Woot offers DEWALT DCBP520 Powerstack 20V MAX Battery, Rechargeable, 5Ah, Lithium Ion (1Pack) for $77.99 (orig. $229) free shipping.

Product Description:
Keep your DEWALT 20-Volt tools running longer with the 20-VoltMAX Lithium-Ion Premium Battery Pack. This battery is compatible withthe complete line of DEWALT 20-Volt MAX tools

• 5.0 Ah capacity
• 3-LED fuel gauge system for immediate feedback on the state of charge
• No memory and virtually no self-discharge for maximum productivity and less downtime
• Lightweight and portable at only 1.4 lbs.
• Part of the DEWALT 20-Volt Max system