Overview

The Blue Sea Systems Add-A-Battery Kit is a dual-battery management kit for boats that need a separate engine start battery and house battery bank without making battery switching complicated. It pairs manual battery control with automatic charging management, so the start and house circuits can stay isolated during normal use while both banks can receive charge when a charging source is active.

This range covers 65A and 120A configurations. The 65A Mini Add-A-Battery versions suit smaller boats and outboard installations with alternator output up to 65A, while the 120A versions cover higher-output systems up to 120A. The automatic charging relay versions support 12/24V DC systems, which makes them useful across many common marine electrical layouts.

On board, the kit helps reduce the risk of draining the engine start battery with electronics, lighting, pumps, or other house loads. It is a practical upgrade for fishing boats, center consoles, small cruisers, and sailboats moving from a single-battery setup to a cleaner two-bank system.

Key features

• Manages two battery banks from a single charging source, helping keep engine starting and house loads separated during normal operation.
• Dual Circuit Plus battery switch can isolate engine and house circuits or combine battery banks for emergency starting.
• Automatic Charging Relay combines batteries during charging and isolates them when not charging or during engine cranking.
• 65A Mini Add-A-Battery configurations are built around the m-Series switch and 65A m-Series ACR for compact outboard-powered boats.
• 120A configurations use the e-Series Dual Circuit Plus switch with a 120A SI-ACR for boats with alternator output up to 120A.
• Plus configurations add a 3-stage, 10A BatteryLink charger with integrated 65A ACR functionality for charging at the dock as well as from the engine alternator.
• BatteryLink Plus configurations include start isolation to help protect sensitive electronics from voltage dips and spikes during engine starting.
• BatteryLink charger versions carry an IP67 rating for temporary immersion up to 1 meter for 30 minutes, and the m-Series switch in Plus versions is IP66 rated against powerful water jets.

Variants

This Add-A-Battery range varies by amperage and package style. The 65A options are intended for smaller alternator outputs and compact installations, while the 120A options match boats with alternator output up to 120A. Standard and boxed versions use the battery switch and ACR layout, while the Plus styles add a BatteryLink charger with plug or bare-wire formats for different shore-power installation needs.

The Bare Wire, Europe, and North America styles in the 65A Plus group are charger-connection variations, so the right version depends on the vessel’s shore-power setup and regional installation requirements. The shared purpose across the range is the same: cleaner switching, automatic charge sharing, and better separation between start and house battery duties.

Installation and setup

This kit is typically installed near the battery banks and main DC distribution area, where short cable runs and clear access to the battery switch are practical. The ACR or integrated charger wiring should be matched to the vessel’s battery layout, cable sizing, overcurrent protection, and charging sources.

The switch is meant to be simple at the helm or battery compartment: turn it on when using the boat and turn it off when leaving. The automatic charging relay handles the charging connection between banks, reducing the need for manual combining during normal operation.

Compatibility and fit

The key buying decision is alternator output. A 65A configuration fits installations with alternator output up to 65A, while the 120A configuration fits systems up to 120A. Boats with larger charging systems should use a battery management setup rated for that higher output.

This product family is best matched to two-bank DC systems where one bank is used for engine starting and the other supports house loads such as electronics, lighting, pumps, refrigerators, or accessories. It is not a substitute for proper battery fusing, cable sizing, or installation practices required for marine DC systems.

Use cases on board

For day boats and fishing boats, the kit helps keep electronics and livewell or bilge loads from leaving the start battery low after time at anchor or drifting. For small cruisers and sailboats, the Plus versions add dockside charging through the BatteryLink charger while still supporting alternator charging underway.

This is a good match for boat owners adding a second battery bank or cleaning up a manual two-battery switch setup where automatic charge sharing and start-battery protection matter.