Overview

The Blue Sea Systems ML-ACR is a 500A automatic charging relay for boats that need to share charging between two large battery banks while keeping those banks isolated when charging is not present or when engine starting could pull voltage down.

It uses a magnetic latching relay, so it draws very low current while monitoring voltage and only uses higher current briefly when changing state. The range covers 12V DC and 24V DC systems, with configurations for standard lead-acid, AGM, and gel batteries, plus LiFePO4-specific versions.

This is a practical fit for dual-bank marine electrical systems where the start bank and house bank need automatic charge management, emergency paralleling for starting, and controlled isolation to help protect sensitive electronics during cranking.

Key features

• 500A continuous rating, with 700A intermittent rating for 5 minutes and 1450A cranking rating for 30 seconds across the ML-ACR range.
• Magnetic latching relay draws less than 40mA while monitoring terminal voltage in either combined or isolated state.
• Dual sensing reads charge on both battery banks, so either bank can trigger automatic combining when a valid charge source is present.
• Start Isolation and Engine Isolation functions can keep house loads or twin-engine circuits separated during cranking or while both engines are running.
• 3/8"-16 (M10) tin-plated copper studs and silver alloy contacts support high-current battery cabling in a marine electrical compartment.
• LED output can show whether the relay is combined, isolated, in voltage lockout, or in Start Isolation or Engine Isolation mode.
• Variant range covers 12V DC and 24V DC systems, standard lead-acid/AGM/Gel batteries, LiFePO4 batteries, tinned-wire control, and Deutsch DTM/Amphenol ATM control connections.

Variants

This ML-ACR range is configured by nominal voltage, battery chemistry, and control interface. The options cover 12V DC or 24V DC electrical systems, standard lead-acid, AGM, and gel banks, or LiFePO4 banks where the lithium-specific version is selected.

Control connection also varies by configuration, with tinned-wire versions and Deutsch DTM/Amphenol ATM versions available. Some configurations include manual control for service lock-off and remote override behavior, while no-manual versions keep the ACR function without that manual-control option.

Electrical and system integration

The ML-ACR belongs near the battery switching and distribution area where the high-current cables can be kept short and properly supported. For 500A installations, the cable size to meet the full rating is 4/0 AWG x 2, and the terminal stud torque is 140 in-lb.

Those numbers matter on boats because loose or undersized connections can add heat and voltage drop under charging and cranking loads. Match the selected voltage and battery-chemistry version to the vessel’s electrical system before installation.

Use on board

This relay is a good match for boats with separate start and house banks that charge from an alternator, shore charger, inverter charger, or other onboard charging source. It suits cruisers, offshore fishing boats, and workboats where high-output charging and controlled battery isolation are part of the electrical plan.

Installation notes

Install it where the control wiring can be protected from vibration and moisture, and where the main battery cables can reach the studs without pulling on the relay body. Follow the vessel’s wiring plan and use installation practices that match the battery type, cable size, and marine electrical requirements for the boat.