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Marine Application Notes:

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Sometimes, It's a long swim!

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Get back to shore with starting power confidence!

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Many boats are equipped with dual batteries and a heavy duty switch with an OFF, 1, both, 2 position.   So that maximum cranking power is achieved, an operator may set the switch to "both" just before starting.  After starting, the switch is left on "both" so that charging both batteries is achieved.   But upon arrival at the desired location, the engine is turned off, leaving the operator to "remember" to switch the battery from the "both position".   It is very easy to forget this step and consequently run down both batteries.   This can leave you in a dangerous situation!  ( i.e. without power to operate your boat or your electronics.)  There are many many ways to prevent this (all with their own pros and cons.)   The discussion below covers some of the best ways to keep your batteries isolated and prevent this "hell of a situation" from occurring. 

One of the first options one normally considers is the basic diode isolator.     But the diode isolator has an inherent voltage drop of about 0.7 volts even at low currents.   This loss of voltage tends to result in undercharged batteries.   Batteries left in undercharged conditions for extended periods will loose capacity and its life will be substantially reduced.    With diode isolators, all of the alternator current must flow through the device resulting in maximum heat generation.  Therefore, for high current alternators, the diode isolator requires very large heat sinks.   This is not so with the Hellroaring battery isolator / combiners.  They can be set up so that the majority of the alternator current bypasses the isolation device.   We recommend that you do not use plain diode isolators.

Combining batteries during charge eliminates this chronic undercharge condition. Of course, you could combine them manually during charge by switching a relay.   However, this still requires that you to remember to switch at the proper time.    Or you could wire a relay to your ignition switch, but this will only give you a false sense of security.   If your alternator quits, you will still run down both batteries!   This is a time when you need your backup battery the most!

We recommend an automatic combiner as the better solution.   But, since mechanical relays/solenoids have their own problems such as sticking contacts, arcing and contact wear, we recommend our solid state automatic battery isolator/combiner as the best solution.

Combiner Installation Methods:

There are many different ways to connect automatic combiners.   Some of them are discussed below.

To eliminate confusion and reduce the risk of alternator damage from battery switching at the wrong time, some people have set up their boats with a system of  three (3) single ON/OFF switches and include an automatic combiner between the batteries.   One battery is connected to the starter circuit through one switch, and the other battery is connected to your alternator and accessories through another switch.  These two switches are always ON but isolated from each other under normal conditions.   The third switch connects the two batteries together when needed and is normally left OFF.   An automatic combiner is also connected between the two batteries that switches ON only when charge voltage/current is available.   The logic is that this switching system is easier to understand and that the risk of accidentally opening the alternator circuit (which can damage your alternator) is substantially  reduced since the switches normally stay in the same position.   However, it is still possible to do this.   Therefore, an alternator protection device is recommended.    We believe that this is a sound dual battery setup.   This setup was conceived as one of the best ways for using an automatic mechanical combiner.   Since the introduction of the Hellroaring Battery Isolator/Combiners, new connection options have become feasible that are less complex and less expensive, even more reliable. 

Our Battery Isolator/Combiners can be well utilized in place of this mechanical combiner circuit application as well as many other circuit configurations.    Our units have additional advantages that mechanical solenoids or relays do not.   The main advantages to our units are:

  • Very low operation current (less than 0.012 amps).  This is great for use with solar panel charging. 
  • No arcing contacts to wear out or get stuck!  
  • Our units are environmentally sealed!   After all, a boat is bound to get wet!
  • Our units have a remote ON switching capability such that you can first switch our combiner ON, then your set your parallel switch ON or your original switch to "both".    This eliminates the high inrush currents that can damage the switch contacts.


In the quest for simplification, you should examine the pros and cons of different wiring methods and choose one that best suit your needs and budget.   With the Hellroaring Battery Isolator/Combiner solid state technology, you have even more choices for a dual battery or three battery system.     Some of the favorite connection schemes with mechanical Combiners may no longer be the best choice!     

Click on these links for a schematic diagram with pros & cons for each installation configuration:

View images of a backup installation on a Bayliner 2050 SS :

Main Battery

Backup install view

BIC View

Remote View

Starter Connection


Also, read some customer feedback.

As you can see, the Hellroaring BIC-95150B and BIC-95300B Battery Isolator Combiners are very versatile in application.   Check back again.   We intend to add additional configuration suggestions and tips including those with multiple engine alternators.    Let us know of your particular application if not covered here.    We will try to include it somewhere on our web site..


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Contact Information

For General Information, Sales, Technical or Customer Support:

Telephone: 406-883-3801
Postal address: P.O. Box 1521, Polson, MT 59860
Electronic mail: sales23@hellroaring.com
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Last modified: 10-31-2023