SportDOG FiledTrainer 425XS Add-A-Dog Collar - The Collar Receiver for FieldTrainer 425XS Stubborn Dog Remote Trainer. Add another dog to your current SportDOG Brand electronic training collar system or replace a Collar Receiver with the SDR-ASX Add-A-Dog Receiver Collar. For dogs 8 pounds or larger with neck sizes 5in - 22in Charges in 2 hours, lasts 40-60 hours per charge