They still exist because of convenience, not chemistry: You can buy them–for a pittance–from New York to Nepal. Perfect for occasional-use devices, such as smoke detectors and TV remotes, because of their long shelf life.
Good for 1,000 discharge cycles. They’re toxic and suffer “memory effect,” when damaging crystals form if the cell isn’t often fully discharged. NiCds are rare except in power tools; their high discharge rates suit big-current draws.
NICKEL-METAL HYDRIDE (NiMH)
Higher energy density than NiCds but only last up to 500 discharge cycles. Must be fully discharged monthly to preserve strength. Leading chemistry for rechargeable AA and other
standard-size cells. Relatively nontoxic.
Unbeatable for energy density. Up to 1,000 discharge cycles with no memory effect, so don’t require discharging. Currently the leading chemistry for laptops, camcorders, cellphones and portable gadgetry in general.
Variation on Li-ion chemistry introduced in 1999. Solid, polymer innards of cell allow designers to mold battery into odd shapes so that they can squeeze it around components, maximizing capacity.