Automatic Charging Relay Systems for Boats

ACR systems protect your starting battery from discharge

Automatic charging relay (ACR) system
Automatic charging relay (ACR) systemCourtesy Blue Sea Systems

An automatic charging relay (ACR) system represents a simple way to charge all of your boat batteries on the fly, so to speak. As long as the engine alternator is churning out amps, like when you’re trolling or moving from one fishing spot to another, the ACR can re-direct excess engine charging capacity to the house battery bank, but only if the starting battery has reached full charge, thus protecting the starting battery from discharge.

Automatic charging relay (ACR) system
Add-a-Battery kit from Blue Sea SystemsCourtesy Blue Sea Systems

If you have a multi-engine boat with a complex system of starting and house battery banks, ask a quality marine electrician to help advice and installation of an ACR.

Yet, for basic two-battery systems, the Add-a-Battery kit ($104.99, westmarine.com) from Blue Sea Systems is pretty easy to install yourself. It includes an ACR, a battery switch and detailed instructions.

Be sure to use wires and fuses of sufficient size to handle the alternator output and starting requirements when running cables for the ACR and battery switch. Check engine owner's manual to determine the amp output and draw, and then use the Blue Sea Systems online Circuit Wizard to select the proper size of tinned copper marine-grade AWG wire or cable.

Blue Sea e-series Dual Circuit Plus Battery Switch and SI-ACR
Blue Sea e-series Dual Circuit Plus Battery Switch and SI-ACRCourtesy Blue Sea Systems

Chose spots close to the batteries to attach both of the main components – a Blue Sea e-series Dual Circuit Plus Battery Switch and SI-ACR. The locations should be dry and well ventilated. Use stainless steel fasteners to install them securely, and check behind bulkheads for wires, hoses and tank before drilling any holes.

Follow the instruction to connect the positive terminal from the starting battery to stud A on the ACR and, with a second wire, the input stud 2 on the battery switch. Similarly, connect the positive terminal from the house battery to ACR stud B and switch input stud 1. Also, connect the engine and accessories switch panel to output studs 2 and 1, respectively, on the battery switch. Install fuses in the positive connections to the ACR and the switch panel. Connect the negative terminals of both batteries to a common bus bar. Connect the engine’s negative cable directly to the negative terminal of the starting battery. Connect the accessory switch panel and the terminal marked “GROUND” on the ACR to the common negative bus bar. Install a fuse in the ACR ground wire. Make sure all terminal connections are tightened properly, and snap the ACR cover in place when finished.

Blue Sea Systems ACR
SI-ACRCourtesy Blue Sea Systems

Once the system is installed, turn the e-series Dual Circuit Plus Battery Switch to “ON” when you get on the boat. The SI-ACR will manage the charging of both batteries when underway, and also when plugged into a charger. In case you need the house battery to start the boat in an emergency, turn the switch to “COMBINE BATTERIES.” Rotate the battery switch to “OFF” when you leave the boat.