Blue Sea Systems Add-A-Battery Kit 120A Dual Circuit Plus 7650 — Automatic Charging Relay & Emergency Parallel

The Blue Sea Systems 7650 Add-A-Battery Kit is a complete 120 A automatic two-battery management system that combines a Dual Circuit Plus switch and an Automatic Charging Relay to charge, protect, and optionally parallel starter and house batteries on boats.

Designed for inboards and larger outboards, the 7650 simplifies battery control by automatically combining batteries during engine charging and isolating them at rest. This reduces wiring complexity and minimizes user intervention while preserving starting capacity and protecting sensitive electronics. The kit includes corrosion-resistant tin-plated copper studs and an emergency parallel feature so you can reliably start the engine even if one bank is depleted.

How does the 7650 manage charging between starter and house batteries?

The system uses an Automatic Charging Relay (ACR) that detects charging voltage from your alternator or charger and automatically connects the starter and house banks while charging is present. When charging stops and voltage falls, the ACR opens to keep banks isolated, preventing house loads from draining the starter battery.

• Automatic Charging Relay senses voltage and combines batteries during charging.
• Separates banks at rest to protect starter capacity and electronics.
• No external isolators or separate regulators required for basic two-bank control.

What does the Dual Circuit Plus switch do and why is it useful?

The Dual Circuit Plus switch controls starter and house banks independently while allowing a single ON/OFF action for everyday use. It streamlines operation, reduces the chance of switching errors, and maintains proper distribution of power to critical loads and the starter system.

• Independent management of starter and house batteries for safer power distribution.
• Single ON/OFF position for routine operation—simpler for owners and crew.
• Integrated emergency combine option to link batteries for starting if one bank is low.

Can I rely on this kit in a marine environment?

Yes. The 7650 is built for harsh conditions with tin-plated copper studs that resist corrosion and maintain reliable conductivity over time. Components are rated for continuous 120 A service, making the kit suitable for typical inboards and larger outboards found on pleasure and workboats.

• Tin-plated copper studs minimize corrosion and ensure consistent electrical contact.
• Continuous 120 A rating supports common marine charging loads and alternators.
• Robust construction designed for vibration and saltwater exposure aboard boats.

What happens if one battery is discharged—can I still start the engine?

The Add-A-Battery kit includes an emergency parallel (combine) function that lets you link both batteries to provide the extra current needed for engine starting. This is a manual or integrated function depending on installation, intended as a last-resort aid rather than a long-term operating mode.

• Emergency combine links starter and house banks to deliver starting current.
• Useful for one-off starting events when a battery is discharged.
• Designed to preserve normal isolating behavior outside of emergency use.

Who is this for? The Blue Sea Systems 7650 Add-A-Battery Kit is aimed at boat owners and installers who want a dependable, low-maintenance two-bank battery management solution for inboard and larger outboard installations. It’s ideal for vessels that need automatic charging coordination, simple daily operation, protection for starter capacity, and a built-in emergency parallel capability without adding complex isolation hardware.