It depends on the battery type, or more specifically how exposed the battery posts are on the system. The Vigilant has 2 main connection types to the battery which determine if the installation can occur with the battery in service: Clamp Connections: If the battery has an exposed square post, the Vigilant can be installed by connecting steel clamps to each battery post. The clamp is fixed to the post with 2 screws that do not interfere with the battery. These posts are typically seen on VLA/flooded batteries, but also some VRLA/sealed designs. Tab washer Connections: If the post is recessed, covered, or otherwise not accessible then a tab washer is typically used for connection. Installation of tab washers requires the system to be taken offline for 30-40 min. This is typically seen on most VRLA/sealed, front facing, and OPzS battery designs.