The Basement Watchdog Big Combo can utilize 2 batteries. The 2 batteries must be connected in parallel—positive to positive and negative to negative. A special cable (PJC) can be ordered by calling Glentronics at 800-991-0466, option 3, and also is carried by some of our retail partners in the U.S. and Canada. It is highly recommended to use batteries that are the same size and age. Connecting 2 batteries in parallel will double your pumping time during a power outage. For safety, each battery should be housed in a battery box. Our backup systems come with a battery box, but if you are hooking up a second battery, call Glentronics to purchase a battery box for that battery.