в вики пишут что ΔV-зарядники определяют по падению напряжения полностью заряженной батареи (хвостик в конце charging curve). http://en.wikipedia.org/wiki/Nickel-metal_hydride_battery#Charging
After the cell is fully charged, and as it begins to overcharge, the voltage polarity of the electrodes inside the battery will begin to reverse, and this will cause the battery voltage to decrease slightly. A ΔV type battery charger ends the charge cycle by switching off the charging current when it senses this drop in voltage.
абсолютное значение напряжения зависит от множества факторов и показателем не является
The "charge curve" graph also shows that the charge voltage will change depending on the charge current (it also changes with temperature and battery age). This generally means that a constant-voltage charging method cannot be used automatically, because it will either be unsafe, or it will not charge batteries reliably and consistently.