DESC: Innotek...The Innovator of In-Ground Fencing Systems! ,,,, Features a sleek, streamlined collar that's 30% smaller than other containment receivers. ReadyTest collar produces a brief audible ''click'' when you've got a perfect collar fit. Waterproof collar runs on rechargeable lithium ion batteries (included). System covers 1/4 acre (expandable to 25 acres). Built-in 24/7 battery backup.,,,, Extra system collars for IUC-4100 sold separately and can be found through item search at item number 38-884-779-01. These collars (item #38-884-779-01) are also compatible with Innotek Ultra Smart systems IUC-4200 and IUC-5200 ,,,, For technical assistance with this product, please call Innotek customer service at 800-826-5527.