Normal Charge: Low cost chargers operate with low charging currents, who are about one tenth of the rated battery capacity (eg 0.4A current at a 4Ah battery). During charge electric energy is converted in chemical energy. If the battery is fully charged, the electric power supplied to the cells cannot further be converted in chemical energy. It is converted into heat instead. The vast majority of NiCd and NiMH cells survive such an overload for some time without significant damage. Some cell types are even designed for a trickle charge eg in an emergency power supply without any effort for state-of-charge surveillance.
Fast Charge: Other applications require shorter charging times. Hand-held-tools for professional use should be recharged in less than one hour, lamps for divers require a recharge time of 3-4 hours (during a surface interval). For this much higher charge currents than for normal charge are required. This much higher currents, if not switched off or at least reduced drastically when the first cell of a pile of cells is fully charged causes a sharp rise in temperature at that cell. This will permanently damaged or even destroyed it within a very short time. Finding a safe end-of-fast-charge point for this reason is the ultimate goal. Modern chargers use the "Minus-Delta-V" method: if a cell is almost fully charged, the cell voltage does not increase further. It slightly drops instead. This slight drop of the cell voltage is used as a criterion to switch off the fast charging mode. The "Minus-Delta-V" effect at NiCd cells is much more distinct than at NiMH cells. Older chargers, designed for NiCd cells therefore can not be used to charge NiMH cells. Furthermore chargers for NiMH should be able to deliver a sufficient charge current of at least 30% of the rated cell capacity because the Minus-Delta-V effect otherwise is too small for detecting a safe end-of fast-charge point.