage drop exceeds a predetermined NDV, indicating a fully-charged battery. The invention is useful in battery chargers, battery conditioners, and the like.