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.