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Charging Relays & Why You Need One

Updated: Sep 22, 2023

Automatic Charging Relay, or ACR as Blue Sea Systems refers to them, are a great addition to any boat electrical system with multiple battery banks. They work by combining one battery bank in parallel with another battery once the voltage of the first bank reaches a specified voltage.

So what's the big deal? Well to begin, if you have a charging system (solar, wind generation, etc) that is charging your house bank and you want to charge other battery banks on the boat (start battery, generator battery), by adding an ACR in the system between the two battery banks will take care of this need. Once the battery draws below another set threshold, the ACR will disconnect preventing both batteries from drawing down.

But before going any further and ordering an ACR to install in your boat, there are a few items to consider.

Options Available

Multiple companies in addition to Blue Sea Systems offer their own version of these charging relays, but when designing a job, we often stick to Blue Sea Systems and Victron's available options.


Mini Automatic Charging Relay (M-ACR) are a great option for smaller alternators found on most outboards. They are only limited to 65amps, so when selecting one of these units its important to ensure the maximum charging output to avoid issues. These units also have start isolation which acts by disconnecting the parallel function during engine cranking preventing sags or spikes in the voltage. These units are also ignition protected which means you can safely mount these units in a gasoline engine compartment. These units also come in 12V and 24V models.


The SI-ACR is almost identical to the M-ACR, with the exception that they can handle 120amps of continuous charge. The units are ignition protected and also have the ability to be wired to disconnect during engine cranking. Additionally, these units are dual sensing, so they will combine if either of the batteries connected reach the voltage setpoint, which can be a great option if your alternator is wired to you start battery and your solar system in connected to your house bank. These units also come in 12V and 24V models.


The SL-ACR, or Magnetic Latching Automatic Charging Relay, is by far the most adaptable charging relay we use. These units are rated to 500 amps and have the ability to manually combine or disconnect your batteries. When redesigning a DC electrical system, we use these often with the addition of an ON/OFF battery switch to greatly simplify battery bank management on a boat. These units simplify the management of the battery bank as they help to keep all batteries at full capacity with, while also allowing you to force combine if a start or house battery gets too low to use. Paralleling battery switches are great for a two battery bank boat, but when you have three or more battery banks, the use of an ML-ACR give you a lot of options if you every run into problems. These units also have the dual sensing abilities of the SI-ACR, are ignition protected, and come in both 12V and 24V models.


The Cyrix, made by Victron Energy, is another Automatic Charging Relay we highly recommend and use often. These units operate very similarly to Blue Sea System's ACRs, as they will combine when a certain voltage is reached and disconnect when the voltage drops below threshold. Cyris units come in 120amp and 230amp versions, and additionally offer Lithium models that can be wired to communicate with the Lithium systems BMS. Similar to the ML-ACR, these units can also be wired to force combine to assist with engine cranking when a start battery becomes too low. In addition to coming in multiple amperage ratings, these units are come in 12V/24V/48V models.

Additional Information

When installing Automatic Charging Relays, it is very important to follow ABYC's E-11 code on properly fusing each side of the ACR battery cable runs as they are considered as a charging source. Ideally, this should be done within 7 inches of the battery.

It is very important to check the charging amperage and voltage when selecting these units. Sources of charge to consider would be the alternator, an inverter/charger, or any renewable energies such as solar or wind generation.

Automatic Charging Relays are not the best option for linking to a dedicated bow thruster or windlass battery. This is because these batteries often get draw down significantly during use, and the voltage difference between could cause an amperage surge causing potentially blowing the fuses or damaging the units. For these applications, a DC/DC charger is recommended to avoid these issues.

And finally, it should be noted that Automatic Charging Relays act in either an open or closed circuit, so they do not follow a charging profile like a dedicated charger would provide. For this reason, it is recommended that the battery banks connected via an ACR should be a similar battery chemistry, and if used with a Lithium battery system a professional marine electrician should be consulted.


Automatic Charging Relays are a great component to have on your boat to ensure all of your battery banks are kept at full health while using your boat. As with anything related to boats, there are always tradeoffs with any system or component, but often time the pros outweigh the cons with these units.


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