Boat Battery Isolators and Charging Relays

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Boat Battery Isolators & Charging Relays How-Tos

Selecting the Correct Automatic Charging Relay

When selecting a particular Automatic Charging Relay (ACR), there are many things to consider in order to get the correct system for your application. You will need to take into account the continuous rating and the intermittent rating. The continuous rating will need to reach levels above the highest alternator output rating, and the intermittent rating will need to be above the biggest load for the battery. You will then need to carefully consider which preset ACR settings to choose, followed by the product features.

Preset ACR Settings

Combine Voltage:

  • Batteries will share the present charge.
  • Voltage:13.0 at 2 minutes.
  • Closed relay and combined batteries.

Open Voltage:

  • No present charge; charge input loads will not exceed set limit.
  • Voltage: 12.75 at 30 seconds.
  • Batteries do not share the charge.

Under Voltage Lockout:

  • Charge may or may not be present.
  • Voltage: 9.5.
  • Batteries do not share the charge.

Product Features

Auxiliary Battery Priority (Optional):

  • Combines batteries and closed relays for extended length of time.
  • Open voltage ranging between 12.25 and 12.75 volts.
  • Engine must be running.

Start Isolation (Optional):

  • Batteries are isolated, and the relay is open.
  • Vulnerable products will be shielded by isolated batteries.
  • Engine must be starting.

Start Assist:

  • Closed relay, combined batteries.
  • Batteries share power.
  • Engine must be starting.

Engine Isolation:

  • One relay will be open, while the other will be closed.
  • Two engines must be running.

We do recommend consulting with a mechanic or technician in the process of choosing an ACR and its features.

Blue Sea Systems Tech Tip: Automatic Charging Relays Explained

Here you can learn about Automatic Charging Relays, Battery Isolators, and Zero Drop Isolators.

Infographic: Automatic charging Relays Explained

Automatic Charging Relays allow two banks on a vessel's alternator to be charged simultaneously from one unit. Doing so, keeps the batteries isolated when not charging. This benefit is useful in the case of one battery failing and having another battery as an emergency back-up. There are a few main styles to choose between:

  • Automatic Charging Relays:Combined voltage and relays that reach over 13V DC will charge two batteries at the same time. When the charge is inactive or overloaded, the voltage will drop to around 12.75V DC. The relay will open, and the two batteries will be isolated. An Automatic Charging Relay allows the current to pass easily from one battery to the next.
  • Battery Isolators:Currents will flow more swiftly to the battery. However, because of the presence of diodes, the voltage is more likely to decrease, which consumes more energy and does not charge batteries quite as quickly. Also, the heat levels will most likely rise, and the currents will split.
  • Zero Drop Isolators:These isolators are useful in addressing the problem of voltage- dropping. These will cost more than the other options due to limited market acceptance.

Applying one of these three concepts to your alternator will bring about different advantages and will inevitably improve the lifespan and quality of your batteries. We recommend, for the sake of safety and accuracy, consulting with a technician or a mechanic in choosing the appropriate method before making any purchases.

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