Charge the 18-volt batteries used by power drills and similar implements as soon as you notice a loss of power. These batteries can remain in an 18-volt battery charger indefinitely without damage to keep them ready for use. Fully discharging most 18-volt batteries shortens their effective lifespan, while storing them outside the charger causes them to lose a substantial portion of their charge within a few days.
Most 18-volt batteries used for power tools are of the nickel-cadmium type, commonly known as NiCad. The long-term power capacity of a NiCad battery suffers after complete discharges, unlike the lithium-ion batteries used in cellular phones, music players and other small electronics. This is largely because the drain on 18-volt batteries tends to be more sporadic and high-intensity than the drain on lithium batteries, preventing NiCad batteries from developing the so-called "memory" that reduces the capacity of lithium batteries that are not fully discharged periodically. Therefore, it is usually wise to recharge an 18-volt battery as soon as it can no longer provide enough power to run a specific tool. However, letting the battery cool to room temperature before inserting it in to the charger is better for its long-term capacity than immediately placing it in the charger.