DEWALT 18-Volt 2-Pack 2.4 Amp-Hour; 2.4 Amp-Hour Nickel Cadmium (Nicd) Power Tool Battery Kit in the Power Tool Batteries & Chargers department at Lowes.com
Dewalt 20V Max = 18V Nominally
Review: DeWalt Battery Adapter for 18 Volt tools to 20V MAX Batteries — AZ DIY Guy
Makita 18V Slider Li-ion Battery Convert to DEWALT 20V(Max) Power Tools Adapter | eBay