74 BATTERY MANAGEMENT promariner.com 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 used on boats: 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. 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. 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. 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 12.75 in this example ACR relay opens, isolating batteries while discharging. Automatic Charging Relay vs. Battery Isolator Battery Isolator .6V Drop - Batteries Under Charged Automatic Charging Relay A lower voltage drop replacement for battery isolators 05V Drop - Batteries Fully Charged An ACR passes the current from one battery to the other An isolator splits the current Engine Alternator Battery 1 Battery 2 Engine Alternator Battery 1 Battery 2 Intro to Automatic Charging Relays Automatic Charging Relay Operation
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