Lithium-ion batteries are the up-and-coming favourite for cordless tools, and for good reason. They pack more power into a given size and weight of battery, though they do deteriorate in a unique way. The capacity of Li-ion cells declines slowly and steadily with time, regardless of how often they’re used.
tool batteries you’re using are nickel cadmium (the most common type at the moment), then it helps to give them a top-up charge every month or so, regardless of whether you’ve used the tool or not. Also, try not to make any cordless tool work harder than necessary. Overloading the tool causes a high current draw on the battery, which can lead to overheating and premature cell failure. Using and recharging the battery when it’s far above or below room temperature will also shorten working life.