Automatic Charging Relays Explained

What do you do when the number of battery banks on your boat is greater than the number of charging outputs from your system’s battery charger, and you want to provide a charge to all batteries? How do you design a boat electrical system that charges two battery banks without adding the cost of a dual output charger? Instead of upgrading your charging system, or installing a system with multiple outputs, install an automatic charging relay (ACR). The cost of a charging system with multiple outputs is considerably more expensive than one with single output, making the installation of the ACR a less expensive option.

For example, consider a typical marine electrical system with two battery banks, a battery charger with one output, and an alternator. To charge both battery banks from your single output charger, connect the ACR between the battery banks. Connect the alternator to charge battery 1 (start battery), and connect the charger to charge battery 2 (house battery). When the charger is charging battery 2, the ACR will combine battery 2 and battery 1 for charging. When the alternator is charging battery 1, the ACR will connect battery 1 and battery 2, and both batteries will be charged. With the installation of the ACR, when at dock and plugged into shore power, the charger is supplying a charge to both battery banks; when the engine is running, the alternator is charging both battery banks.

Similarly, if you are adding an auxiliary battery to power a windlass, you can connect the auxiliary battery through an ACR to a battery bank that is attached to a charger. In this way, you are able to charge an additional battery bank without upgrading the charger to one with multiple outputs.

The ACR automatically connects batteries during the charging cycle and disconnects them under discharge.

When combined with Blue Sea Systems Dual Circuit Plus™ Battery Switch (PN 5511e), the SI ACR model fully automates the charging of two batteries. The combination of the Dual Circuit Plus™ Battery Switch and ACR provides a practical and less expensive solution to manage:

Isolated battery circuits
Emergency parallel backup operation
Automated charge management

Simply turn the battery switch to the ON position when arriving on the boat, and turn it to the OFF position when leaving. You no longer have to worry about which batteries are charging or discharging.

Before replacing your charger or inverter/charger, consider the benefits of an ACR; and for a fully automatic system, add a Dual Circuit Plus™ Battery Switch.

In a boat or vehicle with two battery banks, it is useful to be able to charge both banks while underway. Charge management devices connect two battery banks when charging, and isolate them from each other when not charging. If one battery becomes depleted, there will be a charged bank available for emergency starting.

There are two main 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.

When choosing an ACR, consider:
• Alternator output: The first consideration when choosing an ACR is the output rating of the alternator, described in Amps. A good rule of thumb is to select an ACR that has a Continuous Rating above the rating of the alternator output.

• Auxiliary Battery Loads: A second factor to consider is the largest load on the Auxiliary battery such as a bow thruster. Since the current from loads on the Auxiliary battery could pass through the ACR and damage the relay, select an ACR that has an Intermittent Rating higher than the largest load on the auxiliary battery.

• ACR Additional Features: Finally, there are a number of unique features on the five models of ACRs that should be considered. These features range from Auxiliary Battery Priority that shares the alternator charge with the auxiliary battery longer when the engine is running, Start Isolation that protects sensitive electronics from voltage spikes and sags during starting, Start Assist which provides combining the auxiliary battery with the start battery to share power in the event of a low start battery, Engine Isolation which isolates two engine batteries to protect engine electronics, and Manual Override providing an added level of control in emergencies and for service lockout.