Woot offers DEWALT DCB205 20V MAX 5 Ah Lithium Ion Battery for $49.99 (orig. $69.99). Free shipping for Prime Members. 

Product Description:
Power your 20V MAX* tools with this 20V MAX* 5Ah Battery that features the runtime and performance needed for tough jobs. This battery is compatible with 20V MAX* tools and chargers. 

• Reliable runtime - take on tough jobs with this battery that delivers a 5ah 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
• Lightweight and Compatible: Weighing only 1.4 lbs, this battery is lightweight and compatible with the entire line of 20V MAX* tools from DEWALT
• Part of the 20V MAX* system - these batteries are compatible with 20V MAX* tools and chargers
• Includes: (1) 20V MAX* 5Ah Battery