Battery backup sump pump systems: field service manual

The controller plugs into 120 V, keeps the battery charged and monitors it. It switches battery power to the DC pump when the backup float rises, and sounds an audible alarm with lights that identify the issue (low battery, fluid level, pump running, etc.). Basement Watchdog uses dual redundant floats on its switch.

The backup float must sit higher than the primary pump's on level so the backup only runs when the primary can't keep up; Zoeller specifies that the backup float's off level must be above the DC pump itself.

Plumbing: the DC pump has its own check valve and tees into the primary's discharge above the primary's check valve, so neither pump short-circuits water back through the other. Zoeller ships a tee, a check valve for each pump and an adapter in its kits.

Run time depends on battery capacity and how often the pump cycles; sources cite roughly 5-12 hours of intermittent operation, and retailers cite single-battery run times of several hours of continuous pumping. Batteries generally need replacing every 3-5 years.