home page logo


High output Motorhomes

Popular Auxiliary battery Isolation

Trailer, Slide-in Camper mounted isolation

Tow Vehicle Backup system


Camper / Trailer Application Notes

rvpark.jpg (19800 bytes)When it comes to RV or trailer camping, the usage on your battery differs substantially from that of your vehicle primary starting battery.   When you set up for camp, you need lights, power for your water pump, maybe a fan, radio, or TV etc...

You operate these during times when the engine is not running.    During this time, your RV battery slowly empties its charge.   With your trailer or RV battery connected to your charging system, your starting battery will share the current to your loads.   The problem is, your starting battery was never designed to perform in this manner and, as a result, will fail early in its expected life.   The other problem is a little more serious.   You may be enjoying your stay so much that you stay put for a while.   Eventually you may run out of battery charge, leaving you stranded without starting power!   

People have utilized many different ways to save your starting battery from this annoying situation.   They have used diode isolators, relays with switches, relays wired to ignition circuits or accessory circuits, high current switches, or automatic  mechanical combiners.   Each of these methods have their own advantages and disadvantages.   Some are more complex than others.    Now, you have the choice of using a system with more advantages than ever before!    The Hellroaring BIC-95150B and BIC-95300B Battery Isolator/Combiners are the best solution on the market today.  

RV Starting Battery Isolation

For motorhome types with high output alternators and large house loads while operating, consider the RV starting battery isolation technique as a simple solution to multiple use batteries.  With this setup, a high output alternator can supply maximum charge current to large house battery banks.  All batteries receive proper alternator voltage from an internally sensed alternator due to the very low ON resistance of the BIC-95150B (less than 0.004 ohms).  Since there is no diode drop, the starting battery stays fully charged.  Electrical loads on either battery will not draw from the other.  If you choose this method, we recommend that you contact our technical support staff at our sales email for assistance.

RV-starting battery.jpg (47013 bytes)


Basic Auxiliary Battery & Trailer Application

For most applications, (i.e. slide-in camper, trailer, RV's with required continuous loads under 50 amps on the auxiliary side during engine operation, etc...) consider the basic Auxiliary Battery setup. Because it is so simple, it has become one of our most popular setups!  Your primary objective is to prevent your  starting battery from draining while you consume power from your house (deep cycle) battery.   (Don't be fooled by low cost battery disconnect devices as a solution.)    You also want your house battery to obtain full charge as quickly as practical while operating your vehicle.   With the BIC-95150B installed as shown below, you will obtain those two objectives!   The BIC-95150B has very little resistance (less than 0.004 ohms) when ON during charging and it automatically isolates the two batteries when charge voltage BIC_small_photo.JPG (11235 bytes) stops, even if your alternator fails.   This setup is NOT for Full Backup starting. But, if desired, you will have the ability to combine the batteries manually for a starting boost (i.e. the main battery must still supply substantial cranking current such as cranking slowly, or almost turning over).   The BIC has no mechanical moving parts to degrade or wear out!  For those on a budget and who want the simplest installation, the BIC-95150B basic Auxiliary setup makes an excellent choice for full charging without the voltage drop or bulky physical size associated with diode isolators!  Works great for alternators sensed at the alternator output (i.e. internal sense or single wire alternator.) 


Auxiliary Battery Isolation.jpg (53544 bytes)


Trailer / Camper mounted battery isolation

Although the BIC's can be mounted in a camper or trailer, we recommend mounting a BIC-95150B in the Tow vehicle instead for best performance.

With a Hellroaring BIC-95150A, you can install it in the camper itself. There is no need to alter the tow vehicle alternator system. Since the BIC-95150A is basically a DC Solid State Relay, like what others may call a battery separator, it has integrated control electronics designed for dual battery systems, and it does not require ratings based on the alternator, like a diode isolator does. This is because it never receives all of the alternator output. Rather, the BIC rating need only be higher than the maximum continuous current expected in the camper charge line.  And then, you normally protect it and the wiring by installing an in-line fuse or circuit breaker, where we recommend 50 or 60A rating with 8 AWG wire.   

We aim to limit the peak demand to the Auxiliary battery to about 60 to 80A and average under about 50A by recommending a minimum length of 20 feet total length of not larger than 8 AWG, or 6 AWG if your length exceeds 35 feet. This also provides a good margin, well within the BIC-95150A continuous capability of 95 amps.  For those applications where even more continuous current is required, the BIC's can be wired in parallel for double or more capability. 

One key point about mounting in the camper rather than the tow vehicle: You can expect reduced performance, i.e. reduced charge rate, due to factory tow harness charge wires being long and small prior to reaching the BIC mounted in the camper. This may, at times of higher demand, see the voltage dropped across that long charge line and sufficient resistance exists in that input path, the BIC will sense that drop in voltage and go into a cycling mode which would effectively reduce your charge rate by about half. That is, unless you run an external Sense wire from the BIC sense terminal, through a spare pin or other connector to the truck, and back towards and connects near the main battery or alternator end. This way the BIC will not sense the voltage drop across the small and long factory tow harness wire.

For this reason, and because most customers do not have a spare wire to use or don't want to deal with adding a new wire in the tow vehicle, we would recommend mounting the BIC under the hood of the tow vehicle instead for BEST performance. And, in this case, the BIC-95150B provides the simplest and cleanest installation. The BIC-95150A has the external sense terminal and requires and external sense resistor and TVS diodes mounted externally on the terminals. The BIC-95150B has all that built in internally for a cleaner installation, but does not have the ability to run the separate sense line. So, again the BIC-95150B is best mounted in the tow vehicle under the hood.

With this setup, you can connect your trailer to the tow vehicle and leave it connected for as long as you want.  The trailer loads will not drain your vehicle battery.   If your tow vehicle is already equipped with a towing package, you should have a charge wire available near the trailer hitch.  However, if your factory charge wire is smaller than 10 AWG, and If you have sufficient alternator capability, you can increase your peak and average charge rates by upgrading the charge wire to 8 AWG, or 6 AWG if your length exceeds 35 feet from the BIC. 

With the AC charger connected as in the diagram, you will charge your camper battery only.  If you connect it to terminal "A" instead, then you will charge the vehicle battery also.

Diagram to be updated...

trailer.gif (64112 bytes)


Tow Vehicle Backup Battery System

If you like to disconnect your trailer and drive around to enjoy the scenery, you will not have your trailer loads to worry about.   But, suppose that, while you are out and about, your alternator fails.   They always seems to fail at the most inopportune time.   You may not notice the problem until it is too late and your engine dies or will not start.   You may be parked 20 miles from nowhere, walk back to your vehicle after a nice afternoon fishing, and find your vehicle unable to crank.   It sure would be nice to have a spare battery at a time like this.   Then consider the Basic Backup Battery setup illustrated below.   With this setup you have an emergency backup battery that is always fully charged, even when you disconnect your trailer and your alternator quits.  

If you look closely at why you need an isolation system anyway, you will discover that it is for one primary reason.  That is; to have the ability to start and operate your engine when you have inadvertently drained your starting battery.     Most people only foresee the possibility that their house loads will drain the starting battery.   Because they aren't aware of other possibilities, their focus goes toward preventing that condition only.   We believe that you should not limit your security to only one of the possible ways to drain you starting battery.  When you spend time in the field, you should cover all likely possibilities, including alternator failure.   Those who connect relays to your ignition system obtain a false sense of security.  If their alternator unknowingly fails, their vehicle will continue to operate until both batteries are drained.    All the while they believed that their backup battery was being charged!    Our BIC units will protect a spare backup battery from discharge due to either draining your house/trailer battery, if your alternator quits, or even if your main battery effectively gets disconnected due to corrosion!

Consider this installation suggestion:

Suppose you install a super combination deep cycle/starting battery such as an Optima Yellow top as your starting battery (It is rated for deep cycle use, yet it has plenty of cranking power - 750 CCA.)   Then install, in your vehicle, a regular starting battery as a spare backup battery isolated with a Hellroaring BIC-95300B configured for high current.   With this setup, you don't need isolation tobic75300 -small.JPG (10033 bytes) your trailer or house battery.   You will always have a spare capable of cranking your engine!   If your house loads drain your starting (deep cycle) battery, you can, with the simple flip of a micro-switch, crank your gasoline engine  (You will need two units in parallel and two Backup batteries for a Diesel Engine that requires two main batteries.)  An added benefit to this system is that you will gain battery capacity by the amount of your main battery.    This will add life span to your batteries because you will not need to discharge either one as deeply than as with a single battery.   With this setup, you will need less circuit breakers, less hardware, less space, and will receive more security.

Sure, there may be less expensive individual isolation products available.    But, none stack up to all the benefits that the BIC-95300B is capable of.   After considering all the parts and labor needed to perform the same functions, We believe this setup is one of the simplest, most convenient to use, most reliable, most economical and practical system available for a Backup Start application, and the BIC-95150B is the best for an Auxiliary or Camper battery application.

Basic Backup with Trailer Schematic.jpg (57989 bytes)


Here is one very nice benefit from this setup.   If you use your tow vehicle in cold climates, then you can have double the cranking power when you need it the most as the temperature drops to below -10F.   There is no need to even get out of the vehicle nor open the hood!   Simply flip a micro-switch on the dashboard and you are on your way!   We believe that this is one of the very best setups available!

With this setup, we believe that you do not need isolation between the trailer battery and main battery for two reasons.  1) You will have the backup starting capability, and 2) The trailer battery usually is not connected all the time.  However, if you feel you must have isolation here in addition to having the backup capability, it is very simple to add a BIC-95150B between the alternator and trailer battery or in the trailer itself as described above.

If your application is extreme such that you want multiple and different isolated batteries/loads, you can add a BIC-95150B for each Auxiliary battery bank in addition to the BIC-95300B with a backup battery. 

Yes, I want one today! - Click on the Sales link at the bottom of this page.

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
Sales/Pricing info: Sales page

Privacy Statement

We have the utmost respect for your privacy.  We do not collect information from you other than that required for us to conduct business and communicate with you.   We do not sell, share, nor distribute any information or addresses collected from you.  We do not create nor use mass mailing lists.  We communicate with you one to one and utilize Phone or voice mail if necessary and e-mail.   We do not use automated, time consuming, non-personalized phone systems.  We ask for and obtain your permission prior to using any communication quoted in our marketing information. 

Send mail to sales23@hellroaring.com with your orders, questions, or comments.

Copyright 1998-2023    Hellroaring Technologies, inc.
Last modified: 10-31-2023