Rechargeable batteries come in three varieties of chemical compositions. NiMH (Nickel Metal Hydride), NiCad (Nickel Cadmium) and LiIon (various Lithium Ion technologies. The newest, lightest and longest lasting are made with Lithium (click here for article on Lithium Ion Batteries). Some power tool manufacturers have made their new Lithium Ion Battery power tools backwards compatible (click here for more info). If you are not sure which battery to purchase, check the manufacturers website.