Blue Sea Systems Add-A-Battery Kit 120A — Automatic Dual-Bank Marine Battery Manager, Corrosion-Resistant

Blue Sea Systems Add-A-Battery Kit, 120A is an automatic dual-bank battery management kit that combines a Dual Circuit Plus switch with an Automatic Charging Relay to charge both starter and house batteries while isolating them at rest.

Designed for inboard engines and larger outboards, this kit provides a straightforward, reliable way to manage two battery banks without external isolators or complex regulators. It senses charging voltage, links batteries while the engine or charger is running, and separates them when voltage falls—protecting electronics and preserving starting power. Built with marine-grade components and a 120 amp continuous rating, the system is meant to handle the demands of harsh, corrosive marine environments.

How does the Add-A-Battery Kit keep my starter and house batteries properly managed?

The kit uses a Dual Circuit Plus switch to let the starter and house banks operate independently while providing a single ON/OFF control for routine use. An integrated Automatic Charging Relay detects when charging voltage is present and momentarily parallels the banks so both receive charge. When charging stops, it isolates the banks to prevent house loads from depleting the starter battery.

• Dual Circuit Plus switch separates starter and house circuits with one simple control
• Automatic Charging Relay senses voltage and combines banks only during charging
• Prevents parasitic discharge onto the starter battery when at rest

What happens if one battery is dead and I need to start the engine?

The kit includes an emergency combine or parallel feature that lets you link both batteries temporarily for engine starting if one bank is discharged. This manual combine protects against being stranded while still allowing normal isolation during everyday operation. Use the combine only briefly to start, then return the switch to normal to restore isolation.

• Integrated emergency combine connects both banks for starting power
• Manual operation ensures you control when batteries are paralleled
• Return to normal position after start to prevent unnecessary battery drain

Is this kit durable enough for marine conditions and heavy use?

Yes — the Add-A-Battery Kit is built specifically for marine exposure, using corrosion-resistant tin-plated copper studs and components designed for continuous current up to 120 A. Materials and finishes help reduce corrosion and maintain conductivity in salty, humid environments, so the kit continues to perform reliably season after season. Its construction supports installation on inboards and larger outboards where higher continuous currents are common.

• Tin-plated copper studs resist corrosion and maintain good conductivity
• 120 A continuous rating suits inboards and larger outboard setups
• Rugged design for marine vibration and moisture conditions

How does installation simplify my boat's electrical layout?

The Add-A-Battery Kit reduces wiring complexity by combining switching and automatic charging relay functions into a single, coherent kit. Instead of separate isolators or complex regulator wiring, this solution centralizes battery management and provides clear ON/OFF and emergency combine control. That decreases the chance of switching errors and makes routine operation easier for owners and crew.

• Combines switch and relay functions to cut down on extra components
• Single ON/OFF operation streamlines everyday battery control
• Clear manual combine option reduces confusion during emergencies

Who this is for: The Blue Sea Systems Add-A-Battery Kit, 120A is ideal for boat owners with separate starter and house banks—especially those with inboard engines or larger outboards who need reliable automatic charging and straightforward emergency paralleling. It suits sailors, cruisers, and powerboat operators seeking a durable, low-complexity way to keep both batteries charged while protecting starting capacity and sensitive electronics.