Figure 1

Now, lets examine pros and cons for this setup. 

Pros:

• Optimized configuration for emergency backup battery reliability.
• Extreme simplicity!   Heavy Duty Switches are NOT needed!   Zero!     None!    Zip!   Minimal wiring needs.   Alternator protection device NOT needed!
• External solenoids NOT required.   Can be used with 200 Amp alternator!
• Low Cost!   With heavy duty switches costing as much as $30.00 to $90.00 each, consider your savings by eliminating up to 3 of these.   Save about $25.00 for an alternator protection device.   Save money by reducing the amount of wiring required.   Save labor dollars by simplifying installation.
• Easy, simple to understand, installation!   Eliminate all the hole drilling for those switches.
• If you draw down your primary battery, you will have a fully charged backup battery.    No normal loads are on the backup battery.  Therefore, it is always fully charged and not cycled, resulting in very long life.   When your primary battery wears out, move this battery to become your primary battery (if it is a deep cycle unit).   Then replace the backup with a new battery.   This way, you always keep the best battery for when you need it most!
• Simple automatic operation!   Virtually free of human intervention!    No hassle, worry free.
• Simple manual parallel combination via a remote micro-switch.    From a remote console, you can use very small wire running a long distance to switch the batteries in parallel.  
• In this configuration, the BIC-75300 handles the full cranking current (normal 120 to 180 Amps) for a 350ci V8 gas engine (up to 300 Amps) for 15 to 30 seconds.   (For diesel applications, we recommend you add a second BIC-75300 in parallel to share the load current.)
• The Hellroaring BIC-75300 and BIC-75150 limit inrush currents when switching.    No arcing to wear out the device!
• No diode voltage drop effect when ON!   Super low ON resistance (less than 0.002 ohms!)

Cons: (these are not all cons)

• If you connect your main starting battery to the accessory (house) buss, your starting battery will, by default (*except as noted below), be utilized in a deep cycle application.   This has historically been and still is not good practice.  A starting battery will readily be damaged by deep discharge applications.   In the past, deep cycle batteries were not designed to handle large cranking currents and would be damaged by starting applications.   But, today, there are more choices.   Many deep cycle batteries can handle cranking currents.   An example is the Optima "D" Blue Top or Yellow Top.  They are rated for at least 750 CCA.   This is only 50 CCA less than their starting battery.   A typical 350 cubic inch engine, for example, only requires a battery rated at 540 CCA or better.   (*  Note: When this circuit is utilized in an automotive application where there are no normal "engine off" loads, this is not a problem and a starting battery is acceptable.)

• Mechanical Relay Con:  If you drain your main battery and then parallel your fully charged backup battery, you will initially draw substantial current from it.     One reason people have avoided this system is because, when switching, high inrush currents can damage mechanical contacts.   Also, many people believe that this will suck valuable energy needed for cranking.    The truth is, unless the main battery is damaged with internal shorts, this is better than using the fully charged battery alone.   This is especially true when you allow a few minutes before cranking.     It is true that when you connect them in parallel, current will flow from the fully charged battery to the discharged battery.  As it does, charge builds up in the discharged battery.    The longer you allow this condition to exist, the more charge transfer takes place until equalization (which you don't require in order to crank).   A fully discharged battery at rest has a voltage of about 11.5 volts (no load and no charge).   When you crank your engine, your (fully charged) battery terminal voltage will normally drop to less than this 11.5 volts.   Therefore, current will no longer flow into the weak battery but will only flow to the starting load.   Furthermore, whatever charge has transferred to the weak battery will cause it to supply additional power to the starting load!   So this is a Con only with mechanical relays.   PRO:  The Hellroaring solid state isolator/combiners limit inrush currents when switching.   There is no arcing to wear out the device!

• Mechanical Relay Con: If you drain your main battery and you want to continue operating accessories with the remaining power from your backup battery and you energize a mechanical combiner, it will consume energy that you want to conserve.  Therefore, some people would install an additional heavy duty switch.     PRO:  With a Hellroaring Battery Isolator/Combiner, the energy consumed is insignificant (less than 0.012 amps when ON) allowing this setup to be practical! 

• The dc load buss voltage will drop while starting.   Electronic devices sensitive to this may experience trouble.