Automatic Charging Relay

[Johmas50]

Can someone explain to me in easy terms what the ACR does? I currently have 2 batteries on a Blue Sea Systems switch similar to this one:

http://www.wholesalemarine.com/blue-sea-systems-e-series-battery-switch-selector-4-position.html

But I don't have an ACR, what does the ACR do, and what advantage would this give me if I installed one?

 

[tdonoughue]

I kinda had the same question of late. I found this, which was helpful:

http://www.bluesea.com/resources/1366/Automatic_Charging_Relay_[ACR]_Explained

Also this:

 

[txav8r]

An ACR is a switch but has to be satisfied to close, so it is called a relay. If a charge of sufficient and correct voltage is sensed, it closes and lets that charge flow through to the other battery. The benefit is keeping both batteries similar in state of charge, and allowing the engine charging system to help keep both charged instead of just the ship battery. The negative can be letting the house battery get significantly lower than the ship battery, and that can set up a significant heat transfer with them trying to charge and stablize. The good news, when a battery is severely depleted, it will be below the threshold of the ACR and it won't close anyway. It is possible to put the ACR on either the battery side or the load side of the switch. If it is on the battery side, it is the same, as just placing it between both batteries directly...but off the switch takes significantly less cable than to run between the two batteries. Here is a diagram explaining the ACR a little more...

 

[David Analog]

An ACR/VSR is a voltage sensing relay (also auto charging relay) that separates the two battery banks when the voltage on the sensed side drops below a predetermined threshold. This allows you to discharge one bank (call it the stereo bank) when at rest and operating electronics while keeping one bank (call it the starting bank) isolated, fully charged and in reserve for subsequent starting.
When the boat is running and the rpms are up , the combined voltage of the charging system and starting battery pass a different predetermined threshold, then the voltage sensing relay combines the two banks allowing the charging system to service both banks.
All of this you can do with a manual dual battery switch. But the ACR/VSR adds the element of convenience by doing it automatically. It also can add an element of safety for your boat's charging system. A 'stereo bank' that has a large reserve capacity and is deeply discharged represents an instant and tremendous current load on the charging system (alternator or stator). When combine with the draw of boat operations and the continued play of a large stereo, this can significantly exceed the current and thermal capacity of the alternator. An ACR/VSR will not allow the two banks to combine in this case until there is a sufficient charge. The ACR/VSR may attempt to do so based on an adequate surface charge, but as the largely depeleted battery is added into the circuit the voltage will quickly sag and immediately trigger the separation. Thus, protecting the charging system. Since a Yamaha does not have a robust charging system, this scenario can occur more often than it would in other boats.
In the basic sense an ACR/VSR works simply. However, in reality it's behaviors do change when presented with different systems. You have to know that one approach does not always fit all systems. The initial designers envisioned usage with low draw instrumentation (fishfinders, GPS, etc.). They didn't intend these to be used with megawatt stereos and massive battery banks. That's why you never see an ACR/VSR used with a battery bank serving a high current trolling motor.
Also worth mentioning with ACR/VSRs, is that from brand to brand, voltage sensing and charging is done very differently. Some are bi-directional. Some are single side sensing and priority charge. Pre-packaged kits with manual switches also use different switch circuits with different schemes. A Blue Sea and BEP Marine kit can be different.