Battery Isolators and Automatic Charging Relays (ACRs): A Comparison

Posted on May 12, 2021 by Robert Iversrud

When it comes to battery management, battery isolators and automatic charging relays (also called ACRs or voltage sensitive relays) are both used to safeguard reserve battery power to prevent breakdowns and protect other vital vehicle or boat functions.

So, how are these two components different? Is one better than the other? When should you choose one over the other? Here are some things to keep in mind.

Battery isolators and automatic charging relays (ACRs) are both designed to distribute charging current to multiple batteries, but each manipulates 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 is a slight voltage drop (about 0.7 volts) which can result in undercharged batteries unless alternators with external voltage sensing are used. Isolators with solid state construction, such as many in the Eaton Sure Power product line, have fewer 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 close 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 thus won’t work with an isolator. ACRs can also simplify system design because chargers and regulators that work with a single battery bank will still work with a multiple battery bank.

Both types of charge management devices—battery isolators and ACRs—are commonly used on boats as well as work trucks and other commercial vehicles. 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.


Eaton Sure Power Multi-Battery Isolator

Blue Sea Manual Control Automatic Charging Relays


  • 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 combine batteries during charging and isolate batteries when discharging and starting engines
  • Start Isolation (SI) protects sensitive electronics by temporary isolation of house loads from engine circuit during engine cranking
  • Hermetically sealed to IP67
  • Dual sensing – senses charge on both battery banks
  • LED indicator light displays functionality and battery status

For a third battery management option—using battery separators—see the article How to Choose Between Battery Isolators and Separators.

See our entire selection of battery management products.