The free battery promotion is available with the purchase of either the M18ONEID2-0 or M18ONEPD2-0X Milwaukee tools. The M18ONEID2-0 is a Milwaukee M18 FUEL brushless impact driver, and the M18ONEPD2-0X is a Milwaukee M18 FUEL brushless percussion drill. When you add one of these tool items to your cart, a compatible Milwaukee M18 battery will be automatically added to your order at no additional cost.