Inside that battery pack is a lot of little rechargeable batteries. Often they are wrapped tightly together with plastic Rechargeable power tool batteries come in mainly three types. Nickle-Cadmium, Nickle-Metal Hybride, and Lithium-Ion. You may be choosing a tool brand and have no idea what. Always use the charger that came with the tool. Do not mix and match chargers. No type of battery likes getting hot! If your batteries are hot from use, let them cool before charging. Store batteries in a cool dry place, not touching metal or other batteries. Do not. With all these different power levels for storage, how do I know what's left in my power tool battery? You can use a Multi-Meter to check power levels. You will use the DCV setting on your multimeter, DCV is Direct Current Volts. Set the meter to at least the voltage.