Collar Receiver only, for stubborn dogs; expand your current system or replace an old shock collar Compatible with the SportDOG Brand FieldTrainer 425XS (SD-425XS), 425S (SD-425S) Waterproof and submersible to 25 feet using DryTek technology Fits dogs 8 pounds or larger with neck sizes 5"-22" Rechargeable lithium-ion battery with 2-hour quick charge; features a low battery indicator to let you know when to charge