dknguyen has almost perfectly described charging lead-acid batteries, which have a predictable rise of voltage at end-of-charge. A lot of other battery chemistry types do not have a reliable ΔV at end-of-charge. Rather, under a constant current charge, after the power input stops going into the chemical reactions (which end when the battery is fully charged), the power begins heating the battery, instead. The charger measures the temperature rise of the battery to decide when to quit charging...