Rather than use the term 'isolator' which is often confused with a diode-steered device, I use Automatic Charging Relay, Voltage Sensing Relay or Combiner/Separator. The benefits are:

  • Its automatic and therefore convenient.
  • It can protect your alternator from abruptly seeing an inordinate load.

The cons are:
  • ACR & VSRs manifest in a number of wiring schemes according to the individual manufacturer. Many of these schemes are suitable for preserving the starting battery against drain while using instrumentation (chart plotter, fish finder, GPS, etc) that draws very little current. These schemes often contradict what is really needed for large audio system usage. You'll note that ACR/VSRs are not usually used in circuits applicable to trolling motors which closely parallels a large stereo at rest.
  • Oftentimes, low voltage will keep the relay or solenoid from closing or staying closed which results in a lack of alternator charge and deeper cycling which reduces the stereo battery lifespan. An ACR/VSR makes an AC shore charger essential.
  • None of the ACR/VSR manufacturers have a scheme to facilitate the use of an AC shore charger. They expect two battery banks to be charged via a single-bank charger or the ACR/VSR stays closed with high voltage thereby circumventing the individual battery bank profiles and charge program.
  • ACR/VSR schemes must be flexible to fit the system size. There isn't a single prescription that fits all systems. As the stereo current draw increases, and as battery amp/hours increase and depending on the size of your alternator, your design should be altered to fit the situation.
  • Many of the ACR/VSRs are dual-sensing rather than using a directional priority charge system. And this dominoes into additional complexities.
  • In most cases you have to supplement an ACR/VSR with some form of switch or additional relay to provide a.) emergency bypass, b.) AC charging isolation and/or c.) elimination of small current draw while the boat is in storage. Again, it differs with each system.

In summary, my take is if you're doing it on the cheap or you don't have the tolerance for the added technical issues then just stick with a simple switch. Know when to connect the stereo bank to the alternator in order to strike a predetermined balance between preserving your alternator or extending your battery(s). Keep in mind I'm still a major advocate of an ACR/VSR if you're going to do it right.

Earmark Marine