Automatic Charging Relays m-ACR

m-Series Automatic Charging Relay - 12/24V DC 65A


The Mini Automatic Charging Relay (M-ACR) is designed to manage the charging current from alternators up to 65 Amps found on most outboards and many inboard engines.


Start Isolation (SI) protects sensitive electronics by temporarily isolating house loads from the engine circuit during engine cranking, protecting the house circuit from voltage sags and spikes.


The compact design allows for three mounting styles: surface, rear, or panel mount. The M-ACR is also available as a module in a Custom 360 Panel.

  • Senses charging on two battery banks
  • Pull off and snap on insulating cover
  • LED indicates ACR status
  • 1 ∕4" x .032" male quick connect terminal for ground
  • 7∕ 16" (11 mm) stud length to accept multiple cable terminals
  • Waterproof – rated IP67 for temporary immersion
MSRP: $85.99
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Frequently Asked Questions

Q: Will an ACR manage the charge of my individual battery banks?

An ACR does not direct the charge to the battery that “needs it the most” or has the lowest terminal voltage. If there is a charge present on either battery, indicated by a high enough voltage, the ACR will combine the batteries.

Q: What Charge Sources will an ACR work with?

An ACR will work with all charge sources, including an alternator, AC charger, or solar panel. However, low current charge sources might not produce the voltage rise required to force the ACR to combine.

Q: How does an ACR work?

An ACR senses when the voltage of either of the batteries rises to a level indicating that a charge source is active (13.0V for 2 minutes). The ACR′s contacts then connect and the ACR applies the charge to both batteries. If the voltage on both of the batteries subsequently drops to 12.75V for 30 seconds, the ACR will disconnect, isolating the batteries.

Q: How does an ACR differ from a battery isolator?

Battery isolators use one-way electrical check valves called diodes that allow current to flow to, but not from, the battery. ACRs use a relay combined with a circuit that senses when a charging source is being applied to either battery. ACRs are more efficient than battery isolators because they create little heat and consume minimal charging energy.

Q: Why didn't the ACR disconnect when my engine was turned off?

The ACR will not disconnect until the low voltage threshold is reached to isolate the circuit. It may take several minutes for the voltage of the batteries to drop to this level. Since the ACR incorporates a delay, additional time (up to 2 minutes) is required before the ACR disconnects.

Q: How many ACRs do I need?

To combine two battery banks, one ACR is needed; to combine three battery banks, two ACRs are required.

Q: Do you have an ACR for more than two battery banks?

One ACR will manage the charge between two battery banks. Three battery banks will require 2 ACRs, four battery banks will need three ACRs.

Q: Can I ground to the chassis when installing in a vehicle?

Run the negative wire for the ACR directly to the battery negative terminal of one of the batteries. A chassis ground is often insufficient to allow proper operation.

Q: What does the flashing light mean on my ACR?

A slow flashing LED means that the start isolation wire is energized. A fast flashing LED means that one or both batteries are below 9.5V (12V System) / 19V (24V System)

Q: What are the minimum number of connections I need to make my ACR work?

Three: One wire to each battery, and one for a ground (GND) connection. For safety reasons, remember to disconnect the negative battery connections before beginning any ACR installation. For more information read this article on selecting the right fuses for Blue Sea Systems ACRs.

Q: Do I need a ground wire for the ACR?

Yes, the ACR is an electronic device and needs the ground wire in order to operate. The instructions show an optional switch in the ground wire however the wire is not optional.

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