What are the various sorts of rechargeable cells?
Nickel Cadmium (NiCd) used to be the normal type of cell for mobile phones. They contain Cadmium, which is poisonous and a pollutant. Charging uses constant current. The voltage increases rapidly, stays on a plateau and then rises again until fully charged. Further charging causes an increase in temperature, which causes the voltage to drop. This drop can be detected and charging terminated. Nickel Metal Hydride (NiMH) came next. These store 30-50% more energy density than NiCd, but the price is also higher. The charge/discharge cycle is essentially the same as NiCd. Lithium ion (Li-ion) cells are the most recent, offering the highest energy densities, but at a price. Charging is complex in order to prevent cell degradation or explosion (in the 70s the police tried using Lithium cells in their handheld PMR sets. They were discontinued due to explosions on chargers in police stations!). Charging requires low constant current at the start, followed by high constant current. Cell voltage
