The H2O Add-On Collar unit can be added to any existing H2O 1800 PLUS series training system (excluding Beeper models) to expand your unit up to a 3-dog system. This Add-On Collar can also replace a lost or damaged collar for all H2O 1800 and 1800 PLUS series systems (excluding Beeper models). Programming to your existing system is fast and simple.Key Features:• Digital Technology• Adds to any H2O 1800 PLUS series training system(excluding Beeper models)• Replaces any lost or damaged H2O 1800 seriestraining system (excluding Beeper models)• Up to 1800 yard (1 mile) range• Lightweight and waterproof• MAXX-Range Internal Antenna System• Rechargeable Ni-MH batteries• Built-in safety shut off system• Available Limited Lifetime WarrantyPackage Contents:• 1 x Collar Unit w/ Green collar belt• Double-jack Cable