bluesea.com BATTERY MANAGEMENT 44 Automatic Charging Relay Operation 1. ACR relay is open and batteries are isolated. Voltage begins to rise slowly after engine starts or battery charger is turned on. 2. When voltage rises to COMBINE voltage 13.0V in this example, ACR relay closes, connecting and charging both batteries. 3. When engine stops or battery charger is turned off, voltage rapidly begins falling. 4. When voltage falls to ISOLATE voltage 12.75V in this example ACR relay opens, isolating batteries while discharging. Intro to Automatic Charging Relays Back Cove Yachts installs the SI ACR as original equipment aboard their yachts, including the Back Cove 37. Automatic Charging Relays In a boat or vehicle with two battery banks, it is useful to be able to charge both banks while underway. Charge management devices allow two battery banks to be charged from a single source, such as an alternator, but keep batteries isolated when not charging. If one battery becomes depleted, there will be a charged bank available for emergency starting. There are two types of charge management devices: Automatic Charging Relays (ACR) use a relay combined with a voltage sensing circuit. When a charge is being applied to a battery and the voltage rises over 13V DC, the relay closes and combines the two batteries. When the charge is taken away or the load on the battery is greater than the charging input causing the voltage to drop to 12.75V DC, the relay opens and isolates the two batteries. Isolators 1. Battery Isolators are one-way electrical check valves that allow current to flow to, but not from, the battery. Their disadvantage is that they use diodes, which cause a voltage drop that consumes charging energy, creates heat, and causes batteries to be undercharged. Although alternators with external voltage sensing can correct for undercharging, voltage drop and heat remain a problem. 2. Zero Drop Isolators have more recently been developed to address the voltage drop issue of the traditional isolator but often have a higher price than either of the other two options mentioned above. Automatic Charging Relay vs. Battery Isolator Battery Isolator Splits the current between batteries. 6V Drop - Batteries Under Charged Automatic Charging Relay Passes the current from one battery to the other resulting in a lower voltage drop than battery isolators. .05V Drop - Batteries Fully Charged Engine Alternator Battery 1 Battery 2 Engine Alternator Battery 1 Battery 2 TECH TIP
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