Make your H2O 1850PLUS a 2-dog system. Green is recommended to expand your H2O 1850 PLUS unit to a 2-dog system. Lightweight. Waterproof. 16 levels of intensity adjustable from the transmitter. Nick and continuous stimulations. Beeper with locate run and point and point only modes. MAXX-Range Internal Antenna System. Rechargeable Ni-MH batteries.