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Friday, October 23, 2009

Battery Separation Anxiety

I have an oldie, a 1973 Lifetime motorhome with a Dodge chassis. It looks like the batteries are connected together with one 10-guage wire from the alternator charging circuit going to some kind of box. Another wire supplying 12-volts from the auxiliary generator and one from the batteries goes to the same box. Does this sound right? I just bought the motorhome and the first thing I had to do is rebuild a five foot section of the wiring harness because of a short circuit that melted that section of the harness. I am a good mechanic but don't know anything about dual charging circuits. The box I am referring to is square in shape with aluminum fins on either side with three electrical contact studs which form a normal triangle. What exactly is this box?
Wilfred, (Elizabethtown, KY)


Wilfred, the device you have located is called a dual battery isolator. It is constructed with two diodes which allow the alternator to charge both battery systems while keeping them totally separated. The two batteries should not be connected together for the simple reason that each battery is designed for a different purpose; one to start the engine, the other to power the 12-volt circuits inside the motorhome. Typically an automotive start battery is used to crank the engine and a deep cycle or RV/marine battery is used to power the RV. By design, these two battery systems must remain separate. The dual battery isolator performs that function. The center terminal (or top of the triangle) receives voltage from the output of the alternator and each side terminal should be wired directly to each battery system. The key is to keep each system “isolated” from each other; hence the name.

To test the existing dual battery isolator, connect an ohmmeter from the center terminal to each of the battery terminals. The meter should indicate continuity only in one direction from the center (alternator) terminal to each battery terminal. Reverse the test leads to verify it’s only in one direction. There should be no continuity whatsoever between the two battery terminals. The isolator is faulty if you can read continuity in both directions between any two terminals or if you have continuity between the two outside terminals. When replacing a dual battery isolator, be sure it is rated higher than the total output of the alternator.

Another method of battery separation is accomplished by the use of “smart” devices such as the Sure Power Industries Smart Solenoid. These devices incorporate a high capacity, electronically controlled solenoid switch within a well monitored charging system. The Sure Power Smart Solenoid comes in two varieties; one begins charging the auxiliary system only after the engine battery has reached a minimum 13.2-volts. The other couples the two systems together in parallel when either battery has reached this pivotal voltage. Until then, the battery systems are kept separate. This is a much better alternative than a standard solenoid, but I personally like to see each battery system completely isolated from each other all the time.

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