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Battery Isolators vs. Automatic Charging Relays (ACRs): Thursday Throwdown

Posted by K.E. Gregoric on Jun 16, 2016 3:56:20 PM

When it comes to battery management, both Battery Isolators and Automatic Charging Relays (also called ACRs or voltage sensitive relays) both safeguard reserve battery power, so as to prevent breakdowns and protect other vital vehicle or boat functions.

So then, how are these two components different? Is one better than the other? Who should choose one over the other? Here, we examine these questions.

Both battery isolators and automatic charging relays (ACRs) are designed to distribute charging current to multiple batteries, but manipulate current differently.

Battery isolators use silicon diodes to split a one-way flow of current that charges both batteries evenly so that accessory batteries cannot drain power from the main starting battery. While the isolator is the simpler and more familiar of the two methods, one drawback of this process is a slight voltage drop (about 0.7 volts) which can result in undercharged batteries, unless alternators with external voltage sensing are used. However, isolators with solid state construction, such as many of those by Surepower, means less moving parts that can malfunction or wear out, though these can generate heat.

Isolators vs. ACRs

ACRs are automatic switches that sense when a charging source is being applied to either battery, and closes when the voltage on one rises high enough to indicate it is at or near full charge, normally around 13.3V, until it decreases to a safe level. This prevents both batteries from draining when there is not enough current to charge both. One advantage of the ACR is that it can be used with outboard motors and inverter/chargers that also absorb starting current and therfore can't work with an isolator. Blue Sea also asserts that ACRs simplify system design because "chargers and regulators that work with a single battery bank will still work with a multiple battery bank".

Both of these types of charge management devices are commonly used on boats, as well as work trucks and other commercial vehicles. But according to Waytek Account Manager Robert Iversrud, the preference for which one is used often boils down to the application and electrical design used. 

Whichever of the system you select, Waytek carries some standout options in both categories.


Sure Power Multi-Battery Isolators

Blue Sea Manual Control Automatic Charging Relay, 500A

battery isolator

  • Eliminate multi-battery drain when two or more battery banks are in a charging system
  • Perform as well or better than existing factory installed components
  • Designed to exceed OEM specifications
  • Solid-state electronics
  • Isolates each battery circuit and allows each battery to discharge and charge according to its own needs      








  • Automatically manages the charging of two large battery banks
  • Allows paralleling of battery banks for emergency starting
  • Magnetic Latch (ML) - ACR draws very low current (<40mA to monitor terminal voltage) in the "ON" or "OFF" states, and draws moderate current for very short time when changing state
  • Dual sensing - senses charge on both battery banks
  • Start Isolation (SI) - can be configured for temporary isolation of House loads from Engine circuit during engine cranking to protect sensitive electronics
  • Engine Isolation (EI) - can be configured for isolation of two engines while both are running to protect engine electronics and maximize alternator output 

To learn more about these products, contact us


See our entire selection of battery management products.


Topics: Resources & Tools, Buying Guides, Battery Management, Battery Isolators, ACRs


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