Lowes offers DEWALT 20 -Volt 2 -Pack Lithium-ion ( 4 Ah 4 Ah Battery ) for $79 (reg. $219) with free shipping. 

Product Description:
Power your 20V MAX* tools with these 20V MAX* 4Ah Batteries that feature the runtime and performance needed for tough jobs. These batteries are compatible with 20V MAX* tools and chargers.

• Reliable runtime - Take on tough jobs with this battery that delivers a 4Ah capacity for consistent runtime on the jobsite
• State-of-charge - Get quick visibility to your battery's state-of-charge with the LED indicator to help avoid interruptions
• Part of the 20V MAX* system- This battery is compatible with 20V MAX* tools and chargers
• Includes: (2) DCB240 20V MAX* 4Ah Batteries