NiCd (Nickel Cadmium): are primarily used in cordless power tools. Key advantages include fast & simple charging, good shelf-life, good job-site performance and economical pricing. NiCd batteries have a low energy density and hence are used in 2.4V-18V power tools. Keep in mind that although a voltage higher than 18V might provide more power, the result will be a more cumbersome tool due to the extra weight. NiCd batteries last between 500-800 cycles when properly maintained. NiCd batteries contain toxic materials and recycling is mandatory. Some countries, such as the European Union, even restrict usage.
NiMh (Nickel-Metal Hydride): higher energy density, more environmentally friendly and can be used in higher voltage tools to include 19.2V, 24V & 36V. However, they are bulkier, heavier, have less cycle time (lasting 200-300 cycles) and a shorter shelf-life than the NiCd batteries. Performance in extreme weather can be compromised, thus their usage in power tools is limited.
Li-ion (Lithium-ion): pack more energy in a single cell than all other batteries. This allows cordless power tools to be more compact with a lighter weight and a longer run-time while still offering the same or more power. For instance, a 28V battery offers up to 2 times the run-time of an 18V battery but at a lighter weight. Other advantages include longer life cycle, more consistent power performance, no self-discharge, environmentally "green" and performs well in extreme weather conditions. Li-ion batteries require circuit protection to maintain safe voltage and current levels, so be sure to check for this type of protection. Some power tool manufacturers use this circuit protection to provide additional battery features such as a fuel gauge and over-load protection. Use only manufacturer's suggested battery and/or specified charger.